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MANUAL 


INORGANIC  CHEMISTRY, 


ARRANGED  TO 


FAC'ILlTATE'fHE 


EXPERIMENTAL    DEMONSTRATION 


FACTS  AND  PRINCIPLES  OF  THE  SCIENCE. 


CHARLES  W.  ELIOT, 


PROFESSOR 


FRANK  H.  STOKER, 


PROFESSOR  OF    GENERAL  AND   JNDUSTRIAJ 
METALLURGY,  CHEMISTRY, 

IN  THE  MASSACHUSETTS  INSTITUTE   OF  TECHNOLOGY. 


BOSTON : 

IPRIIN'TIEID    FOR    THE    AUTHORS. 

1867. 


X 


Entered  according  to  Act  of  Congress,  in  the  year  1866,  by 

F.  H.  STOKER  AND  C.  W.  ELIOT, 
In  the  Clerk's  Office  of  the  District  Court  of  the  District  of  Massachusetts. 


ROCKWELL    &    ROLLINS,    PRINTERS 
122  WASHINGTON  STREET,  BOSTON. 


THE    AUTHORS 

INSCRIBE    THIS    BOOK    TO    THEIR    TEACHER     IN     CHEMISTRY, 

PKOF.  JOSIAH  P.  COOKE, 

OF     HAEVAKD     COLLEGE, 
IX  TOKEN  OP 

GRATITUDE  AND  FRIENDSHIP. 


I'm 


ERRATA. 


Page  7,  line  3,  for  10,  read  20. 

" .   43,  lines  5  and  14,  for  kilogramme,  read  gramme. 
*'     49,  line  2.  for  hydrogen,  read  oxygen. 
"    51,  line  29, /br  a  crust  begins  to  form  on  its  surface,  read  a 
drop,  taken  out  on  a  glass  rod,  becomes  almost  solid 
on  cooling. 

"     60,  line  3,  for  a  small  glass  retort,  read  an  ignition-tube. 
"     100,  line  3,  for  K,  read  Na. 
"     144,  line  21,  for  Ni2O,  read  Ni2O3. 
"     194,  line  9,  for  in,  read  an. 
"     200,  line  21,  for  H2S,  read  H2Se. 
"     200,  line  38,  for  23,  read  32. 
"     202,  last  line,  for  norganic,  read  inorganic. 
"     210,  line  19,  for  vapor,  read  paper. 
"     221,  ltoe4,/orU,  read  G. 

"     xix.  of  the  Appendix,  line  10,  for  cork,  read  cock. 
To  page  xxxvii.  of  the  Appendix,  add  The  United  States  coin, 
composed  of  copper  and  nickel,  of  the  denomination  5  cents,  and 
date  1866,  is  2  centimetres  in  diameter  and  weighs  5  grammes. 


PREFACE. 


ERRATA. 

P.  3,  line  11  from  foot,  for  "sixty-one"  read  "sixty-five." 

P.  250,  line  4  from  foot,  for  "  polish  "  read  "  potash." 

P.  258,  line  17,  for  AsCl3  read  2AsCl3. 

P.  311,  line  11,  after  the  word  "  potash"  insert  "and  4  or  5 
parts  of  slaked  lime." 

P.  311,  line  11.  Insert  the  sentence,  "  The  mixed  materials 
should  be  dried  at  a  gentle  heat  on  an  iron  pan  before  being 
put  in  the  tube." 

P.  311,  line  15.  Strike  out  the  sentence  beginning  with 
"  instead"  and  ending  with  "  used." 

P.  499,  line  12,  for  "  quantity"  read  "gravity." 

P.  538,  line  14,  for  "  prevent"  read  "  permit." 

"        "   17,  insert  after  "  amount"  the  words  "  of  fuel." 


iui   umistui,  me  ciuuiurs   iiujye   tmii   LUIS  minium 

will  make  it  easy  for  the  teacher,  even  if  he  be  not  a  professional 
chemist,  to  exhibit  to  his  class,  in  a  familiar  and  inexpensive  man- 
ner, experiments  enough  to  supply  ocular  demonstration  of  the 
leading  facts  and  generalizations  of  the  science.  Judging  from 
their  own  experience,  the  authors  venture  to  hope  that  even  pro- 
fessional chemists  may  find  it  convenient  to  have  always  at  hand 

ill 

\ 


PREFACE. 


IN  preparing  this  manual,  it  has  been  the  authors'  object  to  facil- 
itate the  teaching  of  chemistry  by  the  experimental  and  inductive 
method.  The  book  will  enable  the  careful  student  to  acquaint 
himself  with  the '  main  facts  and  principles  of  chemistry,  through 
the  attentive  use  of  his  own  perceptive  faculties,  by  a  process  not 
unlike  that  by  which  these  facts  and  principles  were  first  estab- 
lished. The  authors  believe  that  the  study  of  a  science  of  obser- 
vation ought  to  develop  and  discipline  the  observing  faculties, 
and  that  such  a  study  fails  of  its  true  end  if  it  become  a  mere 
exercise  of  the  memory. 

The  minute  instructions,  given  in  the  descriptions  of  experiments 
and  printed  in  the  smaller  type,  are  intended  to  enable  the  student 
to  see,  smell,  and  touch  for  himself;    these  detailed  descriptions 
are  meant  for  laboratory  use.      In  order  to  mark  as  clearly  as 
possible  the  distinction  between  chemistry  and  chemical  manipu- 
lation, the  necessary  instructions  on  the  latter  subject  have  been  put 
in  an  Appendix.     In  cases  in  which  it  is  impossible  for  every  stu- 
dent to  expeririient  for  himself,  the  authors  hope  that  this  manual 
will  make  it  easy  for  the  teacher,  even  if  he  be  not  a  professional 
chemist,  to  exhibit  to  his  class,  in  a  familiar  and  inexpensive  man- 
ner, experiments  enough  to   supply  ocular  demonstration  of  the 
leading  facts  and  generalizations  of  the  science.     Judging  from 
their  own  experience,  the  authors  venture  to  hope  that  even  pro- 
fessional chemists  may  find  it  convenient  to  have  always  at  hand 

III 


IV  PREFACE. 

the  details  of  several  hundred  experiments,  covering  the  ground 
of  an  extensive  course  of  chemical  lectures. 

The  student  of  this  manual  is  supposed  to  be  already  acquainted 
with  the  rudiments  of  physics.  The  chemist  must  often  depend 
upon  physical  properties  for  his  means  of  characterizing  the  numer- 
ous substances  with  which  he  deals,  and  he  is  nearly  concerned 
with  the  physical  properties  of  gases  and  vapors ;  but  chemistry 
has  now  so  wide  a  scope  and  so  great  an  importance  as  to  deserve 
to  be  studied  by  itself,  and  not  merely  as  an  appendix  to  the  sub- 
ject of  molecular  physics. 

Like  all  elementary  text-books,  this  manual  is -a  mere  compila- 
tion ;  it  embodies  in  a  somewhat  new  form  previously  existing 
statements  of  well  recognized  facts  and  principles  which  have 
become  the  common  stock  of  the  science.  There  is  little  original 
in  the  book,  except  its  arrangement  and  method,  in  part,  and  its 
general  tone.  The  authors  have,  of  course,  drawn  largely  from 
the  invaluable  compilations  made  by  Gmelin,  Otto,  and  Watts, 
and  they  have  also  availed  themselves  freely  of  the  text-books  of 
Stoeckhardt  and  Miller  and  the  writings  of  Hofmann. 

The  book  is  not  written  in  the  interest  of  any  particular  theory 
or  system  of  notation,  but  is  intended  to  exhibit,  so  far  as  is  pos- 
sible within  the  limits  proper  to  an  elementary  manual,  the  present 
state  of  the  scienpe. 

The  authors  will  feel  very  grateful  to  any  one  who  will  com- 
municate to  them  errors,  detected  in  using  the  book^  or  suggestions 
for  its  improvement. 

BOSTON,  June,  1867. 


TABLE  OF  CONTENTS. 


PAGE 

Introduction.  —  Subject  matter  of  Chemistry.  Chemical  change .  Analy- 
sis and  synthesis.  Fact  and  theory.  •  1-4 

Chap.  I.  — Air.  Atmospheric  pressure.  Properties.  Analysis.  -Air  a 
mixture.  Composition  of  air.  -  4-10 

Chap.  II. —Oxygen.  Preparation  and  properties  of  oxygen.  Oxygen 
supports  combustion.  Oxides.  Oxidation.  Wide  diffusion  of  oxygen.  -  11-15 

Chap.  III.  — Nitrogen.    Preparation  and  properties  of^nitrogen.  -      15-18 

Chap.  IV.  — Water.  Properties  of  water.  Steam.  Analysis,  electroly- 
sis and  synthesis  of  water.  Atoms  and  molecules.  Molecular  hypothesis. 
Atomic  weights.  Chemical  combination.  Water  in  nature.  Distillation. 
Preparation  of  pure  water.  Solution. 

Chap.  V.  —  Hydrogen.  Preparation  and  properties  of  hydrogen.  Diffu- 
sive power.  Inflammability.  Heat  from  burning  hydrogen.  Unit  of  heat. 
Oxyhydrogen  blow-pipe.  Form  of  gas-flames.  Explosive  mixtures  of  hydro- 
gen and  oxygen.  Oxygen  burns  in  hydrogen  as  well  as  hydrogen  in  oxygen.  37-47 

Chap.  VI.  —  Compounds  of  oxygen,  hydrogen,  and  nitrogen.  Peroxide  of 
hydrogen.  Oxidizing  and  reducing  agents.  Nitric  acid.  Meaning  of  the 
terms  acid  and  alkaline.  Neutralization.  Nitrous  oxide.  Nitric  oxide. 
Hyponitric  acid.  Nitrous  acid.  Analysis  of  nitric  acid.  Synopsis  of  the 
oxides  of  nitrogen.  Law  of  multiple  proportions.  Definite  and  obscure  chem- 
ical action.  Air  a  mixture.  Anhydrous  and  hydrated  nitric  acid.  Atomic 
weights  and  combining  weights.  Molecular  formulae.  Combining  weight  of 
nitric  acid.  Nitric  acid  reactions.  Symbols  and  notation.  Empirical  and 
rational  formulae.  Dualistic  formulae.  Typical  formulae.  Uses  of  symbolic 
formulae.  Nitrogen  and  hydrogen.  Ammonia.  Analysis  and  synthesis  of 
ammonia.  Nascent  state.  Composition  of  ammonia.  Ammonium.  Salts 
of  ammonium.  Sources  of  ammonia.  Ammonia  water.  -  48-87 

Chap.  VII.  — Chlorhydric  acid.  Properties,  analysis,  and  composition  of 
chlorhydric  acid.  Atomic  weight  of  chlorine.  Synthesis  of  chlorhydric  acid. 
Manufacture  of  chlorhydric  acid.  Chemical  affinity.  Preparation  and  uses  of 
chlorhydric  acid.  Aqua  regia.  •  88-100 


VI  CONTENTS. 

Chap.  VIII. —  Chlorine.  Preparation  and  properties  of  chlorine.  Chlo- 
rine water.  Metals  burn  in  chlorine.  Chlorine  burns  imhydrogen  and  hydro- 
gen in  chlorine.  Combustion  in  chlorine.  Uses  of  chlorine.  How  chlorine  acts 
in  bleaching  and  disinfecting.  Action  of  chlorine  on  ammonia.  Chloride  of 
nitrogen.  Oxides  of  chlorine.  Hypochlorous  acid.  Chlorous  acid.  Hypo- 
chloric  acid.  Chloric  acid.  Perchloric  acid.  -  -  88-115 

Chap.  IX.  — Bromine.  Bromhydric  acid.  Bromic  acid.  Hypobromous 
acid.  Chloride  of  bromine.  Bromide  of  nitrogen.  -  -  11G-120 

Chap.  X.  — Iodine.  Extraction  and  properties  of  iodine.  Tests  for 
iodine.  lodohydric  acid.  lodic  acid.  Periodic  acid.  Iodide  of  nitrogen. 
Chlorides  of  iodine.  Bromides  of  iodine.  The  chlorine  group  of  elements.  120-132 

Chap.  XI.  — Fluorine.    Fluorhydric  acid.    Etching  glass.  -132-135 

Chap.  XII.  — Ozone  and  Antozone.  Allotropism.  Preparation  and 
properties  of  ozone.  Tests  for  ozone.  Ozone  a  disinfectant.  Ozone  in  the 
atmosphere.  Preparation  and  properties  of  antozone.  The  antozone  cloud. 
Antozone  formed  in  all  processes  of  combustion.  Antozone  oxidizes  water. 
Differences  between  ozone  and  antozone.  -  130-150 

Chap.  XIII.  -Sulphur.  Crystallization  of  sulphur.  Crystalline  struc- 
ture. The  six  systems  of  crystallization.  Sulphur  a  dimorphous  element. 
Change  of  prismatic  into  octahedral  sulphur.  Methods  of  obtaining  crystals. 
Soft  sulphur.  Milk  of  sulphur.  Metals  burn  in  sulphur  as  in  oxygen.  Sul- 
phydric  acid.  Preparation,  analysis,  and  properties  of  sulphydric  acid.  Sul- 
phuretted hydrogen  water.  Sulphydric  acid  as  a  reagent.  Heady  decomposi- 
tion of  sulphydric  acid.  PersuTphide  of  hydrogen.  Oxides  of  sulphur. 
Sulphurous  acid.  Preparation,  properties,  and  composition  of  sulphurous 
acid.  Liquid  sulphurous  acid.  Bleaching  power  of  sulphurous  acid.  Oxida- 
tion of  sulphurous  acid.  Sulphites.  Sulphuric  acid.  Manufacture  and  prop- 
erties of  sulphuric  acid.  Its  behavior  towards  water  and  ice.  Hydrates  of 
sulphuric  acid.  Sulphates.  Fuming  sulphuric  acid.  Anhydrous  sulphuric 
acid.  Preparation  and  properties  of  anhydrous  sulphuric  acid.  Hyposulphur- 
ous  acid.  Chlorides  of  sulphur.  -  -  151-11)1 

Chap.  XIV.  —  Selenium  and  Tellurium.  Properties  of  selenium.  Iso- 
morphism. Atomic  volume.  Tellurium.  Compounds  of  tellurium.  The  sul- 
phur group  of  elements.  -  -  195-199 

Chap.  XV. — Combination  by  volume.  Synopsis  of  the  gaseous  com- 
pounds previously  studied.  Condensation  ratios.  Combining-weight  and 
volume-weight.  Double  or  product  volume.  Molecular  condition  of  element- 
ary gases.  Molecular  formulae.  -  -  -  195-205 

Chap.  XVI.  —  Phosphorus.  Allotropic  modifications  of  phospWorus. 
Friction  matches.  Phosphorescence.  Solutions  of  phosphorus.  Poisonous 
properties  of  phosphorus.  Manufacture  of  phosphorus.  Red  phosphorus. 
Amorphous  and  crystallized  red  phosphorus.  Safety  matches.  Phosphu- 
retted  hydrogen.  Preparation  and  analysis  of  phosphuretted  hydrogen  gas. 
Composition  of  phosphuretted  hydrogen  as  compared  with  that  of  ammonia. 


CONTENTS.  VII 

Liquid  and  solid  phosphuretted  hydrogen.  Compounds  of  phosphorus  and  oxy- 
gen. Bed  oxide  of  phosphorus.  Hypophosphorous  acid.  Phosphorous  acid. 
Spontaneous  combustion.  Phosphoric  acid.  Hydrates  of  Phosphoric  acid. 
Meta-  and  pyro-phosphoric  acids.  Chlorides  of  phosphorus.  Dissociation. 
Bromides,  iodides,  and  sulphides  of  phosphorus.  -  -205-237 

Chap.  XVH.  —  Arsenic.  Sources  and  properties  of  arsenic.  Arseniu- 
retted  hydrogen.  Isomerism.  Properties  and  uses  of  arsenious  acid.  Solubil- 
ity of  arsenious  acid.  Arsenic  acid.  Salts  of  arsenic  acid;  their  analogy  to 
the  phosphates.  Detection  of  arsenic  in  cases  of  poisoning.  Diffusion  of 
liquids.  Dialysis.  Crystalloids  and  colloids.  Marsh's  test.  Chloride  of  ar- 
senic. Sulphides  of  arsenic.  Sulphur  salts.  Sulpharsenites  and  Sulphar- 
seniates.  -  -  -  237-261 

Chap.  XVTTT.  —  Antimony.  Sources,  properties  and  alloys  of  antimony. 
Antimoniuretted  hydrogen.  Testing  for  antimony.  Teroxide  of  antimony. 
Antimoniate  of  antimony.  Antimonic  acid.  Metantimonic  acid.  Chlorides 
of  antimony.  Sulphides  of  antimony.  -  -  262-276 

Chap.  XIX.  —  Bismuth.  Fusible  metal.  Teroxide  of  bismuth.  Bis- 
muthicacid.  Chloride  of  bismuth.  Sulphide  of  bismuth.  The  nitrogen  group 
of  elements.  -  .  .  -  277-283 

Chap.  XX. — Carbon.  Allotropic  modifications  of  carbon.  Diamond.  Graph- 
ite or  plumbago.  Graphitic  acid.  Gas-carbon.  Coke.  Anthracite.  Charcoal. 
Preparation  of  charcoal.  Distillation  of  wood  and  of  coal.  Illuminating  gas. 
Lampblack.  Properties  of  charcoal.  Reducing  power  of  charcoal.  Deflagration. 
Stability  of  charcoal.  Charcoal  absorbs  gases.  Induces  combinations.  Dis- 
infects. Decolorizes.  Compounds  of  carbon  and  hydrogen.  Organic  chemis- 
try. Homologous  series.  Marsh  gas  or  light  carburetted  hydrogen.  Atomic 
weight  of  carbon.  Typical  hydrogen  compounds.  Composition  of  illuminat- 
ing gas.  Carbonic  acid.  Preparation  and  properties  of  carbonic  acid.  Venti- 
lation of  wells.  Diffusion  of  gases.  Solubility  of  carbonic  acid.  Carbonic 
acid  produced  in  the  processes  of  fermentation,  respiration,  decay,  and  combus- 
tion. Liquid  and  solid  carbonic  acid.  Decomposition,  analysis,  and  synthesis 
of  carbonic  acid.  Carbonates.  Carbonic  oxide.  Preparation  and  properties 
of  carbonic  oxide.  Carbonic  oxide  a  deoxidizing  agent.  Heat  evolved  in  the 
combustion  of  carbonic  oxide.  Dissociation  of  carbonic  oxide.  Combustion. 
Luminosity  of  flames.  The  Bunsen  burner.  Quantity  and  intensity  flames. 
All  flames  gas  flames.  Form  of  luminous  flames.  Blast-lamps  and  blow- 
pipes. Oxidizing  and  reducing  flames.  Chimneys.  Indestructibility  of  mat- 
ter. Kindling  temperature.  Flames  and  fires  extinguished  by  wire-gauze  and 
other  good  conductors.  Safety-lamps.  Flaming-fires.  Loss  of  heat  from  in- 
complete combustion.  Chlorides  of  carbon.  Compounds  of  carbon  and  ni- 
trogen. Bisulphide  of  carbon.  -  -  284-300 

Chap.  XXI.  —  Boron.  Allotropism  of  boron.  Boracic  acid.  Chloride  of 
boron.  Fluoride  of  boron.  Fluoboric  acid.  Sulphide  and  nitride  of  bo- 
ron. »  -  361-308 

Chap.  XXII.  —  Silicon.  Modifications  of  silicon.  Silicon  and  hydrogen. 
Oxide  of  silicon.  Silicic  acid.  Soluble  silicic  acid.  '  Silica  in  natural  waters. 
Silicates.  Formulas  of  silicates.  Decomposition  of  silicates.  Composition 
and  formula  of  silicic  acid.  Chloride  of  silicon.  Fluoride  of  silicon.  Fluo- 
silicic  acid.  Fluosilicates.  Sulphide  of  silicon.  The  carbon  group.  -  309-387 


VIII  CONTENTS. 

Chap.  XXIII.  — Sodium.  Chloride  of  sodium.  Sea  salt.  Bromide  and 
iodide  of  sodium.  Sulphate  of  sodium.  Glauber's  salt.  Supersaturated  solu- 
tions. Carbonate  of  sodium.  Leblanc's  process.  Bicarbonate  of  sodium. 
Yeast  powders.  Sulphides  of  sodium.  Metallic  sodium.  Hydrate  of  sodium. 
Caustic  soda.  Bases  and  acids.  Replacement.  Direct  combination.  Phos- 
phates of  sodium.  Borax.  Borax  as  a  blow-pipe  test.  Silicates  of  sodium. 
Waterglass.  Glass^  Devitrified  glass.  Hyposulphite  of  sodium.  -  -388-411 

Chap.  XXIV.  —  Potassium.  Sources  of  potassium.  Carbonate  of  potas-  - 
ium.  Bicarbonate  of  potassium.  Hydrate  of  potassium.  Caustic  potash. 
Strength  of  bases.  'Alkalimetry.  Volumetric  analysis.  Valuation  of  potash 
and  soda-ash.  Metallic  potassium.  Cream  of  tartar.  Chloride  of  potassium. 
Bromide  of  potassium.  Iodide  of  potassium.  Cyanide  of  potassium.  Sulph- 
ides of  potassium.  Liver  of  sulphur.  Sulphates  of  potassium.  Nitrate  of  po- 
tassium. Refining  of  saltpetre.  Saltpetre  not  explosive.  Gunpowder.  Chlo- 
rate of  potassium.  -  .  -411-433 

Chap.  XXV.  —  Ammonium  salts.  The  hypothetical  metal  ammonium. 
Hydrate  of  ammonium.  Chloride  of  ammonium.  Sulphate  of  ammonium. 
Nitrate  of  ammonium.  Carbonates  of  ammonium.  Sulphides  of  ammo- 
nium. -  .  433-439 

Chap.  XXVI.  —  Lithium.  Spectrum  analysis.  Rubidium  and  Caesium. 
Thallium.  -  439-445 

Chap.  XXVII.  —  Silver  and  its  relations  to  the  alkali  metals.  Quantiva- 
lence  or  atomicity.  Extraction  of  silver.  Metallic  silver.  The  term  metal. 
Silver  coin.  Nitrate  of  silver.  Oxides  of  silver.  Fulminating  silver.  Pho- 
tography. The  daguerreotype.  Photography  on  glass.  Photography  on  pa- 
per. Chloride  of  silver.  Atomic  weight  of  silver.  Bromide  of  silver.  Iodide 
of  silver.  Cyanide  of  silver.  Sulphide  of  silver.  Sulphate  of  silver.  The 
alkali  group.  Quantivalence.  Atomicity.  -  445-463 

Chap.  XXVIII.  —  Calcium,  strontium,  barium,  and  lead.  Carbonate  of 
calcium.  Calcareous  petrifactions.  Calc-spar  and  arragonite.  Oxide  of  cal- 
cium. Hydrate  of  calcium.  Milk  of  lime.  Slaked  lime.  Mortar.  Caustic 
lime.  Sulphate  of  calcium.  Plaster  of  Paris,  or  calcined  gypsum.  Plaster 
casts.  Incrustation"  of  steam-boilers.  Hard  water.  Testing  water.  Phos- 
phates of  calcium.  Chloride  of  calcium.  Hypochlorite  of  calcium.  Bleach- 
ing powder.  Preparation  of  chlorine  and  oxygen  from  bleaching  powder. 
Chlorimetry.  Nitrate  of  calcium.  Sulphydrate  of  calcium.  Sulphite  of  cal- 
cium. Metallic  calcium.  —  Strontium  and  barium.  Peroxide  of  barium. 
Strontium  salts.  The  calcium  group.  The  alkaline  earths.  — Lead.  Extrac- 
tion of  lead.  Separation  of  silver  from  lead.  Oxides  of  lead.  Corrosion  of 
lead  by  water.  Leaden  water  pipes.  Suboxide  of  lead.  Protoxide  of  lead  or 
litharge.  Cupellation.  Peroxide  of  lead.  Red  lead  or  minium.  Sesquioxide 
of  lead.  Sulphides  of  lead.  Chloride  of  lead.  Sugar  of  lead.  White  lead. 
Silicate  of  lead.  -  464-4'.)'.) 

Chap.  XXIX.  — Magnesium,  zinc,  and  cadmium.  Metallic  magnesium. 
Oxide  of  magnesium..  Calcined  magnesia.  Hydraulic  magnesia.  Chloride 
of  magnesium.  Sulphate  of  magnesium  or  Epsom  salts.  Carbonate  of  mag- 
nesium. Citrate  of  magnesium.  Phosphate  of  magnesium,  and  of  ammonium.— 


CONTENTS.  IX 

Zinc.  Granulated  zinc.  Galvanized  iron.  The  galvanic  current.  The  lead 
tree.  Oxide  of  zinc.  Chloride  of  zinc.  Sulphate  of  zinc.  Alloys  of  zinc. 
Cadmium.  Properties  of  cadmium.  The  cadmium  atom.  The  magnesium 
group.  ...  .  ...  499-511 

Chap.  XXX.  — Aluminum,  glucinum,  chromium,  manganese,  iron,  cobalt, 
nickel,  and  uranium.  Metallic  aluminum.  Alloys  of  aluminum.  Aluminum 
bronze.  Oxide  of  aluminum.  Hydrate  of  aluminum.  Aluminates.  Mor- 
dants in  dyeing.  Lakes.  Chloride  of  aluminum.  Sulphate  of  aluminum. 
Alum.  Silicates  of  aluminum.  Earthenware,  bricks  and  pottery.  Hydraulic 
cement.  Concrete.  —  Glucinum. — Chromium.  Oxides  of  chromium.  Chlorides 
of  chromium.  Sulphate  of  chromium.  Chromic  acid.  Chromates.  —  Manga- 
nese. Protoxide  of  manganese.  Sesquioxide  of  manganese.  Alums.  Bin- 
oxide  of  manganese.  Manganic  acid.  Manganates.  Permanganic  acid. 
Permanganate  of  potassium  or  chameleon  mineral.  —  Iron.  Ores  of  iron. 
Extraction  of  iron.  The  bloomary  process.  The  blast  furnace.  Fluxes. 
Cast-iron.  Impurities  of  iron.  Wrought-iron.  The  puddling  process.  Steel. 
The  Bessemer  process.  Protoxide  of  iron  or  ferrous  oxide.  Sesquioxide  of 
iron  or  ferric  oxide.  Oxidation  by  iron-rust.  Ferric  hydrate.  Sulphide  of 
iron.  Iron  pyrites.  Chlorides  of  iron.  Sulphate  of  protoxide  of  iron,  or  cop- 
peras. Ink.  Dyeing.  Ferric  sulphate.  Nitrates  of  iron.  Silicates  of  iron. 
Cyanides  of  iron.  Prussian  blue.  Tests  for  iron.  —  Cobalt  and  nickel.  Ura- 
nium. Salts  of  the  sesquioxides.  The  Sesquioxide  group.  Atomic  volume  of 
alums.  Rare  elements  allied  to  aluminum  and  iron.  -  -  511-564 

Chap.  XXXI.  —  Copper  and  mercury.  Extraction  of  copper  from  its  ores. 
Alloys  of  copper.  Dinoxide  of  copper.  Protoxide  of  copper.  Sulphides  of 
copper.  Chlorides  of  copper.  Sulphate  of  copper.  Verdigris.  —  Mercury.  Its 
extraction  and  properties.  Oxides  of  mercury.  Red  oxide  of  mercury  as  a 
source  of  oxygen.  Vermilion.  Calomel.  Corrosive  sublimate.  Mercuric 
iodide.  Sulphates  and  nitrates  of  mercury.  Amalgams.  -  -504-580 

Chap.  XXXII.  — Titanium.  Tin.  Its  ore.  Its  extraction  and  proper- 
ties. Tinning.  Protoxide  of  tin.  Binoxide  of  tin.*  Stannates.  Sulphides  of 
tin.  Protochloride  of  tin.  Its  reducing  power.  Bichloride  of  tin.  Alloys  of 
tin.  The  tin  group.  -  -580-589 

Chap.  XXXIII.  — Molybdenum.  Bisulphide  of  molybdenum.  Oxides  of 
molybdenum.  —  Vanadium.  Its  occurrence.  Its  oxides.  —  Tungsten.  Its  occur- 
rence and  properties.  Oxides  of  tungsten.  Wolfram.  Tungstate  of  so- 
dium. -  -  -  589-591 

Chap.  XXXIV.  —  Gold.  Its  wide  diffusion.  Its  physical  properties.  In- 
destructibility of  gold.  Refining  of  gold.  Chloride  of  gold.  Gilding.  Alloys 
of  gold.  Platinum.  Chlorides  of  platinum.  Platinum  sponge.  Platinum  black. 
The  platinum  group.  -  -  591-600 

Appendix.  —  Glass  tubing.  Cutting  and  cracking  glass.  Bending  and 
closing  glass  tubes.  Blowing  bulbs.  Lamps.  Blast-lamps  and  blowers. 
Caoutchouc,  stoppers,  tubing,  and  sheets.  Corks.  Cork-cutters.  Putting 
tubes  through  corks.  Supports  for  vessels.  Iron  stand.  Sand-bath.  Wire- 
gauze  supports.  Pneumatic  trough.  Collecting  gases.  Safety  tubes.  Gas- 
holders. Deflagrating  spoons.  Platinum  foil  and  wire.  Filtering.  Filters. 


X  CONTENTS. 

Drying  of  gases.  Drying  tubes.  Chloride  of  calcium  tubes.  Spring  clip. 
Screw-compresser.  Water-bath.  Iron  retort.  Self-regulating  gas-generator. 
Glass  retorts.  Flasks.  Beakers.  Test-tubes.  Test-glasses.  Measuring- 
glasses.  Burettes.  Reading  Burettes.  Pipettes.  Wash-bottle.  Porcelain 
dishes  and  crucibles.  Kings  to  support  round-bottomed  vessels.  Crucibles. 
Tongs.  Pincers.  Mortars.  Spatulae.  Thermometers.  Furnaces.  The 
metrical  system  of  weights  and  measures.  Table  for  the  conversion  of  degrees 
of  the  centigrade  thermometer  into  degrees  of  Fahrenheit's  thermometer. 
Table  for  the  conversion  of  grammes  into  grains,  and  centimetres  into 
inches. I-XL 


MANUAL 


INORGANIC  CHEMISTRY. 


INTRODUCTION. 

1.  THE  various  objects  which  constitute  external  Nature  pre- 
sent to  the  observing  eye  an  infinite  variety  of  quality  and  cir- 
cumstance. Some  bodies  are  hard,  others  soft ;  some  are  brittle, 
others  tough  or  elastic ;  some  natural  objects  are  endowed  with 
life,  —  they  grow  ;  others  are  lifeless,  —  they  may  be  moved,  but 
never  move  themselves ;  some  bodies  are  in  a  state  of  incessant 
change,  while  others  are  so.  immovable  and  unchangeable  that 
they  seem  everlasting.  In  the  midst  of  this  infinite  diversity  of 
external  objects,  where  lies  the  domain  of  Chemistry  ?  What  is 
the  subject-matter  of  this  science  ? 

When  air  moves  in  wind,  when  water  moves  in  tides,  or  in  the 
fall  of  rain  or  snow,  the  air  and  water  remain  air  and  water  still ; 
their  constitution  is  not  changed  by  the  motion,  however  frequent 
or  however  great.  A  bit  of  granite,  thrown  off  from  the  ledge 
by  frost,  is  still  a  bit  of  granite,  and  no  new  or  altered  thing.  If 
a  solid  piece  of  iron  be  reduced  to  filings,  each  finest  morsel  is 
metallic  iron  still,  of  the  same  substance  as  the  original  piece,  as 
will  appear  to  the  eye  if  a  morsel  be  sufficiently  magnified  under 
the  miscroscope.  The  melted,  fluid  lead  in  the  hot  crucible,  and 
the  solid  lead  of  the  cold  bullet  cast  from  it,  are  the  same  in 
substance,  only  differing  in  respect  to  temperature.  In  all  these 
cases,  the  changes  are  external  and  non-essential,  not  intimate 

and  constitutional ;  they  are  called  physical  changes. 

l 


2.  When  if  on  is "  exposed,  to  .the  weather,  it  becomes  covered 
with  a  brownish,  earthy  coating  which  bears  no  outward  resem- 
blance to  the  original  iron ;  and,  if  exposed  long  enough,  the 
metal  completely  disappears,  being  wholly  changed  into  this  very 
different  substance,  rust.  A  piece  of  coal  burns  in  the  grate, 
and  soon  it  vanishes,  leaving  nothing  behind  but  a  little  ashes. 
Dead  vegetable  or  animal  matters,  buried  in  the  ground,  soon 
putrefy,  decay,  and  disappear.  So,  too,  the  fragment  of  granite 
which  frost  has  broken  from  the  ledge,  exposed  for  centuries 
to  the  action  of  air  and  rain,  becomes  changed ;  it  "  weathers," 
and  after  a  time  could  no  longer  be  recognized  as  granite.  All 
these  changes  involve  alterations  in  the  intimate  constitution  of 
the  bodies  which  undergo  them  ;  they  are  called  chemical  changes. 

Experiment    1.  — Mix  thoroughly  3   grammes   (for  tables  of  tV 
Metrical  System  of  Weights  and  Measures,  see  Appendix)  of  coarse! 
powdered  sulphur  with  8  grammes  of  copper  filings,  or  fine  turning 
FIG.  l.  Put  the  mixture  into  a  tube  of  hard  glass,  No. «. , 

about  12  centimetres  long,  and  closed  at  o. 
end.     (For  the  manipulation   of  tubing, 
Appendix,  §§  1-4.)     Hold  the  tube  by  the 
open    end,  with   the  wooden    nippers,  as  in 
Fig.  1 ,  and  heat  the  mixture  over  the  gas-lamp 
(Appendix,  §5),  until  both  copper  and  sr; 
phur  have  disappeared. 

Before  heat  was  applied,  the  mixture  of  t. 
two  substances  was  simply  mechanical,  and  t! 
copper  might  have  been  completely  separat 
from  the  sulphur,  by  due  care  and  patienc 
but  during  the  ignition  the  copper  and  sulpl 
have  united  chemically,  and  there  has  be 
formed   a   substance   which,   while   containing   both,  has  no  exten 
resemblance  to  either.    In  the  new  body,  the  eye  can  detect  neitln. 
copper  nor  sulphur. 

If  ten  grammes  of  metallic  iron  be  allowed  to  rust  away  completely 
in  moist  air,  the  pile  of  rust  which  remains  when  the  metal  has  disap- 
peared, will  weigh  much  more  than  ten  grammes.  The  iron  has  com- 
bined with  two  of  the  constituents  of  the  atmosphere,  the  gas  called 
oxygen  and  the  vapor  of  water.  The  weight  of  the  rust  is  the  sum 
of  the  weights  of  the  metallic  iron,  and  of  the  water  and  oxygen  with 
which  the  iron  has  combined. 


ANALYSIS    AND    SYNTHESIS.  3 

Processes  by  which  the  whole  character  and  appearance  of  the 
bodies  concerned  are  changed,  as  in  these  experiments,  so  that 
essentially  new  bodies  are  formed  from  the  old,  are  chemical 
processes.  It  is  the  function  of  the  chemist,  on  the  one  hand,  to 
investigate  the  action  of  each  substance  upon  every  other,  to  take 
note  of  the  phenomena  which  attend  this  action,  and  to  study  the 
properties  of  the  combinations  which  result  from  it ;  and  on  the 
other,  to  contrive  the  resolution  of  compound  bodies  into  their 
simpler  constituents.  He  further  seeks  the  general  laws  by 
which  the  intimate  combinations  of  matter  are  controlled.  With 
these  ends  in. view,  the  chemist  endeavors  to  pull  to  pieces,  or  in 
technical  language,  to  analyze,  every  natural  substance  on  which 
he  lays  hands.  Having  thus  found  out  the  composition  of  the 
*iabstance,  he  seeks  to  put  it  together  again,  or  to  recompose  it 
'.tot  of  its  constituent  parts.  By  one  or  both  of  these  two  pro- 
•^sses,  analysis  (unloosing)  and  synthesis  (putting  together),  the 
Chemist  studies  all  substances. 

'T  3.    There  are  two  questions  which  the  chemist  asks  himself 
Concerning  every  natural  substance.     First,  of  -what  is  it  com- 
posed ?     With  the  means  at  his  disposal  the  chemist  may,  or  may 
not,  be  able  to  resolve  the  substance  into  simpler  constituents. 
Tjf  he  succeeds  in  decomposing  it,  he  obtains  the  answer  to  this 
rst  question  ;  if  the  body  cannot  be  decomposed  by  any  known 
ulethod  of  analysis,  the  substance  must  be  regarded  as  being 
ftready  at  its  simplest.     Such  simple  bodies  are  called  elements. 
Secondly,  the  chemist  asks,  —  how  does  this  new  substance  com- 
fort itself  when  brought  into  contact  with  other  substances  already 
imiliar  ?     There  are  sixty-one  substances  which  are,  at  present, 
jmitted  to  be  simple,  primary  substances,  or  elements  ;  of  com- 
..^mnd  bodies,  formed  by  the'  union  of  these  elements  with  each 
Other,  we  find  a  series,  numbering  many  thousands,  in  the  inor- 
ganic-kingdom of  Nature,  comprising  all  the  diversified  mineral 
constituents  of  the  earth's  crust ;  while  another  series,  far  more 
complex  in  composition,  and  almost  innumerable  in  multitude, 
exists  in  the  vegetable  and  animal  world.     The  task    of   the 
chemist   in   thoroughly   answering   his    second   question    would 
clearly  be   endless,  were   it   not   for   the  existence  of  general 
properties  common  to  extensive  groups  of  both  elementary  and 


FACT   AND    THEORY. 

compound  bodies,  and  of  general  laws  which  chemical  processes 
invariably  obey. 

While,  therefore,  the  chemist  seeks  the  answers  to  the  two 
fundamental  questions  above  stated,  he  is  at  the  same  time 
inquiring  what  relations  exist  between  the  properties  of  a  body 
and  its  composition,  and  he  is  also  studying  that  natural  and 
invariable  sequence  of  chemical  phenomena,  which,  when  fully 
known,  will  constitute  the  perfect  science  of  chemistry. 

4.  Generalizations  from  observed  facts,  so  long  as  they  are 
uncertain  and  incomplete,  are  called  hypotheses  and  theories ; 
when  tolerably  complete  and  reasonably  certain,  they  are  called 
laws.  The  attention  of  the  student  should  be  constantly  directed 
to  the  keen  discrimination  between  facts  and  the  speculations 
founded  upon  those  facts  ;  between  the  actual  evidence  of  our 
trained  senses,  brought  intelligently  to  bear  upon  chemical  phe- 
nomena, and  the  reasonings  and  abstract  conclusions  based  upon 
this  evidence  ;  between,  in  short,  that  which  we  may  know,  and 
that  which  we  may  believe. 


CHAPTER    I. 

AIR. 

5.  WE  are  everywhere  surrounded  by  an  atmosphere  of 
invisible  gas,  called  air.  All  objects  upon  the  earth's  surface 
are  bathed  with  it.  In  motion,  it  is  wind,  and  we  recognize  its 
existence  by  its  powerful  effects;  but  in  the  stillest  places  it  exists 
as  well. 

The  presence  of  air  in  any  ordinary  bottle,  flask,  or  other 
hollow  vessel  which  is  empty,  in  the  sense  in  which  this  word  is 
ordinarily  applied,  can  be  shown  very  simply  by  attempting  to 
put  some  other  substance  into  the  vessel,  under  such  conditions 
that  the  air  cannot  pass  out  from  it.  Or  the  air  can  actually  be 
pumped  out  of  the  bottle  ;  and  it  can  be  removed  by  other 
means,  both  mechanical  and  chemical. 


ATMOSPHERIC    PRESSURE.  0 

Exp.  2*  —  Wrap  around  the  throat  of  a  funnel,  with  narrow 
outlet,  a  strip  of  moistened  cloth  or  paper,  so  that  the 
funnel  shall  fit  tightly  into  the  neck  of  a  bottle.  After  the 
funnel  has  been  fitted  to  the  bottle,  as  is  shown  in  Fig.  2, 
fill  it  with  water  and  observe  that  this  water  does  not  run 
into  the  bottle.  The  bottle  which  we  have  called  empty  is 
in  reality  filled  with  air,  and  it  is  this  air  which  prevents 
the  water  from  entering  the  bottle.  If,  now,  the  funnel  be 
lifted  slightly,  so  that  the  mouth  of  the  bottle  shall  no  longer 
be  completely  closed  by  it,  the  air  within  the  bottle  will 
pass  out,  and  the  water  in  the  funnel  will  instantly  flow  down. 

In  order  to  pump  the  air  out  of  any  hollow  vessel,  an  appa- 
ratus known  as  the  air-pump  is  commonly  employed.  Descrip- 
tions of  this  machine  may  be  found  in  the  text-books  on  physics. 
For  purposes  of  illustration,  a  portion  of  the  air  can  always 
be  removed  by  suction,  or  by  mechanically  displacing  the  air  by 
some  other  substance,  as  when  the  finger  thrusts  the  air  out  of  a 
thimble. 

Exp.  3.  —  Fit  to  any  small  flask  or  phial  a  perforated  cork  (for  the 
manipulation  of  corks,  see  Appendix,  §  8),  to  which  has  been  adapted 
a  short  piece  of  glass  tubing,  No.  7.  Tie  upon  this  glass  tube  a  short 
piece  of  caoutchouc  tubing.  Suck  part  of  the  air  out  of  the  flask,  and 
then  nip  the  caoutchouc  tube  with  thumb  and  finger,  so  that  no  air 
shall  reenter.  Immerse  the  neck  of  the  flask  in  a  basin  of  water,  and 
release  the  caoutchouc  tube.  Water  will  instantly  rise  into  the  flask 
to  take  the  place  of  the  air  which  has  been  sucked  out. 

7.  The  water,  in  this  experiment,  is  forced  into  the  flask  by 
the  pressure  of  the  superincumbent  atmosphere.  Air  has  weight. 
Jt  is  subject  to  the  law  of  gravitation,  and  is  attracted  towards 
the  earth's  centre  in  the  same  way  as  other  ponderable  matter. 
It  has  been  found  by  experiment  that  a  litre  of  dry  air,  at  the 
temperature  of  0°,  and  under  a  pressure  of  760  millimetres, 
weighs  1.2932  grammes.  It  has  also  been  determined  by  the 
physicists  that  the  force  with  which  the  air  is  attracted  to  the 
earth  is  on  an  average  equal  to  a  weight  of  1.033  kilogrammes 
to  the  square  centimetre  of  surface.  That  is  to  say,  the  ocean  of 
air  above  us  presses  down  upon  every  square  centimetre  of  the 
earth's  surface  with  a  force  equal  to  that  which  would  be  ex- 
erted by  a  bar  of  met^l,  or  other  substance,  a  centimetre 


6  PROPERTIES    OF    AIR. 

square    in    section,   and    long    enough    to   weigh    1.033    kilo- 
grammes. 

If  such  a  bar  were  constructed  of  iron,  it  would  be  1.3  metres 
long ;  if  of  water,  and  a  bar  of  this  substance  can  readily  be 
made  by  enclosing  the  water  in  a  tube,  it  would  be  10.33  metres 
long;  if  of  mercury,  76  centimetres  long.  A  clear  conception 
of  the  atmospheric  pressure,  is  important  to  the  chemist  because 
of  its  bearing  upon  the  mechanical  collection,  manipulation,  and 
measurement  of  gases. 

8.  In  addition  to  the  qualities  already  mentioned,  we  find  air 
to  be  tasteless,  and  odorless,  colorless  when  in  small  depths,  but 
exhibiting  a  blue  tint  when  seen  in  large  masses,  as  when  in  the 
absence  of  clouds  we  look  at  the  sky,  or  at  a  distant  mountain. 
Several  of  its  properties  are  not  peculiar,  but  are  common  to  all 
gases.     Dry  air  obeys  precisely  the   law  of  Mariotte  up  to  a 
pressure  of  several  atmospheres ;   that  is  to  say,  its  volume  di- 
minishes or  increases  in  proportion  to  the  pressure  to  which  it  is 
subjected ;    but   it   has   never   yet  been  condensed  to  a  liquid. 
Upon  being  heated  one  degree  centigrade,  it  expands  ^-^,  or  in 
other  terms  0.003665,  its  volume  at  0°.     At  the  temperature  of 
0°,  air  is  773  times  lighter  than  water  at  4°  ;  that  is,  than  water 
at  its  maximum  density. 

These  physical  properties  of  air  have  been  enumerated  in 
order  to  a  distinct  acquaintance  with  this  gas,  and  that  we  may 
more  clearly  comprehend  its  chemical  properties  as  we  now  pro- 
ceed to  study  them. 

9.  Of  what  is  air  composed?     When  a  bar  of  iron  is  heated 
in  the  air,  as  at  a  blacksmith's  forge,  it  becomes  covered  with  a 
coating,  which  flies  off  in  scales  when  the  iron  is  beaten  upon  the 
anvil.     If  a  bit  df  tin  be  melted  in  a  shallow  crucible,  its  surface 
soon  becomes  covered  with  a  white  earth,  or  ashes.     At  a  high 
temperature  both  iron  and  tin  slowly  burn  in  the  air,  and  are 
converted  into  earth  or  ashes.     With  the  exception  of  gold,  silver, 
platinum,  and  a  few  other  exceedingly  rare  metals,  all  the  metals 
burn,  or  rust,  when  heated  in  the  air.     If  no  air  be  present,  this 
rust  or  ashes  will  not  be  formed,  however  long  or  intensely  the 
metal  may  be  heated.     But  in  what  manner  is  the  rust  formed  ? 
Is  something  driven  out  of  the  metal  into  the  air,  or  does  some- 


ANALYSIS    OF    AIR.  7 

thing  come  out  of  the  air  and  unite  with  the  metal  ?     Experi- 
ment shall  answer. 

Exp.  4.  —  Place  10  grammes  of  tin-foil  in  a  porcelain  dish,  4  or 
5  c.  m.  in  diameter.  Counterbalance  the  dish,  with  its  contents,  upon 
scales  which  will  turn  promptly  with  0.5  grin,  when  thus  loaded. 
Heat  the  dish,  placed  upon  the  wire-gauze  of  the  iron-stand  (see  Ap- 
pendix, §  9),  over  the  gas-lamp,  moderately  and  continuously,  until  a 
large  part  of  the  metal  has  been  converted  into  ashes.  To  facilitate 
the  change,  move  the  dish  frequently,  so  that  a  new  surface  of  metal 
shall  be  often  exposed  to  the  air.  Replace  the  dish,  when  perfectly 
cool,  upon  the  pan  of  the  balance,  and  observe  that  the  weight  of  the 
dish  and  its  contents  has  very  decidedly  increased. 

10.  It  is  possible  that  during  the  heating  the  metal  may  have 
lost  something,  but  it  is  certain  that  it  has  gained  more.  This 
additional  matter  has  not  come  from  any  alteration  in  the  dish, 
for  it  is  made  of  materials  expressly  adapted  to  resist  such  treat- 
ment, and  a  little  cleaning  will  restore  it  to  exactly  its  original 
condition.  We  have,  then,  taken  something  out  of  the  air, 
which,  gaseous  in  the  air,  has  become  solid  in  the  white  ashes  of 
the  tin. 

If  this  something  with  which  the  tin  has  united  can  be  sepa- 
rated again  from  the  metal,  and  restored  to  the  gaseous  condition, 
it  will  be  easy  to  compare  it  with  common  air,  and  so  learn 
whether  the  matter  which  combined  with  the  heated  tin  is  air 
itself,  or  only  a  part  of  the  air.  It  is  quite  possible  to  recover 
the  gas  which  enters  into  the  composition  of  the  rust  of  iron,  or 
copper,  or  tin,  but  the  processes  required  are  too  circuitous  for 
the  present  purpose.  From  the  rust  of  other  common  metals, 
such  as  lead,  manganese,  or  mercury,  the  absorbed  gas  can  be 
very  easily  expelled.  The  rust  of  mercury  is  the  most  easily 
decomposed.  Mercury -rust  may  be  prepared  by  the  long  heating 
of  the  metal  in  the  air,  in  a  manner  strictly  analogous  to  the 
method  already  applied  to  the  preparation  of  tin-rust. 

Exp.  5. — Put  into  a  tube  of  hard  glass,  No.  3,  about  12  c.  m. 
long,  10  grammes  of  the  rust  of  mercury,  a  substance  which  is  sold 
under  the  name  of  red  oxide  of  mercury.  Tubes  of  hard  glass, 
for  such  purposes,  will  be  hereafter  designated  as  "  ignition-tubes." 
Attach  to  this  ignition-tube,  by  means  of  a  perforated  cork,  or  caout- 


ANALYSIS    OF    AIR. 

chouc    stopper,   a   delivery-tube  of 
FlG>3-  glass,   No.   8,  of   such    shape    and 

length  that  it  shall  reach  beneath 
the  inverted  saucer  in  the  pan  of 
water,  as  represented  in  Fig.  3.  The 
point  of  departure,  for  the  construc- 
tion of  the  apparatus,  is  the  top  of 
the  gas-lamp  placed  upon  the  foot 
of  the  iron-stand ;  the  end  of  the 
ignition-tube  should  be  about  4  c.  m. 
above  the  top  of  the  lamp.  The 
other  details  of  the  apparatus  may 
be  learned  from  the  figure. 

Upon  lighting  the  lamp,  the  air  within  the  ignition-tube  will  expand, 
and  a  .portion  of  it  will  pass  out  through  the  delivery-tube.  This  air 
should  be  collected  in  a  small  bottle  by  itself,  and  thrown  away.  The 
volume  of  air  thus  thrown  away  should  of  course  not  be  much  greater 
than  that  of  the  tubes.  (For  a  description  of  the  pneumatic  trough,  see 
Appendix,  §  10.) 

As  the  ignition-tube  becomes  hot,  gas  will  be  freely  given  off  from 
the  red  oxide  of  mercury  contained  in  it.  It  is  necessary  to  avoid 
heating  intensely  a  single  small  spot  of  the  ignition-tube,  lest  the  glass 
soften,  and,  yielding  to  the  pressure  from  within,  blow  outward,  and  so 
spoil  the  tube  and  arrest  the  experiment.  The  gas-flame  should  be 
so  placed  and  regulated  as  to  heat  3  or  4  c.  m.  of  the  tube  at  once. 
Collect  the  escaping  gas  in  bottles  of  100  to  150  c.  c.  capacity. 

As  soon  as  the  disengagement  of  gas  slackens,  lift  the  iron-stand  up, 
and  take  the  delivery-tube  out  of  the  water,  taking  care  that  no  water 
shall  remain  in  the  end  of  the  tube.  Then,  and  not  till  then,  extinguish 
the  lamp.  (See  Appendix,  §  10.)  In  the  upper  part  of  the  ignition- 
tube,  and  sometimes  in  the  delivery-tube  also,  metallic  mercury  will  be 
found  condensed  in  minute  globules.  The  liquid  metal  is  volatile  at 
the  temperature  to  which  it  has  been  subjected,  and  has  distilled  away 
from  the  hot  part  of  the  tube,  and  condensed  upon  the  cooler  part. 

Thus*  is  recovered  the  metallic  mercury  from  which  the  red 
mercury-rust  was  originally  prepared  by  long  heating  in  contact 
with  air.  Is  the  gas,  which  the  mercury  originally  took  from  the 
atmosphere,  air  itself,  or  something  different  ? 

Exp.  6. — Introduce  a  lighted  splinter  of  soft  wood  into  a  bottle 
of  the  gas  collected  in  the  last  experiment.  It  will  burn  with  much 
greater  brilliancy  than  in  the  air.  Attach  a  bit  of  wax  taper  to 


AIR    A    MIXTURE. 

a  piece  of  wire ;  light  the  taper,  blow  it  out,  and  while  the  wick  still 
glows,  introduce  it  into  a  second  bottle  of  the  gas  of  Exp.  5.  The 
glowing  wick  will  burst  into  flame,  and  the  taper  will  burn  with  extraor- 
dinary brilliancy. 

11.  It  is  very  obvious,  from  these  experiments,  that  the  gas 
which    enters   into  the  composition  of  mercury-rust  is   not   air 
itself.     But  since  it  came  originally  from  the  air,  if  it  is  not  the 
whole  of  air,  it  must  be  a  part  or  constituent  of  air.     This  gas, 
which  causes  combustible  substances  to  burn  with  such  intensity,  is 
indeed  a  constant  constituent  of  the  air,  and  a  thorough  study  of  all 
its  properties  will  hereafter  convince  us  that  it  is  a  chemical  element 
of  very  various  powers  and  great  importance.     It  is  called  oxygen, 
and  under  this  name  will  form  the  subject  of  the  next  chapter. 

12.  But  if  oxygen  be  not  air  itself,  but  only  a  constituent  of 
air,  it  follows  that  air  must  have  other,  or  at  least  another,  con- 
stituent.    If  mercury  be  long  heated  in  contact  with  a  certain 
confined  portion  of  air,  it  will  abstract  from  this  air,  as  we  have 
seen,  one   of   its    ingredients,  namely,  oxygen,    and   there   will 
be  left  behind  whatever  of  air  is  not  oxygen.     This  experiment, 
the    original   one  by   which   the   illustrious   chemist   Lavoisier 
demonstrated  that  air  is  a  constant  mixture   of  two  different 
gases,  deserves  careful  study,  both  for  its  philosophical  value  and 
its  historical  importance.     The  actual  experiment  lasts  several 
days,  and  is  therefore  unsuitable  for  repetition  by  the  student. 
A  description  of  it  will  suffice. 

Into  a  flask,  provided  with  a  long  neck,  some  metallic  mercury  was  in- 
FIG.  4.  troduced ;  the  neck  of  the  flask  was 

then  bent,  as  shown  in  Fig.  4,  the 
flask  placed  upon  a  furnace,  and 
the  end  of  the  neck  plunged  into 
a  basin  of  mercury ;  a  jar  was  then 
placed  over  the  end  of  the  tube, 
and  a  portion  of  the  air  within  the 
jar  was  sucked  out  by  means  of  a 
bent  tube ;  the  mercury  thereupon 
rose  in  the  jar,  and  the  point  at 
which  it  stood  was  accurately 
noted.  The  thermometer  and  barometer  were  also  simultaneously 
observed.  Fire  was  then  lighted  in  the  furnace,  and  the  heat  main- 


10  COMPOSITION    OF   AIR. 

tained  for  twelve  days  at  a  point  just  below  that  required  to  make  the 
mercury  boil.  The  mercury  became  gradually  covered  with  red  scales, 
and  the  air  in  the  jar,  which  at  first  expanded  from  the  action  of  the 
heat,  slowly  decreased  in  bulk  until  fresh  scales  were  no  longer  formed. 
From  these  red  scales  Lavoisier  obtained,  by  the  method  already  ex- 
hibited (Exp.  5),  the  element  oxygen.  The  residual  air  in  the  jar 
proved,  on  examination,  to  be  unfit  for  the  support  of  combustion  and 
of  animal  life ;  a  candle  was  instantly  extinguished  by  it,  as  if  plunged 
in  water,  and  small  animals,  thrust  into  the  gas,  died  in  a  few  seconds. 
The  gas  is,  in  reality,  a  second  elementary  substance,  distinguished  by 
marked  chemical  and  physical  peculiarities.  It  is  called  nitrogen,  and 
under  this  name  will  be  more  completely  studied  in  another  chapter. 

13.  The  experiment  of  Lavoisier  not  only  affords  the  means 
of  separating  the  two  different  gases  of  which  air  is  composed, 
but  also  determines  the  proportions  in  which  they  are  mixed  in 
air.  If  the  diminution  in  bulk  which  the  air  in  the  jar  undergoes 
during  the  whole  progress  of  the  experiment,  be  accurately 
measured,  it  will  be  found  that  the  bulk  of  the  residual  gas,  the 
nitrogen,  is  only  four-fifths  of  the  original  volume  of  air.  The 
lost  fifth  is  the  oxygen  which  has  combined  with  the  mercury. 
The  air,  then,  is  not  an  element,  but  is  compound,  and  its  two 
principal  ingredients  are  the  elementary  bodies,  oxygen  and 
nitrogen,  mixed  in  the  proportion  of  four  measures  of  nitrogen 
to  one  of  oxygen.  It  is  quite  possible  to  prove  by  synthesis 
what  analysis  has  thus  taught.  On  putting  together  four  meas- 
ures of  nitrogen  and  one  measure  of  oxygen,  a  mixture  is  ob- 
tained, which,  except  by  very  refined  experiments,  is  jiot  to  be 
distinguished  from  pure  air.  Aqueous  vapor  is  another  normal 
constituent  of  the  actual  atmosphere,  and  small  traces  of  other 
gases  than  nitrogen  and  oxygen  are  always  present  in  it,  as  will 
be  set  forth  hereafter. 


OXYGEN.  11 

CHAPTER    II. 

OXY  GEN. 

14.  OXYGEN  gas  may  be  prepared  by  heating  red-oxide  of 
mercury,  as  described  in  Exp.  5  ;  or,  far  more  conveniently,  by 
heating  a  mixture  of  chlorate  of  potassium  and  black  oxide  of 
manganese.  For  the  present  we  have  to  regard  these  substances 
merely  as  materials  suitable  for  the  preparation  of  oxygen. 
Their  constitution  will  be  studied  hereafter. 

Exp.  7.  —  Mix  intimately  5  grms.  of  chlorate  of  potassium  with 
5  grms.  of  black  oxide  of  manganese,  which  has  been  previously 
well  dried.  Place  the  mixture  in  a  tube  of  hard  glass,  No.  1, 
12  or  15  c.  m.  in  length.  Attach  to  this  ignition-tube,  by  means  of  a 
perforated  cork  or  caoutchouc  stopper,  a  delivery  tube  of  glass,  No.  7, 
as  represented  in  Fig.  3,  and  described  upon  page  8.  Heat  the 
mixture  in  the  ignition-tube  and  collect  the  gas  which  will  be  given 
off,  in  bottles  or  jars  of  the  capacity  of  about  250  c.  c.  The  first  100 
c.  c.  or  so  of  gas  should  be  rejected,  since  it  will  be  contaminated  with 
the  air  originally  contained  in  the  apparatus. 

It  is  easy  to  determine  the  moment  at  which  the  evolution  of  oxygen 
commences,  by  noting  the  increased  size  of  the  bubbles  of  this  gas  as 
contrasted  with  those  of  the  expanded  air,  and  the  greater  rapidity 
with  which  the  bubbles  of  oxygen  come  over.  For  every  grm.  of  chlo- 
rate of  potassium  taken,  about  230  c.  c.  of  oxygen  gas  should  be  obtained. 

Besides  the  general  precautions  described  under  Exp.  5,  the  follow- 
ing should  here  be  observed.  1.  Both  the  chlorate  of  potassium  and 
the  oxide  of  manganese  should  be  perfectly  dry  and  pure,  that  is,  free 
from  moisture,  dust,  or  particles  of  organic  matter.  2.  So  soon  as  the 
oxygen  begins  to  be  delivered,  the  heat  beneath  the  ignition-tube  should 
be  diminished,  if  need  be,  and  so  regulated  that  the  evolution  of  gas 
shall  be  tranquil  and  uniform.  3.  The  uppermost  portions  of  the  mix- 
ture should  be  heated  before  the  lower.  The  oxygen  thus  obtained  is 
to  be  employed  in  the  experiments  shortly  to  be  described. 

In  case  large  quantities  of  oxygen  are  needed,  a  similar  mixture  of 
equal  weights  of  chlorate  of  potassium  and  black  oxide  of  manganese 
is  heated  in  a  retort  of  iron  or  copper,  and  the  gas  is  collected  in  large 
metallic  vessels,  called  gas-holders,  such  as  are  described  in  §  11  of  the 
Appendix. 


12  PROPERTIES    OF    OXYGEN. 

Besides  the  methods  above  described,  there  are  many  other 
ways  of  preparing  oxygen.  Several  of  these  methods  will  be 
described  hereafter,  when  the  materials  employed  can  be  intelli- 
gently studied. 

15.  Oxygen  is  a  transparent  and  colorless  gas,  not   to   be 
distinguished  by  its  aspect  from   atmospheric  air.     Like  air,  it 
has  neither  taste  nor  smell.     It  is,  however,  somewhat  heavier 
than  air.     If  the  weight  of  a  measure  of  air  be  taken  as  1,  then 
the  weight  of  the  same  measure  of  oxygen  is  found  to  be  .1.1056. 
At  0°,  and  a  pressure  of  760  m.  m.  of  mercury,  1  litre  of  oxygen 
gas  weighs  1.4298  grms. 

Since  oxygen  is  thus  heavier  than  air,  it  is  not  absolutely  necessary, 
in  collecting  it,  or  transferring  it  from  one  vessel  to  another,  that  we 
should  operate  over  water,  as  has  been  directed.  When  a  gas  is  much 
heavier,  or  much  lighter,  than  atmospheric  air,  it  may  often  be  con- 
veniently collected  by  displacement.  A  bottle  can  readily  be  filled 
with  oxygen  from  the  gas-holder  by  carrying  the  delivery-tube  to  the 
bottom  of  the  upright  bottle,  and  allowing  the  gas  to  flow  in  slowly,  as 
if  it  were  water.  In  a  short  time  the  air  will  be  wholly  displaced,  and 
the  bottle  filled  with  oxygen.  The  progress  of  the  operation  -can  be 
followed  by  testing  the  contents  of  the  upper  part  of  the  bottle  from 
time  to  time,  with  a  glowing  match;  when  this  bursts  sharply  into 
flame  the  gas  may  be  assumed  to  be  pure  enough  for  all  ordinary 
purposes. 

1 6.  Oxygen  has  never  yet  been  reduced  to  the  liquid  condition. 
Of  all  known  substances  it  exerts  the  smallest  refracting  power 
upon  the  rays  of  light.     Compared  with  that  of  atmospheric  air, 
its  refractive  power  is  as  0.830  to  1.000.     The  specific  heat  of 
oxygen,  compared  with  that  of  an  equal  weight  of  water  taken 
as  unity,  is  0*2182  ;    it  has  a  lower  capacity  than  other  gases 
for  absorbing  and   radiating   heat.     Water  dissolves  it  in  small 
proportion;    100  volumes    of    water   dissolve,  at   the    ordinary 
temperature,  about  3  volumes  of  oxygen.     It  exhibits    decided 
magnetic  properties,  like  those  of   iron  ;  and,  as  with  iron,  its 
susceptibility  to  magnetization  is  diminished,  or  even  temporarily 
suspended,  by  a  sufficient  elevation  of  temperature.     Its  mag- 
netic force,  compared  with  that  of  the  air,  is  as  17.5  to  3.4,  that' 
of  a  vacuum  being  taken  as  0. 


OXYGEN    SUPPORTS    COMBUSTION.  13 

17.  A  striking  characteristic  of  oxygen  is  its   power  of  sup- 
porting combustion.     This  has  already  been  illustrated  in  Exp.  6, 
and  may  be  further  exhibited  by  a  great  variety  of  experiments. 

FIG.  5.  Exp.  8.  —  Place   in    a    deflagrating    spoon    (see    Ap- 

pendix, §  12),  a  bit  of  sulphur  as  large  as  a  pea. 
Light  the  sulphur,  and  thrust  it  into  a  bottle  of  oxygen. 
It  will  burn  with  a  beautiful  blue  flame,  and  much  more 
brilliantly  than  in  air.  An  acid,  suffocating  gas  is  pro- 
duced. 

Exp.  9. —  Place  a  piece  of  charcoal,  that  of  bark 
is  best,  in  a  deflagrating  spoon.  Kindle  the  charcoal 
by  holding  it  in  the  flame  of  a  lamp,  and  then  introduce 
it  into  a  bottle  of  oxygen.  It  will  burn  vividly,  throwing  off  brilliant 
sparks  if  bark  charcoal  has  been  employed. 

In  this  experiment,  as  in  the  preceding,  the  products  of  the  combus- 
tion are  obviously  gaseous,  no  solid  substance  being  formed. 

Exp.  10.  —  A  piece  of  phosphorus,  the  size  of  a  small  pea, 
having  been  well  dried  between  pieces  of  blotting-paper,  is  placed  in  a 
deflagrating  spoon,  touched  with  a  hot  wire  or  a  lighted  match,  and 
then  thrust  into  a  jar  of  oxygen.  It  will  burn  with  intense  brilliancy 
and  formation  of  a  dense  white  smoke. 

It  should  be  observed  that  phosphorus  is  a  substance  which  inflames 
very  readily  in  the  air,  when  subjected  to  friction  or  any  slight  eleva- 
tion of  temperature.  It  is  hence  so  dangerous  that  it  must  always  be 
kept  under  water.  It  should  be  cut,  also,  under  water. 

18.  Many  substances  commonly  called  incombustible,  because 
they  do  not  burn   readily  in  ordinary  air,  burn   vigorously  in 
oxygen.     Of  these,  metallic  iron  may  be  taken  as  an  example. 

Exp.  11.  —  A  piece  of  fine  piano  wire  is  brought  into  a  spiral 
form  by  winding  it  around  a  glass  rod,  or  a  common  lead  pencil, 
a  straight  piece  of  wire  being  left  above,  so  that  it  can  be  stuck  into  a 
cork  or  wooden  cover.  The  lower  end  of  the  spiral  is  immersed  for  a 
moment  in  melted  sulphur,  so  that  upon  withdrawing  it  there  shall  be 
left  a  small  bead  of  sulphur  upon  the  wire.  Kindle  now  this  sulphur, 
and  quickly  place  the  wire  in  a  bottle  of  oxygen,  at  the  bottom  of 
which  has  been  spread  a  layer  of  sand. 

The  burning  sulphur  heats  the  iron  to  redness,  which  then  burns 
brilliantly,  with  scintillation.  From  time  to  time,  glowing  balls  of 
molten  matter  fall  off  from  the  wire,  and  bury  themselves  in  the  sand 
at  the  bottom  of  the  bottle,  or  even  melt  into  the  glass. 


14  OXIDES. 

If  an  abundance  of  oxygen  be  at  hand,  this  experiment  had  better 
be  performed  in  a  jar  of  2  or  3  litres  capacity  ;  a  watch-spring  which 
Fia.  6.  nas  ^en  rendered  flexible  by  igniting  it,  and  then  allow- 
ing it  to  cool  slowly,  being  the  best  material  with  which  to 
form  the  spiral  coil.  In  this  case  it  is  well  to  tie  a  bit  of 
tinder  upon  the  end  of  the  coil  as  the  kindling  material,  or 
to  attach  a  piece  of  twine  to  the  wire,  and  soak  this  in 
sulphur.  But  the  experiment  succeeds  well  even  in  very 
small  bottles  of  oxygen,  provided  the  wire  be  fine,  and 
that  the  quantity  of  sulphur  employed  for  kindling  be  not 
too  large. 

19.  This  experiment  clearly  proves  what  has   been  already 
stated,  that  iron,  when  red-hot,  combines  with  oxygen.     It  is  the 
burnt  or  oxidized  iron  which  falls  in  globules  to  the  bottom  of 
the  bottle.     This  substance  is  called  oxide  of  iron.     The  com- 
pounds which  are  formed   by  the  union  of  oxygen  with  other 
elements  are  called  oxides.     The  substances  which   have   been 
heretofore  mentioned  under  the  more  familiar  name  of  rust,  like 
iron-rust,  tin-rust,  mercury-rust,  are  called  in  chemistry  oxides, — 
as  the  oxide  of  iron,  oxide  of  tin,  and  oxide  of  mercury. 

20.  It  will  have  been  observed  that  the  combinations  obtained 
in  the  foregoing  experiments  are  of  very  various  quality.     Some 
of  these  compounds  are  solid,  others  gaseous ;  some  are  acid  and 
caustic,  while  others  are  tasteless  and  innocuous.     They  agree 
only  in  this,  that  they  all  contain  oxygen.     All  these  bodies  will 
be  studied  in  detail  hereafter.     It  concerns  us  now  more  particu- 
larly to  realize  the  number  and  variety  of  the  bodies  into  which 
oxygen  enters  as  an   essential   ingredient.     In   fact,  the   most 
important  quality  of  oxygen  is  that,  with  a  single  exception,  it 
unites  with  all  the  other  elements  to  form  compounds. 

This  act  of  ^  combination  is  often  accompanied  by  development 
of  light  and  heat,  as  in  the  foregoing  experiments,  and  in  the 
affairs  of  common  life  we  daily  witness  similar  effects.  All  the 
ordinary  phenomena  of  fire  and  light  depend  upon  the  union  of 
the  body  burned  with  the  oxygen  of  the  air.  Indeed,  the  term 
combustion  may  for  all  ordinary  purposes  be  regarded  as  synony- 
mous with  oxidation. 

Combustion  is  less  vivid  in  air  than  in  pure  oxygen,  because 
of  the  nitrogen  with  which  the  oxygen  of  the  air  is  diluted.  If 


NITROGEN.  15 

a  substance  combines  slowly  with  oxygen,  it  may  often  happen 
that  no  evolution  of  heat  or  light  can  be  detected.  Thus,  when 
a  small  piece  of  iron  rusts  at  the  ordinary  temperature  of  the  air, 
there  is  perceived  neither  light  nor  heat,  although  a  combination 
of  iron  and  oxygen  has  been  formed,  as  in  Exp.  11.  Heat  is 
really  disengaged  in  the  slow  rusting  of  iron,  as  in  every  act  of 
chemical  combination,  but  it  is  taken  up  and  carried  away  by  the 
circumambient  air,  at  the  moment  of  its  formation,  so  that  it  can- 
not usually  be  perceived.  Slow  oxidation,  such  as  this,  is  often 
spoken  of  as  slow  combustion. 

21.  Many  of  the  compounds  of  oxygen  are  very  familiar 
bodies.  Indeed,  oxygen  is  the  most  widely  diffused  and  the  most 
abundant  of  all  known  substances.  Not  only  does  it  occur  in 
the  air,  of  which  it  constitutes  about  one-fifth  the  volume,  as  has 
been  already  remarked,  but  at  least  one-third  of  the  solid  crust 
of  tha  globe  is  composed  of  it.  It  is  the  chief  ingredient  of 
wateif  as  will  appear  in  a  subsequent  chapter.  It  enters  largely 
into  the  composition  of  plants  and  animals.  Silica,  in  all  its  va- 
rieties of  sand,  flint,  quartz,  rock-crystal,  &c.,  contains  about  half 
its  weight  of  oxygen,  and  the  same  is  true  of  the  various  kinds  of 
clay,  and  of  chalk,  limestone,  and  marble.  Oxygen  is  absolutely 
essential  to  the  maintenance  of  animal  and  vegetable  life.  The 
chemistry  of  the  respiration  of  animals  depends  upon  the  absorp- 
tion of  oxygen  from  the  air  respired.  In  the  absence  of  oxygen 
suffocation  ensues. 


CHAPTER    III. 

NITROGEN. 


22.  The  common  modes  of  obtaining  nitrogen  depend  upon 
the  removal  of  oxygen  from  the  air. 

Exp.  12.  —  Fill  a  tube  of  hard  glass,  No.  1  or  2,  about  35  centimetres 
long,  with  bright,  not  too  coarse,  copper  turnings  ;  place  this  tube  upon  a 
semi-cylindrical  trough  of  sheet  iron,  and  support  it  upon  a  ring  of  the 


16 


NITROGEN    FROM    AIR. 


iron  stand,  as  shown  in  Fig.  7.  It  is  well  to  interpose  a  thin  layer  of 
asbestos  between  the  tube  and  the  iron  trough,  in  order  to  prevent  the 
glass  from  adhering  to  the  metal  when  it  becomes  soft  by  heat.  By 
means  of  corks,  attach  to  one  end  of  this  tube  a  delivery-tube  leading 
to  the  water-pan,  as  shown  in  the  figure,  and  connect  the  other  end 
with  a  chloride  of  calcium  tube  (see  Appendix,  §  15),  attached  to  a 

FIG.  7. 


gas-holder  filled  with  air.  Light  the  lamps  (for  a  description  of  these, 
see  Appendix,  §  5),  beneath  the  tube  which  contains  the  copper  turn- 
ings, and  wait  until  the  copper  has  become  red-hot ;  then  allow  air 
to  flow  slowly  out  of  the  gas-holder  over  the  hot  metal.  The  heated 
copper  will  combine  with  the  oxygen  of  this  air,  and  retain  the  whole 
of  it,  so  that  nothing  but  nitrogen  will  be  delivered  at  the  water-pan. 
This  nitrogen  may  be  collected  in  small  bottles  and  tested  with  lighted 
splinters  of  wood,  which  should  be  instantly  extinguished  on  being  im- 
mersed in  it. 

23.  A  still  simpler  method  of  preparing  nitrogen  is  to  burn  out 
the  oxygen  from  a  confined  portion  of  air,  by  phosphorus,  or 
by  a  jet  of  hydrogen. 

Exp.  13.  —  Float  a  small  porcelain  capsule  upon  the  surface  of  the 
water-pan ;  a  large  cork  must  be  placed  beneath  the  capsule  if  this 
will  not  float  of  itself.  In  the  capsule  put  about  a  cubic  centimetre  of 
phosphorus,  and  set  it  on  fire.  Invert  over  the  whole  a  wide-mouthed 
bottle,  of  the  capacity  of  a  litre  or  more,  and  hold  this  bottle  so  that 
its  mouth  shall  dip  beneath  the  surface  of  the  water.  During  the  first 
moments  of  the  combustion,  the  heat  developed  thereby  will  cause  the 
air  within  the  bottle  to  expand  to  such  an  extent  that  a  few  bubbles  of 
the  air  will  be  expelled  ;  but  after  several  seconds  water  will  rise  into 
the  bottle  to  take  the  place  of  the  oxygen  which  has  united  with  the 
phosphorus. 


PROPERTIES    OF   NITROGEN.  17 

The  dense  white  cloud,  which  fills  the  bottle  at  first,  is  a  compound  of 
phosphorus  and  oxygen  which  is  soluble  in  water.  It  will,  therefore, 
soon  be  absorbed  by  the  water  in  the  pan,  and  will  disappear,  so  that 
at  the  close  of  the  experiment  nearly  pure  nitrogen  will  be  left  in  ffhe 
bottle.  But,  as  the  phosphorus  ceases  to  burn  before  the  last  traces  of 
oxygen  are  exhausted,  the  nitrogen  obtained  by  this  method  is  never 
absolutely  pure. 

As  soon  as  the  phosphorus  goes  out,  the  bottle  should  be  shaken  in 
such  a  way  that  the  porcelain  capsule  may  be  upset,  and  sunk  in  the 
water-pan.  The  properties  of  the  nitrogen  may  be  now  examined. 

24.  Nitrogen  is   a   transparent,  colorless,  tasteless,  odorless, 
incondensable  gas.     It  is  not  quite  so  heavy  as  air.     If  a  meas- 
ure of  aiij  weigh  1  gramme,  then  an  equal  measure  of  nitrogen  will 
weigh  0.9714  gramme.     At  0°,  and  a  pressure  of  760  millimetres 
of   mercury,  1  litre  of   nitrogen  weighs  1.256  grammes.     The 
specific  heat  of  the  gas  is  0.244,  that  of  an  equal  weight  of  water 
being  1.000.     A  litre  of  water  at  0°   dissolves  only  20  cubic 
centimetres  of  nitrogen.     Its  refractive  power  in  regard  to  light 
is  to  that  of  atmospheric  air  as  1.034  to  1.000. 

25.  In  its  chemical  deportment  towards  other  substances,  nitro- 
gen  is  remarkably  different  from  oxygen.     Whilst  oxygen  is 
active  and,  as  it  were,  aggressive,  nitrogen,  at  least  when  in  the 
condition   in  which   it   exists    in    air,   is  remarkably  inert  and 
indifferent   as   regards   entering    into    combination  with    other 
bodies.     It  is  marked  rather  by  the  absence  of  salient  character- 
istics than  by  any  active  properties  of  its  own.     Many  of  the 
metals,  sulphur,  phosphorus,  and  numerous  other  substances,  may 
be  kept  unchanged  for  any  length  of  time  in  vessels  filled  with 
nitrogen.     A  burning  candle  will  instantly  be  extinguished  when 
thrust  into  a  jar  of  nitrogen  gas,  for  with  the  nitrogen  the  con- 
stituents of  the  candle  have  no  tendency  to  combine. 

As  it  extinguishes  flame,  so  it  destroys  life.  Animals  cannot 
live  in  an  atmosphere  of  pure  nitrogen.  It  may,  indeed,  be 
breathed  for  a  shprt  time  with  impunity,  but  it  does  not  support 
respiration.  It  is  not  poisonous ;  if  it  were,  it  could  not  be 
breathed  in  such  large  quantities  as  it  is  in  air.  An  animal  im- 
mersed in  it  dies,  simply  from  want  of  oxygen. 

26.,  As  a  diluent  of  the  oxygen  in  the  air,  nitrogen  is  essential 

2 


18  WATER. 

to  the  existing  balance  and  order  of  Nature.  All  animal  and 
vegetable  life  —  most  inanimate  matter,  even  —  stands  in  har- 
monious relations  with  the  chemical  composition  of  the  at- 
mosphere. The  presence  of  so  large  a  proportion  of  nitrogen 
in  the  air  prevents  the  too  rapid  action,  as  regards  combustion 
and  respiration,  that  would  take  place  in  an  atmosphere  of  un- 
mixed oxygen. 

Nitrogen  is  widely  diffused  in  nature.  Besides  occurring  in 
the  air,  it  is  a  constituent  part  of  all  animals  and  vegetables,  and 
of  many  of  the  products  resulting  from  the  decomposition  of 
these.  Notwithstanding  the  indisposition  of  nitrogen  in  the  free 
state  to  enter  into  combination,  a  very  large  number  of  interest- 
ing and  important  compounds  can  be  formed  by  resorting  to  in- 
direct methods  of  effecting  its  union  with  other  elements. 


CHAPTER    IV. 

WATER. 

27.  Another  natural  substance,  quite  as  common  as  air,  is 
water.     Three-fourths  of  the  earth's  surface  is  covered  with  it. 
It  is  diffused  through  the  atmosphere  in  the  form  of  vapor,  and 
is   a   constituent  of  all  animal  and  vegetable  substances,  and  of 
many  minerals.     We  take  up  next  this  familiar  substance,  in 
order  that  we  may  gain,  through  the  study  of  it,  a  deeper  insight 
into  chemical  principles,  and  enlarge  our  experience  by  making 
acquaintance  with  a  new  element.     Let  us  first  define  with  pre- 
cision the  external  and  physical  properties  of  water,  and  then 
apply  the  two  chemical  methods,  of  analysis  and  synthesis,  to  the 
closer  investigation  of  its  essential  nature. 

28.  At  the  ordinary  temperature  of  the  air,  pure  water  is  a 
transparent  liquid,  devoid  of  taste  or  smell.     In  thin  layers  it 
appears  to  be  colorless,  but  large  masses  of  it  are  distinctly  blue, 
as  seen  in  mid-ocean,  in  many  deep  lakes  of  pure  water,  and 
in  masses  of  ice,  such  as  icebergs  and  some  glaciers  where  it 
is  possible  to  look  through  the  ice  from  below. 


PROPERTIES    OF    WATER.  19 

This  color  can  be  seen  upon  the  small  scale  by  looking  down  through 
a  column  of  pure  water,  2  metres  long,  upon  pieces  of  white  porcelain. 
The  water  may  be  held  in  a  glass  tube,  5  c.  m.  wide,  which  has  been 
blackened  internally  with  lamp-black  and  wax  to  within  1.25  c.  m. 
of  the  end.  which  is  closed  by  a  cork.  Fill  the  tube  with  chemi- 
cally pure  water,  throw  into  it  a  few  pieces  of  porcelain,  and  place  it 
in  a  vertical  position,  on  a  white  plate.  On  now  looking  through  the 
column  of  water  at  the  bits  of  porcelain,  which  can  only  be  illumined  by 
light  reflected  from  the  white  plate  through  the  rim  of  clear  glass,  it 
will  be  observed  that  they  exhibit  a  pure  blue  tint,  the  intensity  of 
which  will  diminish  in  proportion  as  the  column  of  water  is  shortened. 
The  blue  coloration  may  also  be  recognized  when  a  white  object  is  illu- 
minated through  the  column  of  water,  by  sunlight,  and  seen  at  the  bottom 
of  the  tube  through  a  small  lateral  opening  in  the  black  coating. 

29.  At  4°,  the  temperature  at  which  it  is  densest,  water  is 
773  times  heavier  than  air  at  0° ;  at  15°  it  is  819  times  heavier 
than  air  of  the  same  temperature.     A  cubic  centimetre  of  water 
at  its  greatest  density,  that  is,  at  4°,  weighed  in  a  vacuum,  is  our 
unit  of  weight  —  a  gramme.     One  litre  of  water,  which  measures 
1000  cubic  centimetres,  therefore,  weighs  a  kilogramme.     Water 
is  compressible  and  elastic ;  by  the  pressure  of  one  atmosphere  it 
can  be  reduced  to  the  extent  of  about  47-millionths  of  its  original 
volume,  and  this  is  true  for  every  added  atmosphere  of  pressure 
so  far  as  experiment  has  extended.     Water  expands  upon  being 
heated,  though  at  a  less  rate  than  other  liquids ;   the  rate  of  ex- 
pansion increases  with  the   temperature.     Notwithstanding   the 
fact  that  water  expands  when  cooled  below  4°,  as  well  as  when 
warmed  above  that  temperature,  its  refractive  power  on  light 
continues  to  increase  regularly  below  4°,  as  though  it  contracted. 
The  refractive  index  increases  continuously  between  -J-5°.2   and 
— 1°.3,  the  direction  of  the  variation  not  changing  in  the  pas- 
sage through  the  point  of  maximum  density.     At  0°  the  index 
is  1.333. 

30.  Pure  water  at  0°,  a  temperature  always  to  be  obtained  by* 
melting  ice,  is  taken  as  a  standard  to  which  the  weights  of  equal 
bulks  of  other  substances,  liquid  or  solid,  are  referred.     In  other 
words,  the  specific  gravity  of  water  is  taken  as  1  ;  and  in  terms 
of  this  unit  the  specific  gravities  of  all  other  liquid  and  solid  sub- 
stances are  expressed.     The  specific  gravity  of  gold,  for  example, 


20  PROPERTIES    OF    WATER. 

is  19.3 ;  that  is  to  say,  the  weights  of  equal  bulks  of  water  and 
of  gold  are  to  one  another  as  1  to  19.3. 

31.  Water  is  also  the  standard  of  specific  heat.     By  specific 
heats  are  meant  the  relative  capacities  for  heat  of   the  same 
weights  of  different  substances,  at  the  same  temperature.     For 
example,  to  raise  1  kilogramme  of  mercury  from  0°  to  1°  requires 
only  one-thirtieth  of  the  quantity  of  heat  necessary  to  raise   1 
kilogramme  of  water  from  0°  to  1°.     Water  having  been  made  the 
standard  of  specific  heat,  its  capacity  for  heat  is  denoted  by  1, 
and  that  of  mercury  will  accordingly  be  0.033.     At  the  same 
temperature,  and  for  equal  weights,  water  has  a  greater  capacity 
for  heat  than  any  solid  or  liquid  known.     Hence  it  results  that 
the  specific  heats  of  all  solid  and  liquid  substances  are  expressed 
by  fractions. 

Water  conducts  heat  very  slowly  ;  it  may  be  boiled  many  min- 
utes at  the  top  of  a  test-tube,  which  is  held  all  the  while  by  the 
lower  end,  in  the  fingers,  without  inconvenience. 

32.  When  exposed  to  a  certain  degree  of  cold,  water  crystal- 
lizes, with  formation  of  ice,  or  snow,  according  to  circumstances  ; 
and   upon   being   heated   sufficiently  it  is  transformed  into  an 
invisible  gas,  called  steam.     Both   these  changes,  however,  are 
purely  physical,  and  therefore  do  not  fall  within  the  province  of 
this  manual.     The  chemical   composition  of   the  water  is  the 
same,  whether  it  be  liquid,  solid,  or  gaseous.     The  temperature 
at  which  ice  melts  is  one  of  the  fixed  points  of  the  centigrade 
thermometer,  numbered  0°,  and  the  temperature  at  which  water 
boils,  under  a  pressure  of .  76  c.  m.  of  mercury,  is  the  other 
fixed  point,  numbered  100°.     Water  evaporates  at  all  tempera- 
tures, and  is  therefore  a  constant  ingredient  of  the  atmosphere. 
Even  ice  slowly  evaporates,  at  temperatures  far  below  0°,  with- 
out first  passing  into  the  liquid  condition. 

In  crystallizing,  that  is  to  say,  in  assuming  the  solid  form,  water 
increases  in  volume.  The  specific  gravity  of  ice  is  only  0.916, 
which  is  equivalent  to  saying  that  in  the  act  of  freezing,  916  c.  c. 
(cubic  centimetres)  of  water  will  be  changed  into  a  litre  of  ice. 
From  this  fact  result  many  familiar  phenomena,  such  as  the  float- 
ing of  ice,  the  upheaving  and  disintegrating  action  of  frost,  and  the 
bursting  of  pipes  and  other  hollow  vessels,  when  water  is  frozen 


STEAM.  21 

in  them.  The  crystals  of  ice  belong  to  the  so-called  hexagonal 
system ;  they  are  six-sided  prisms,  with  regular  faces ;  by 
agglomeration  they  produce  .stellar  and  fern-like  forms  of 
infinite  variety  and  great  beauty.  Ice  is  a  slow  conductor 
of  heat,  and  a  non-conductor  of  electricity.  It  becomes  electric 
by  friction. 

Steam  is  a  colorless,  transparent  gas,  as  invisible  as  atmospheric 
air.  It  is  lighter  than  air,  the  weight  of  any  given  volume  of 
steam,  at  the  ordinary  temperature,  being  to  that  of  the  same 
volume  of  air  as  0.622  to  1,  a  ratio  deduced  by  calculation  from 
the  composition  of  steam.  At  100°,  the  boiling  point  of  water, 
the  ratio  of  the  weights  of  equal  volumes  of  steam  and  air  is 
0.455  to  1,  and  one  volume  of  water  furnishes  about  1700  vol- 
umes of  steam  of  100°.  When  steam  is  heated  by  itself,  with- 
out the  presence  of  any  liquid  water,  it  is  called  superheated 
steam ;  but  when  there  is  water  present,  so  that  no  excess  of  heat 
can  accumulate  in  the  steam,  above  the  quantity  needed  for  its 
formation  under  the  pressure  at  which  it  exists,  the  steam  is 
called  saturated,  meaning  saturated  with  water.  When  steam 
escapes  into  the  air,  there  is  formed  a  multitude  of  little  bubbles 
or  vesicles,  composed  of  a  film  of  water  filled  with  air,  precisely 
similar  to  the  vesicles  seen  in  clouds  and  fogs.  This  steam-cloud 
is  sometimes  improperly  spoken  of  as  steam  or  vapor,  an  error 
against  which  the  student  should  be  upon  his  guard.  Similar 
fogs  of  air-filled  vesicles  are  formed  whenever  the  atmos- 
phere is  cooled  to  a  temperature  so  low  that  the  aqueous  vapor 
contained  in  it  can  no  longer  exist  in  the  gaseous  state. 

33.  Let  us  pass  now  to  the  analysis  of  water.  Of  what  is 
water  composed  ?  We  can  determine  this  point  by  methods 
similar  to  those  which  were  adopted  in  the  examination  of  air. 
There  are  several  metals  which,  upon  being  brought  into  contact 
with  water,  will  abstract  from  it  one  of  its  ingredients,  in  the 
same  way  that  we  have  seen  them  abstract  oxygen  from  the 
air.  Some  metals  can  abstract  this  ingredient  even  at  the 
ordinary  temperature.  Thus  the  metal  called  sodium,  on 
being  brought  in  contact  with  water,  decomposes  it,  and,  uniting 
with  one  of  its  constituents,  sets  free  another  as  a  gas.  This 
new  gas  is  called  Hydrogen. 


22 


ANALYSIS    OF    WATER. 


Exp.  14.  —  Make  a  small  cylinder  of  wire-gauze  by  rolling  a  piece 
of  fine  gauze,  about  6  c.  m.  square,  around  a  thick  piece  of  No.  3 
glass  tubing.  Twist  fine  wire  around  the  cylinder  in  order  to 
preserve  its  form,  then  slip  the  cylinder  off  the  glass,  and  close 
one  end  of.  it  by  pressure  with  a  stout  pair  of  pincers.  Within 
this  cylinder  of  wire-gauze  place  a  piece  of  metallic  sodium  as 
large  as  a  pea,  and  then  close  the  upper  end  of  the  cylinder  by 
pressure  with  the  pincers,  as  before.  Place  the  wire-gauze  cylinder 
and  its  contents  in  a  dry  iron  spoon,  thrust  the  spoon  beneath  the 
surface  of  the  water-pan,  and  instantly  place  over  the  sodium  the 
mouth  of  a  large  test-tube  which  has  previously  been  filled  with  water 
and  left  inverted  in  the  pan. 

As  soon  as  the  sodium  touches  the  water,  bubbles  of  gas  will  begin 
to  escape  from  the  wire-gauze  cage,  and  the  test-tube  must  be  so  held 
that  this  gas  shall  be  collected  within  it.  When  the  evolution  of  gas 
has  ceased,  close  the  mouth  of  the  test-tube  with  the  thumb,  turn  it 
mouth  uppermost,  remove  the  thumb,  and  touch  a  lighted  match 
to  the  gas.  The  gas  will  take  fire  at  once,  and  burn  with  a  pale  blue 
flame. 

34.  At  a  low  red  heat  water  can  be  decomposed  by  several  of 
the  metals,  such  as  iron,  tin,  zinc,  and  magnesium.  Iron  is  well 
adapted  for  this  experiment. 

Exp.  15.  —  Provide  a  piece  of  iron  gas-pipe,  about  35  c.  m.  long, 
and  1  c.  m.  or  more  in  internal  diameter ;  fill  it  with  small,  bright 

FIG.  8. 


iron-turnings,  and  support  it  upon  a  ring  of  the  iron  stand  over  one 
or  two  wire-gauze  gas-lamps.  By  means  of  perforated  corks,  connect 
with  the  iron  tube,  on  the  one  hand,  a  glass  delivery-tube  leading 
to  the  water-pan,  as  shown  in  the  figure,  and  upon  the  other  a  delivery- 
tube  coming  from  a  thin-bottomed  glass  flask,  half  full  of  water,  and  sup- 
ported upon  a  ring  of  a  second  iron  stand.  Light  the  lamps  beneath  the 


ELECTROLYSIS    OF    WATER. 


23 


the  iron  tube,  and  wait  until  its  contents  have  become  red-hot ;  then 
heat  the  water  in  the  flask  until  it  boils  slowly.  As  the  aqueous 
vapor  passes  over  the  hot  iron-turnings  it  will  be  decomposed,  one 
of  its  constituents  will  unite  with  the  iron,  and  hydrogen  will  pass 
off  through  the  delivery-tube  and  may  be  collected  in  bottles  at  the 
water-pan,  so  soon  as  the  air  originally  contained  in  the  tubes  and 
flask  has  all  been  expelled. 

If,  at  the  close  of  this  experiment,  and  after  the  tube  has  become 
cold,  the  iron  be  removed  from  the  tube,  it  will  be  found  to  be  covered 
with  a  black  coating  similar  to  that  which  forms  on  iron  heated  in  the 
air. 

35.  By  these  experiments  it  has  been  proved  that  one  of  the 
components  of  water  is  a  gas  called  hydrogen.  But  with  the 
exception  of  the  coating  upon  the  iron  of  Exp.  15,  we  have  as 
yet  nothing  to  indicate  what  other  ingredients  the  water  may 
contain.  Such  evidence  can,  however,  be  readily  obtained  by 
resorting  to  another  method  of  analysis.  If  a  galvanic  current 
is  caused  to  flow  through  water,  the  force  by  which  the  constitu- 
ents of  the  water  are  held  together  will  be  overcome,  and  the 
water  will  be  resolved  into  the  elements  of  which  it  is  composed. 
On  immersing  the  platinum  poles  of  a  galvanic  battery  in  water, 
to  which  a  little  sulphuric  acid  has  been  added  for  the  purpose  of 
increasing  its  conducting  power,  minute  bubbles  of  gas  will  im- 
mediately be  given  off  from  these  poles,  and  will  FIG.  9. 
be  seen  rising  through  the  liquid.  We  have  here 
abundant  proof  of  the  powerful  action  exerted  by 
the  battery  upon  the  water.  But  the  experiment 
will  be  much  more  satisfactory  if  it  be  made  in  a 
vessel  so  arranged  that  the  evolved  gases  may  be 
collected  for  examination. 

For  this  purpose  the  apparatus  shown  in  Fig.  9  can 
be  conveniently  employed.  The  test-glass,  nearly  full 
of  water  which  has  been  'mixed  with  from  ^  to  £ 
of  sulphuric  acid,  carries  two  platinum  wires  ce- 
mented into  the  glass.  These  wires  terminate  above 
in  thin  plates  of  platinum ;  over  each  of  these  plates 
there  is  inverted  a  long,  narrow  test-tube  full  of 
water,  acidulated  in  the  same  way  as  that  in  the 
test-glass.  Upon  connecting  the  wires  with  a  gal- 
vanic battery,  —  two  Bunsen's  cells  of  medium  size  will  be  suffi- 


24  ELECTROLYSIS    OF    WATER. 

cient,  —  the  water  will  be  decomposed  and  the  resulting  gases,  as  they 
are  given  off  at  the  platinum  plates,  will  rise,  transparent  and  color- 
less, into  the  test-tubes.  On  comparing  the  bulks  of  the  two  gases,  it 
will  be  found  that  twice  as  much  gas  has  collected  in  the  one  tube  as  in 
the  other.  If  the  test-tube  containing  the  larger  volume  of  gas  be 
now  closed  with  the  thumb,  turned  mouth  uppermost,  and  the  gas 
within  touched  with  a  lighted  match,  it  will  take  fire  and  burn  with  the 
characteristic  flame  of  hydrogen.  It  is,  in  fact,  hydrogen. 

If  the  smaller  volume  of  gas  in  the  other  tube  be  examined  in 
the  same  way,  it  will  not  inflame,  although  it  gives  intense  brilliancy 
to  the  combustion  of  the  match.  If  a  splinter  of  wood,  retaining 
but  a  single  ignited  spark,  be  immersed  in  the  gas,  it  instantly  ex- 
hibits a  vivid  incandescence,  and  in  a  moment  bursts  into  flame. 
This  gas  is  oxygen.  It  is  thus  proved,  that  out  of  water  may  be  un- 
loosed two  volumes  of  hydrogen  and  one  volume  of  oxygen. 

If,  now,  the  platinum  plates  be  pressed  so  near  together  that  a 
single  large  test-tube,  full  of  acidulated  water,  can  be  placed  over 
both,  the  gas  obtained  by  passing  the  galvanic  current  will  exhibit 
properties  differing  from  those  of  either  hydrogen  or  oxygen.  It 
is  in  fact  a  mechanical  mixture  of  these  gases  in  the  proportions  in 
which  they  would  unite  chemically  to  form  water.  The  mixture  is  pre- 
cisely similar  to  that  which  would  have  been  obtained  if  the  two 
volumes  of  hj'drogen  and  one  volume  of  oxygen,  previously  collected 
in  two  separate  tubes,  had  been  mixed  in  one.  On  touching  a  lighted 
match  to  the  mixed  gas  it  instantly  explodes  with  great  violence,  the 
hydrogen  burning  suddenly,  so  that  for  a  moment  a  flash  of  flame  fills 
the  whole  interior  of  the  tube.  Incited  by  the  burning  match,  the 
hydrogen  and  oxygen  have  combined  chemically  to  form  water,  a 
portion  of  which  is  deposited  as  dew  upon  the  inner  walls  of  the  tube. 

At  the  temperature  of  the  air,  and  under  ordinary  circum- 
stances, oxygen  and  hydrogen  do  not  combine  chemically.  Up- 
on being  brought  together  they  simply  mix  with  one  another 
mechanically  in  conformity  with  the  physical  laws  which  govern 
the  diffusion  of  gases.  But  under  the  influence  of  heat,  of  elec- 
tricity, and  of  certain  other  agents,  the  two  gases  will  unite 
chemically,  and  will  thus  again  be  converted  into  the  water  from 
which  they  were  derived. 

36.  It  remains  to  be  investigated  whether  hydrogen  and  oxy- 
gen, during  their  conversion  into  water,  undergo  any  change  of 
volume;  or  merely  combine  to  produce  a  measure  of  gaseous 


SYNTHESIS     OF    WATER. 


25 


water  exactly  equal  to  the  sum  of  the  measures  of  the  constitu- 
ents. To  determine  this  point  it  is  necessary  to  compare  the 
joint  volumes  of  the  constituents  of  the  water  with  the  volume 
of  the  product  formed,  at  a  temperature  high  enough  to  maintain 
the  latter  in  the  purely  gaseous  condition  known  as  dry  steam. 

Through  the  closed  end  of  a  U  tube  (Fig.  1 0)  two  platinum  wires 
are  passed,  and  welded  tightly  to  the  glass  walls  of  the  tube.  The  outer 
ends  of  these  wires  are  formed  into  loops  for  the  attachment  of  appro- 
priate battery  wires ;  their  inner  ends  are  separated  by  a  distance  of 
two  millimetres.  The  general  arrangement  of  the  apparatus  to  be  em- 
ployed is  shown  in  Fig.  11.  The  U  tube  is  first  completely  filled  with 

FIG.  11. 


FIG.  10. 


mercury,  and  then  the  screw-compressor  (Appendix,  §  16)  at  a  is 
opened  so  as  to  afford  a  gradual  exit  to  the  metal  in  the  open  limb. 
By  means  of  a  delivery-tube  reaching  down  the  open  limb  to  the  bend 
of  the  tube,  we  introduce  from  a  gas-holder  (see  Appendix,  §  11)  a 
quantity  of  a  mixture  of  oxygen  and  hydrogen,  made  in  the  propor- 
tions in  which  they  form  water,  —  namely,  two-thirds  hydrogen,  and 
one-third  oxygen,  —  in  such  a  manner  that  the  gas  shall  bubble  up 
through  the  mercury  into  the  sealed  limb,  from  which,  of  course,  the 
mercury  escapes  as  the  gas  enters.  A  column  of  gas  25  or  30  c.  m. 
high  is  thus  admitted. 

It  must  be  borne  in  mind  that  this  mixture  of  hydrogen  and  oxygen 
is  very  explosive;  fire  should  be  carefully  kept  away  from  the  vicinity 
of  the  gas-holder  which  contains  it,  and  any  remnant  of  the  mixture 
which  is  not  used  should  be  thrown  away  at  the  end  of  the  experiment. 


26  SYNTHESIS    OF    WATER. 

The  gas-filled  limb  of  the  U  tube  is  next  surrounded  by  a  high  glass 
cylinder,  b  c,  the  ends  of  which  are  fitted  with  corks ;  through  the 
lower  cork  pass  the  U  tube,  and  a  small  glass  tube,  which  is  connected 
with  a  condensing  worm,  d,  kept  cool  with  water ;  through  the  upper 
cork  pass  the  wires  which  are  to  carry  the  electric  current  to  the  pla- 
tinum points  at  the  top  of  the  U  tube,  and  a  bent  glass  tube  coming 
from  the  flask,  e.  The  top  of  the  cylinder  b  c  rises  about  5  c.  m. 
above  the  sealed  extremity  of  the  U  tube.  In  the  flask  e,  fusel  oil, 
a  liquid  which  boils  at  132°,  a  point  much  higher  than  the  tem- 
perature at  which  water  becomes  a  gas,  is  kept  in  constant  ebul- 
lition. The  vapor  rising  from  the  flask  penetrates  the  annular  space 
between  the  U  tube  and  the  enclosing  cylinder,  and  quickly  raises  the 
tube  to  its  own  temperature.  These  strong-smelling  vapors  are  not 
allowed  to  escape  into  the  atmosphere,  but  are  carried  out  from  the 
bottom  of  the  cylinder  b  c,  into  the  condenser  d. 

When  thus  heated,  the  column  of  mixed  oxygen  and  hydrogen  in  the 
tube  expands,  and  its  height  is  marked  by  a  caoutchouc  ring,  previously 
slipped  over  the  cylinder  b  c.  Care  must  be  taken,  before  doing  this,  to 
bring  the  mercury  to  the  same  level  in  both  limbs  of  the  U  tube,  by 
adding  or  withdrawing  mercury  as  may  be  required.  A  few  centimetres 
of  mercury  are  next  poured  into  the  open  limb,  which  is  then  closed 
with  a  good  cork.  Between  this  cork  and  the  mercury  intervenes  a 
column  of  air,  8  or  10  c.  m.  in  length,  which  will  act  as  a  spring,  and 
break  the  shock  caused  by  the  explosion  of  the  mixed  gases.  This 
mixture  is  now  to  be  inflamed  by  causing  an  electric  spark  to  pass 
between  the  platinum  points  within  the  tube.  This  spark  may  be 
obtained  from  a  Ruhmkorff  coil,  or  from  an  electrical  machine.  The 
gases  instantly  rush  into  combination,  with  an  intense  energy  which 
produces  the  phenomena  called  explosive,  and  at  the  high  temperature 
which  exists  within  the  tube  (132°)  the  water  formed  retains  the 
gaseous  condition.  On  removing  the  cork,  and  allowing  the  mercury 
to  flow  through  the  screw-compressor  until  it  is  level  in  both  limbs  of 
the  U  tube,  it  becomes  obvious  that  the  original  volume  of  the  gaseous 
mixture  is  diminished  by  one-third  ;  the  residuary  two-thirds  are  dry 
steam.  If  the  U  tube  is  allowed  to  cool,  this  steam  will  condense  into 
liquid  water. 

This  experiment  demonstrates  that  two  volumes  of  hydrogen 
and  one  volume  of  oxygen  are  compacted,  when  chemically 
united,  into  two  volumes  of  steam. 

37.  We  have  thus  established  the  composition  of  water  by 
analysis,  having,  through  the  agency  of  the  electric  current,  re- 


ATOMS    AND    MOLECULES.  27 

solved  water  into  two  gaseous  constituents,  hydrogen  and  oxygen, 
and  we  have  also  demonstrated,  by  the  converse  or  synthetical 
method,  that  hydrogen  and  oxygen  are  its  only  constituents,  since 
we  have  reproduced  water  by  effecting  the  chemical  union  of 
these  two  elementary  materials  mixed  in  due  proportion. 

If  equal  volumes  of  hydrogen  and  oxygen  be  represented 
by  equal  squares,  having  the  initials  of  the  elements  inscribed 
therein,  the  composition  of  water  by  volume,  and  the  condensa- 
tion which  occurs  when  the  chemical  union  of  the  elements  takes 
place,  may  be  thus  expressed  to  the  eye : 

Each  smallest  possible  or 
greatest  conceivable  volume  of 
steam  will  invariably  yield,  on 
decomposition,  its  own  volume 
of  hydrogen,  and  half  its  vol- 
ume of  oxygen. 

38.  It  has  been  agreed  to  call  by  the  name  "  atom  "  the  small- 
est quantity  of  an  element  which  can  be  conceived  to  exist  in 
combination ;  this  technical  term  is  applied  only  to  the  chemical 
elements,  and  to  certain  chemical  knots,  or  groups,  of  elements, 
which,  under  conditions  hereafter  to  be  studied,  play  the  part  of 
an  element. 

It  has  further  been  agreed  among  chemists  to  call  by  the  name 
" molecule"  the  least  quantity  of  a  compound,  or  of  an  element, 
which  can  exist  by  itself  uncombined,  or  take  part  in  any  chemi- 
cal process ;  a  molecule  always  contains  more  than  one  atom,  but 
these  atoms  may  be  either  of  one,  two,  or  of  several  kinds. 

39.  Physical  experiments  upon  the  expansion  and  contraction 
of  numerous  gases,  simple  and  compound,  have  proved  that  all 
gases  comport  themselves  in   sensibly  the  same  manner  under 
like  variations  of  temperature  and  pressure  ;  whence  it  has  been 
inferred  that  the  intimate  mechanical  structure  of  all  gases,  com-- 
pound  as  well  as  simple,  is  the  same.     This  theoretical  concep- 
tion is  expressed  in    the   following   propositions,  of  which    the 
second  is  the  more  general  and  includes  the  first :  — 

The  elementary  gases  contain,  under  like  conditions  of  tem- 
perature and  pressure,  equal  numbers  of  atoms  in  equal  vol- 
umes. 


28  MOLECULAR    HYPOTHESIS. 

Equal  volumes  of  all  gases,  whether  simple  or  compound, 
contain  under  like  conditions,  the  same  numbers  of  molecules. 

The  idea  of  an  atom  is  complete  and  independent  in  itself; 
the  idea  of  a  molecule  is  partly  a  consequent  of  the  idea  of  an 
atom,  and  partly  of  the  physical  facts  which  the  definition  helps 
to  formulate. 

These  definitions  and  hypotheses  have  found  acceptance,  partly 
on  the  strength  of  experimental  evidence,  partly  because  of  their 
adaptation  to  the  mathematical  mode  of  investigating  physical 
problems  which  border  on  the  domain  of  chemistry,  but 
chiefly  on  account  of  the  clearness  and  formal  consistency  which 
they  have  imparted  to  chemical  language  and  modes  of  thought. 
Chemical  symbolization  and  nomenclature  are  mainly  based  on 
the  above  definitions  and  hypotheses,  which  therefore  justly  de- 
mand the  student's  closest  attention.  Let  us  apply  them  to  the 
chemical  compound,  water. 

40.  The  molecule  of  water,  or  least  quantity  of  water  which 
is  conceived  to  exist  by  itself,  must  yield,  like  any  other  quantity 
when  resolved  into  its  elements,  twice  as  large  a  volume  of 
hydrogen  as  of  oxygen.  In  accordance  with  the  physical 
hypothesis  above  explained,  the  molecule  must  consequently 
contain  twice  as  many  atoms  of  hydrogen  as  of  oxygen.  The 
bulk  and  weight  of  the  molecule  and  atom  are  not  absolute 
quantities,  on  account  of  their  assumed  infinitesimal  character. 
None  but  relative  facts  can  be  known  touching  these  hypothetical 
quantities,  which  are  both  less  than  any  assignable  quantity, 
although  one  must  be  smaller  than  the  other.  We  shall  express 
in  the  simplest  terms  all  our  actual  knowledge  of  the  matter,  and 
shall  at  the  same  time  conform  to  our  definitions,  in  saying  that 
a  molecule  of  water  contains  two  atoms  of  hydrogen  and  one 
atom  of  oxygen.  The  symbol  H2O  which  we  have  already  used 
to  indicate  the  volumetric  composition  of  water  (§  37)  will  now 
receive  an  added  meaning ;  the  H  will  represent  for  us  an  atom 
of  hydrogen,  and  the  O  an  atom  of  oxygen. 

When  the  proportions  in  which  two  bodies  combine  by  volume, 
and  their  specific  gravities,  or  equal-volume  weights,  are  known, 
it  is  a  matter  of  easy  calculation  to  determine  the  proportions  in 
which  they  combine  by  weight.  The  specific  gravity  of  oxygen, 


ATOMIC    WEIGHTS.  29 

or  its  density  compared  with  that  of  air,  has  already  been  given, 
namely,  1.1056.  The  specific  gravity  of  hydrogen  likewise 
referred  to  air  as  the  term  of  comparison,  has  been  found  by 
the  most  exact  experiments  yet  made  to  be  0.06926.  Oxygen  is 
therefore  16  times  heavier  than  hydrogen.  If  hydrogen  be 
made  the  standard  of  specific  gravity  for  gases,  its  specific 
gravity  will  be  denoted  by  1,  and  that  of  oxygen  will  be  16. 
Now  any  measure  of  dry  steam  is,  as  we  have  seen,  resolvable 
into  its  own  measure  of  hydrogen  and  half  that  measure  of  oxy- 
gen; the  weights  of  equal  measures  of  hydrogen  and  oxygen 
are  as  1  to  1 6 ;  but  there  is  twice  as  much  hydrogen  as  oxygen 
in  bulk,  therefore  the  weight  of  the  hydrogen  generated  from 
any  quantity  of  water,  small  or  great,  is  to  the  weight  of  the 
oxygen  simultaneously  produced,  as  2  to  16.  In  18  parts  by 
weight  of  steam,  water,  or  ice,  there  are  then  2  parts  by  weight 
of  hydrogen  and  16  of  oxygen,  and  it  matters  not  what  the 
absolute  weight  of  these  parts  may  be  ;  the  proposition  is  as  true 
of  kilogrammes  as  of  grammes,  of  the  milligramme  as  of  the 
millionth  of  the  milligramme  of  water,  in  either  of  its  physical 
states. 

Applying  these  facts  of  observation  to  our^  abstract  definitions 
of  molecule  and  atom,  it  will  appear  that  the  molecule  of  water, 
the  least  proportional  weight  in  which  it  is  conceived  to  exist 
uncombined,  must  be  composed,  like  any  other  mass  of  water,  of 
2  parts  by  weight  of  hydrogen,  and  16  parts  by  weight  of  oxy- 
gen ;  but  in  conformity  with  our  definitions  and  hypotheses  we 
conceive  of  the  molecule  as  consisting  of  two  atoms  of  hydrogen 
and  one  of  oxygen  ;  one  proportional  part  by  weight  of  hydrogen 
is  then,  in  chemical  language,  synonymous  with  one  atom  of 
hydrogen,  and  16  of  the  same  parts  by  weight  is  the  relative 
quantity  of  the  atom  of  oxygen.  As  for  volume  so  for  weight, 
absolute  quantities  are  entirely  unattainable ;  the  numbers  ex- 
press proportions  only.  The  numbers  1  and  16  are  called  the 
atomic  weights  of  hydrogen  and  oxygen  respectively ;  they 
express  the  proportions  by  weight  in  which  these  two  elements 
enter  into  combination.  If  these  numbers  be  borne  in  mind,  the 
symbol  of  water,  H20,  will  always  remind  us  that  water  consists 
of  1  part  by  weight  of  hydrogen  and  8  parts  of  oxygen. 


30  CHEMICAL    COMBINATION. 

That  any  given  weight  of  water,  as  for  example  one  gramme, 
is  one-ninth  hydrogen  and  eight-ninths  oxygen,  is  a  fact  capable 
of  experimental  demonstration.  It  is  not  difficult  to  decompose 
a  convenient  weight  of  water,  and  to  actually  weigh  separately 
the  hydrogen  and  the  oxygen  which  are  produced ;  the  weights 
of  the  two  gases  will  invariably  be  to  each  other  as  1  to  8  or  as 
2  to  1 6.  The  great  value  of  the  symbols  used  in  chemistry  may 
be  well  illustrated  by  the  amount  of  information  condensed  into 
the  concise  expression  H2O ;  we  learn  from  it  the  number  and 
names  of  the  elements  entering  into  the  composition  of  water, 
and  the  ratios  in  which  the  elements  are  united  by  volume  and 
by  weight. 

41.  This  discussion  of   the   constitution  of  water  rests  upon 
two  solid  facts  of  observation,  namely,  the  composition  of  water 
by  volume  and  its  composition  by  weight ;    all  else  is  plausible 
hypothesis  and  convenient  theory.     The  strong  chemical  com- 
pound, water,  admirably  illustrates  the  essential  changes  which 
the   elements   undergo,  when   they  are  joined  together  by  that 
peculiar  force  whose  play  it  is  the  object  of  chemistry  to  study. 
Nothing  can  be  more  striking  than  the  contrast  between  the 
properties  of  hydrogen  and  oxygen  gases,  or  of  a  mechanical 
mixture  of  these  elements,  and  those  of  the  liquid  water  which  is 
produced  by  their  chemical  union ;  even  in  dry  steam,  a  promi- 
nent property  of  hydrogen  (inflammability),  and  a  marked  char- 
acteristic of  oxygen  (power  of  supporting  combustion),  entirely 
disappear.     In   mechanical   mixtures    the    constituents   may  be 
mingled  in  any  proportions  ;  in  chemical  compounds  the  elements 
are  forcibly  united  in  definite  volumetric  and  ponderal  proportions, 
and  the  individuality  of  the  elements  is  lost  in  the  formation  of  a 
new  substance,  with  new  properties.     The  CHEMICAL  FORCE  is 
a  peculiar  power,  distinct  from,  though  akin  to,  the  forces  of 
Light,  Heat,  and  Electricity ;  it  is  the  province  of  chemistry  to 
investigate  the  conditions,  modes,  and  effects  of  its  action. 

42.  Having  thus  succeeded  in  determining  the  constituents  of 
air  and  water,   we  are  naturally  led  to  inquire  whether  it  be 
not  possible  to  resolve  oxygen,  nitrogen,  and  hydrogen  themselves 
into  simpler  forms  of  matter.     To  this  question  but  one  answer 
can  be  made,  —  the  result  of   the  accumulated  experience  of 


WATER    IN    NATURE.  31 

many  philosophers  of  this  and  former  generations,  namely,  that 
oxygen,  nitrogen,  and  hydrogen  are  incapable  of  decomposition 
by  any  means  as  yet  at  our  disposal.  They  resist  the  most  pow- 
erful influences  of  electricity  and  heat,  and  they  issue  unchanged 
from  every  variety  and  form  of  chemical  reaction  hitherto  devised 
in  the  hope  of  resolving  them  into  simpler  forms  of  matter.  W§ 
are,  therefore,  justified  in  regarding  these  gases  as  simple  bodies, 
or  elements,  in  contradistinction  to  decomposable  bodies,  such  as 
air  and  water. 

43.  The  water  which  occurs  in  nature  is  never  absolutely 
pure.     In  the  form  of  ice,  and  as  it  falls  from  the  clouds  as  rain 
or  snow,  it  is,  indeed,  tolerably  free  from  foreign  substances ;  but 
after  having  once  soaked  into  the  ground,  it  becomes  charged  with 
a  variety  of  mineral  and  other  substances  which,  being  soluble 
in  water,  are  dissolved  by  it  as  it  trickles  through  the  earth. 

AVhere  the  proportion  of  soluble  matter  contained  in  the  water 
is  unusually  large,  and  particularly  if  it  possesses  marked  medici- 
nal properties,  the  water  is  called  mineral  water,  and  the  springs 
from  which  it  issues  are  known  as  mineral  springs.  Sea-water 
may  be  regarded  as  a  variety  of  mineral  water. 

44.  For  the   conduct  of   chemical  investigations  it  is   often 
necessary  to  purify  natural  water.     This  is  done   by  a  process 
called  distillation.     As  a  general  rule  distilled  water  is  employed 
in  all  delicate  chemical  operations. 

Exp.  16.  — In  a  retort  of  500  c.  c.  capacity,  put  200  or  300  c.  c.  of 
well-water.     Thrust  the  neck  of  the  re-  FlG  12 

tort  into  a  half-litre  receiver  placed  in 
a  pan  of  cold  water.  Cover  the  receiver 
with  a  cloth,  or  with  coarse  paper,  and 
upon  this  pour  cold  water  from  time 
to  time,  or  pile  upon  it  fragments  of  ice. 
Place  the  retort  upon  wire-gauze,  on  a 
ring  of  the  iron  lamp-stand,  and  adjust 
the  distance  of  the  retort  from  the  lamp 
as  described  in  Exp.  5,  Fig.  3.  Light 
the  lamp  beneath  the  retort,  and  bring 
the  water  to  boiling.  As  fast  as  the  water 
in  the  retort  is  converted  into  steam,  this  vapor  will  pass  over  into  the 
cold  receiver,  and  will  there  be  condensed  again  to  the  liquid  condi- 


32  DISTILLATION. 

tlon.  Continue  to  boil  until  about  three-quarters  of  the  water  in  the 
retort  has  evaporated. 

The  earthy  and  saline  ingredients  of  well-water  are  for  the  most 
part  not  volatile  ;  very  few  of  them  are  capable  of  accompanying  the 
water  as  it  goes  off  in  vapor ;  hence  the  greater  part  of  the  original 
impurity  of  the  water  will  remain  behind  in  the  retort. 

Beside  the  non-volatile  impurities,  there  are  often  contained  in  well- 
water  certain  volatile  substances,  such  as  ammoniacal  salts  and  organic 
matter,  which  pass  over  into  the  receiver  with  the  aqueous  vapor ; 
but  since  it  has  been  found  that  most  of  these  volatile  matters  go  over 
with  the  first  portions  of  the  steam,  it  is  only  necessary  to  throw  away 
that  portion  of  the  distillate  which  is  first  condensed,  in  order  to  obtain 
thereafter  water  of  a  high  degree  of  purity. 

This  experiment  must  not  only  be  so  regulated  that  the  retort  shall 
not  boil  over,  but  care  must  be  taken  that  vapor  alone  shall  pass  off. 
The  ebullition  should  be  so  moderate  that  none  of  the  particles  of 
water  which  are  thrown  up  mechanically  from  the  surface  of  the  liquid 
can  be  projected  into  the  neck  of  the  retort,  or  carried  thither  by  the 
current  of  steam. 

45.   In  the  operation  of  distillation  the  substance  to  be  dis- 
tilled must  in  the  first  place  be  converted  into  the  condition  of 
vapor,  this  vapor  must  next  be  transferred  to  another  vessel,  and 
there,  by  refrigeration,  be  again  condensed  to  the  liquid  state. 
As  will  appear  from  the  foregoing  experiment,  the  vaporization 
is  effected  in  the  retort  or  still,  and  the  refrigeration  in  the  con- 
denser.    In  the  experiment  above  given  the  receiver  acts  at  once 
as  receiver  and  condenser,  but  a  far  more  efficient  apparatus  can 
be  constructed  by  interposing  a  long  tube  between  the  retort  and 
the   receiver.     This   tube   may  be  wrapped  with   cloths   upon 
which  bits  of  ice  are  laid,  or  water  is  poured  ;  or  better,  the  tube 
may  be  enclosed  in  a  larger  tube,  or  a  metallic  pipe,  through 
which  a  current  of  cold  water  is  made  to  circulate.     The  water, 
which  may  be  iced  if  need  be,  is  poured  in  through  the  funnel  at 
the  lower  end  of  the  tube,  and  passes  out  at  the  top  (Fig.  13, 
see  next  page).     This  exceedingly  convenient  and  valuable  form 
of  condenser  was  invented  by  Weigel ;  it  is,  however,  commonly 
called  Liebig's. 

Exp.  17.  —  Place  a  few  drops  of  the  distilled  water  obtained  in  the 
preceding  experiment  upon  a  piece  of  platinum  foil  (Appendix,  §  13). 


PURE    WATER. 


33 


FIG.  13. 


Hold  the  foil  with  iron  pincers  above  the  gas  flame  in  such  a  manner 
that  the  liquid  may  slowly  evaporate  without  boiling  or  spirting.  After 
the  water  has  disappeared, 
no  residue  will  be  found  up- 
on the  foil.  Take  now  the 
same  number  of  drops  of  wa- 
ter from  out  the  retort,  and 
evaporate  them  upon  the 
foil  as  before.  A  very  de- 
cided residue  of  earthy  mat- 
ter will  be  left  upon  the 
foil. 

46.  In  this  country,  where  ice  can  be  had  in  abundance  at  all 
times,  it  may  often  be  employed  as  a  convenient  substitute  for 
distilled   water.     In   freezing,   that    is,   in    crystallizing,    water 
rejects  a  great  part  of  the   foreign  substances  which  were  dis- 
solved in  it.     Hence,  by  collecting  ice  and  remelting  it,  there 
can  be  obtained  water  which  is  nearly  pure. 

Rain-water,  also,  especially  that  which  has  been  collected  in  the 
open  country,  is  often  pure  enough  to  be  used  for  chemical 
purposes. 

47.  But  even  after  all  the  mineral  and  all  the  organic  matters 
have  been  removed,  the  water  is  not  yet  absolutely  pure.     It  still 
contains  oxygen  and  nitrogen  in  solution.     Both  of  these  gases  are 
soluble  in  water  to  a  certain  extent ;  and  since  the  water  upon  the 
surface  of  the  earth  is  all  the  while  in  contact  with  air  it  must  neces- 
sarily become  charged  with  the  constituents  of  the  air.     A  method 
of  collecting  these  gases  for  examination  FIG.  14. 

will  be  described  in  a  subsequent  chapter  ; 
we  are  here  more  particularly  concerned 
with  their  removal.  This  may  be  effect- 
ed by  long  continued  boiling. 

Exp,  18.  —  In  a  common  medicine  phial 
of  thin  glass,  and  of  the  capacity  of  about 
half  a  litre,  place  300  or  400  c.  c.  of  recently 
distilled  water.  Draw  out  and  bend  the 
neck  of  the  phial,  as  is  shown  in  Fig.  14,  and 
tie  upon  its  point  a  short  piece  of  caoutchouc 
tubing.  Boil  the  water  steadily  during  half  an  hour.  Finally,  nip 


34  SOLUTION    IN    WATER. 

the  open  end  of  the  caoutchouc  tube,  at  the  same  moment  remove 
the  lamp  from  beneath  the  flask,  and  instantly  seal  the  neck  of  the 
phial  by  directing  the  flame  of  a  blow-pipe  against  the  narrow  spot. 
This  water  can  now  be  preserved  for  an  indefinite  length  of  time,  with- 
out undergoing  change.  Upon  inverting  the  phial,  the  water,  which 
has  been  thus  thoroughly  freed  from  air,  will  fall  about  as  if  it  were  a 
solid  body,  and  will  strike  against  the  glass  with  a  sudden  shock.  The 
apparatus  is  in  fact  nothing  else  than  the  so-called  water-hammer  of 
the  physicists. 

It  follows,  then,  that  whenever  absolutely  pure  water  is  needed 
for  chemical  investigations,  natural  water  must  first  be  distilled, 
with  the  precautions  above  indicated,  and  this  distilled  water 
must  subsequently  be  thoroughly  boiled,  in  order  to  expel  the 
gases  which  it  holds  in  solution.  Water  so  purified,  though 
necessary  for  chemical  purposes,  is  unfit  to  support  the  life  of 
fishes  or  other  animals  which  breathe  in  water,  and  is  not  suitable 
for  drinking.  It  is  not  only  insipid  and  unpalatable,  but  is  not 
refreshing  Wke  ordinary  water.  Even  if  only  a  part  of  the  dis- 
solved gases  have  been  removed,  as  is  the  case  with  water  which 
has  been  recently  distilled,  the  taste  of  the  water  is  still  flat, 
and  repugnant.  Hence,  on  board  vessels  where  fresh  water  is 
prepared  by  distilling  sea-water,  the  distillate  should  be  left  for 
some  time  in  contact  with  air,  in  order  that  by  absorbing  the 
constituents  of  the  air  it  may  become  fit  for  drinking. 

48.  As  "might  be  inferred  from  the  foregoing,  water  has  the 
property  of  dissolving  many  substances,  solid,  liquid,  and  gaseous. 
Sugar,  for  example,  .dissolves  readily  in  water,  but  sand  is  in- 
soluble therein. 

A  substance  is  said  to  be  soluble  when  it  is  capable  of  being 
divided  in,  and  dispersed  through  water  so  intimately  and  com- 
pletely that  its  particles  become  invisible,  and  can  no  longer  be 
separated  by  filtration ;  the  result  of  this  coalescence,  or  the 
solution  as  it  is  termed,  is  a  transparent  liquid,  as  a  general 
rule  scarcely  less  mobile  than  the  water  itself. 

Of  the  various  substances  soluble  in  water,  some  dissolve  in 
far  larger  proportion  than  others.  With  some  liquids,  as  alcohol 
for  example,  water  can  be  mixed  in  any  proportion ;  but  of  ether 
it  dissolves  but  little,  and  of  oil  none.  The  proportion  of  any 
substance  that  can  be  dissolved  in  a  given  quantity  of  water  is 


SOLUTION.  35 

usually  limited,  and  under  fixed  conditions  is  definite  and  peculiar 
for  each  substance.  When  a  given  quantity  of  water  has  dis- 
solved as  much  of  a  substance  as  it  is  capable  of  dissolving  at 
the  temperature  and  pressure  to  which  it  happens  to  be  exposed, 
the  solution  is  said  to  be  saturated.  Generally  speaking,  solid 
substances  dissolve  in  far  larger  quantity  in  hot  than  in  cold 
water,  though  with  gases  and  some  exceptional  solids  the  con- 
trary obtains.  From  the  saturated  hot  solution  of  any  saline 
substance,  crystals  are  usually  deposited  during  the  process  of 
cooling.  But  so  long  as  a  solution  is  neither  exposed  to  varia- 
tions of  temperature,  nor  changed  by  the  addition  of  another 
substance  or  by  the  abstraction  of  either  of  its  parts,  it  will 
usually  deposit  nothing,  and  will  remain  unaltered  during  an 
indefinite  period  of  time. 

During  the  act  of  solution,  the  first  portions  of  the  solid  dis- 
solve with  comparative  rapidity,  the  subsequent  portions  dis- 
solving more  and  more  slowly,  until  complete  saturation  is  at- 
tained. In  preparing  a  solution  of  any  solid,  at  the  ordinary 
temperature  of  the  air,  it  is  therefore  inadvisable  to  add  a  large 
quantity  of  water  all  at  once ;  a  much  more  satisfactory  result 
will  usually  be  obtained  if  the  substance  be  rubbed  in  a  mortar 
with  repeated  small  portions  of  water,  the  several  portions  of  the 
solution  being  poured  off  into  a  common  receptacle  as  fast  as  the 
water  becomes  nearly  saturated. 

There  are  many  other  liquids  besides  water  which  are  com- 
monly used  as  solvents,  but  as  water  is  the  commonest  solvent  of 
all,,  and  the  most  universally  applicable,  some  of  the  general 
principles  of  solution  may  here  be  appropriately  set  forth. 

49.  Solution,  though  in  many  cases  closely  allied  to  chemical 
action,  is  usually  treated  of  as  a  distinct  process.  From  the  best 
marked  chemical  action  it  differs  in  several  particulars.  In  true 
chemical  combination  the  union  of  the  several  ingredients  is  so 
close  and  intimate,  that  their  properties  are  merged  and  lost  in 
those  of  the  compound ;  while  in  the  solvents  proper,  such  as 
water,  alcohol,  and  benzine,  the  particles  of  the  dissolved  matter 
appear  to  be  merely  mechanically  divided  and  diffused  through 
the  liquid.  The  chemical  properties  of  the  dissolved  matter 
undergo  no  essential  change  during  the  act  of  solution,  but  remain 


36  SOLUTION. 

unimpaired.  When  common  salt  is  dissolved  in  water,  the  brine 
retains  the  peculiar  taste  of  the  salt,  and  behaves  like  salt  itself 
towards  most  chemical  agents  ;  moreover,  the  salt  can  readily 
be  recovered  unchanged  by  evaporating  the  water.  But  if  a 
piece  of  chalk  be  placed  in  muriatic  acid,  chemical  decomposition 
and  combination  will  immediately  occur,  the  first  signalized  by  a 
violent  effervescence,  the  second  resulting  in  the  formation  of  a 
liquid  which  contains  neither  chalk  nor  muriatic  acid,  if  the  ma- 
terials have  been  mixed  in  due  proportion,  but  which  yields,  on 
evaporation,  a  solid  chemical  compound,  containing  one  of  the 
constituents  of  each. 

Chemical  combination,  as  usually  defined,  occurs  in  fixed  pro- 
portions only,  whereas  solution  takes  place  in  indefinite  propor- 
tions ;  not  only  may  many  substances,  as  alcohol  and  glycerine, 
be  mixed  with  water  in  every  proportion,  but  where  the  solubility 
of  a  substance  is  limited  in  one  direction,  as  that  of  common 
salt,  of  which  only  about  0.355  part  is  dissolved  by  one  part 
of  water  at  15°,  the  substance  can  nevertheless  be  dissolved 
in  every  possible  proportion  below  this  maximum.  Chemical 
action  is  most  marked  between  substances  of  unlike  character ; 
but  with  solution  the  rule  is  different.  In  general,  solution 
occurs  most  readily  when  the  solvent  is  not  far  removed  in  com- 
position and  properties  from  the  body  dissolved. 

Extreme  cases  of  chemical  action  upon  the  one  hand  and  of 
solution  on  the  other,  are  readily  distinguishable.  But  there  is  a 
wide  range  between  these  extremes,  and  it  is  well-nigh  impossi- 
ble to  find  a  point  at  which  the  line  of  demarcation  shall  be 
drawn.  Many  cases  which  at  first  sight  seem  to  be  examples  of 
simple  solution  can  readily  be  shown  to  depend  in  part  upon 
chemical  force. 

The  majority  of  chemists  are  now  inclined  to  regard  most 
instances  of  solution  as  feeble  exhibitions  of  the  chemical  force, 
or  at  all  events  as  intermediate  between  purely  chemical  and 
merely  mechanical  action.  Solution  facilitates  chemical  action 
between  heterogeneous  materials  both  by  overcoming  the  force 
of  cohesion  by  which  the  particles  of  homogeneous  solids  are 
held  together,  and  also  by  bringing  the  particles  of  the  unlike 
bodies  into  intimate  contact  with  one  another  through  the  vehicle 


HYDROGEN. 


37 


of  the  common  solvent.  Cohesion  resists  chemical  action  as  it 
does  gravity,  but  solution  overcomes  cohesion,  frees  the  particles 
from  the  bonds  which  held  them,  and,  as  we  may  imagine,  leaves 
them  free  to  enter  into  other  combinations. 


CHAPTER    V. 

H YDR  O  GEN. 

50.  The  commonest  method  of  preparing  hydrogen  is  by 
treating  zinc  or  iron  with  dilute  sulphuric  acid.  Unless  very 
large  quantities  of  the  gas  are  needed  this  method  is  cheaper 
and  more  convenient  than  either  of  those  heretofore  mentioned. 

Exp.  19.  — To  a  bottle  18  or  20  c.  m.  high,  and  of  500  or  600  c.  c. 
capacity,  the  mouth  of  which  has  an  internal  diameter  of  2.5  to  3  c.  m., 
fit  a  caoutchouc  stopper  or  a  FIG.  15. 

sound  cork,  furnished  with  a 
thistle-tube.  Fig.  15,  and  a  gas 
delivery-tube,  of  No.  6  glass. 
Within  ihe  bottle  put  15  or 
20  grms.  of  granulated  zinc, 
or  small  scraps  of  the  sheet  met- 
al, and  as  much  water  as  will  fill 
about  one-quarter  of  the  bottle. 
Replace  the  cork  in  the  bottle, 
taking  care  to  press  it  in  tightly, 
and  gradually  pour  in  common 
muriatic  acid  through  the  thistle- 
tube.  The  thistle-tube  must 
reach  nearly  to  the  bottom  of  the  bottle,  so  that  its  point  may  dip 
beneath  the  water ;  and  the  muriatic  acid  must  be  added  by  small  suc- 
cessive portions,  not  more  than  a  large  thimbleful  at  a  time. 

On  the  addition  of  the  first  portions  of  the  acid,  chemical  action 
will  ensue,  the  contents  of  the  bottle  will  become  warm,  and  gas  will 
be  seen  to  escape  from  the  liquid.  This  gas  is  hydrogen. 

After  all  the  air  has  been  expelled  from  the  bottle  the  hydrogen  may 
be  collected  over  the  water-pan,  in  inverted  bottles  filled  with  water, 


38  PREPARATION    OF   HYDROGEN. 

or  it  may  be  passed  into  a  gas-holder  (Appendix,  §  11).  The  rapidity 
with  which  the  gas  shall  be  evolved  is  easily  controlled  by  regulating 
the  supply  of  acid  ;  and  the  moment  at  which  the  hydrogen  ceases  to 
be  contaminated  with  air  can  be  determined  by  collecting  small  por- 
tions of  the  escaping  gas,  in  wide-mouthed  bottles  of  about  50  c.  c. 
capacity,  and  testing  its  quality  by  means  of  a  lighted  match.  In 
doing  this  the  small  bottle  filled  with  gas  must  not  be  turned  over,  but 
should  be  carefully  lifted  from  the  water  without  changing  its  vertical 
position,  and  the  lighted  match  should  then  be  touched  beneath  the 
mouth  of  the  bottle.  If  the  hydrogen  be  pure,  it  will  buta  tranquilly 
at  the  mouth  of  and  within  the  bottle,  but  in  case  the  gas  is  still  mixed 
with  much  air,  a  sharp  explosion  will  occur  at  the  moment  when  the 
match  is  touched  to  it.  In  order  to  avoid  these  explosions,  which 
would  be  exceedingly  dangerous  if  the  volume  of  mixed  gases  were 
large,  it  is  indispensably  necessary,  in  preparing  hydrogen,  to  take  care 
that  none  of  the  gas  shall  be  admitted  into  the  gas-holder  until  all  the 
atmospheric  air  has  been  expelled  from  the  bottle  in  which  the  gas  is 
generated.  So  too,  in  experimenting  with  hydrogen,  no  light  should 
ever  be  brought  in  contact  with  the  contents  of  the  bottle  in  which  it  is 
generated,  or  with  any  large  quantity  of  the  gas,  until  the  purity  of  the 
sample,  or  rather  its  non-explosive  character,  has  been  demonstrated 
by  applying  to  a  very  small  volume  of  the  gas  the  test  above  described. 
This  experiment,  which  has  here  been  executed  with  zinc,  can  be 
equally  well  performed  with  iron-filings,  and  with  several  other  of  the 
less  common  metals. 

Muriatic  acid,  or,  in  chemical  nomenclature,  chlorhydric  acid, 
is  a  compound  of  hydrogen  and  another  element,  called  chlorine, 
which  will  shortly  be  described.  The  chemical  composition  of  this 
substance  can  be  represented  by  the  symbol  H  Cl,  in  which  H  rep- 
resents, as  before,  the  least  proportional  weight  of  hydrogen 
which  exists  in  combination,  and  Cl  the  least  proportional 
weight  of  chlorine.  We  may  likewise  abbreviate  the  word  zinc 
to  the  symbol  Zn,  and  the  chemical  process,  or  reaction,  by  which 
the  hydrogen  is  liberated,  may  then  be  symbolized  by  the 
equation, 

2  HCl       Zn  =  ZnCl         2  H. 


Since  hydrogen  is  a  gas,  it  escapes  as  such,  and  there  remains 
dissolved  in  the  water  within  the  bottle  a  compound  of  the 
elements  chlorine  and  zinc,  called  chloride  of  zinc.  The  zinc 
which  was  free,  enters  into  combination,  and  the  hydrogen  which 


PROPERTIES    OP    HYDROGEN.  39 

was  in  combination,  is  set  free ;  in  other  words,  the  zinc  has  been 
substituted  for,  or  has  replaced,  the  hydrogen. 

51.  Hydrogen  is  a  transparent,  colorless,  and   tasteless   gas, 
odorless  when  pure.     It  is  not  poisonous,  though  animals  die  from 
suffocation  when  immersed  in  it,  as  they  do  in  an  atmosphere  of 
nitrogen.     It  has  never  been  condensed  to  a  liquid.     It  is  the 
lightest  substance  known,  being  about  14J  times  lighter  than  air. 
If  1  volume  of  air  weighs  1  gramme,  an  equal  volume  of  hy- 
drogen will  weigh  only  0.0693  grm.     It  is  11,160  times  lighter 
than  water,  151,700  times  lighter  than  quicksilver,  and  236,000 
times  lighter  than  platinum.     1  litre  of  hydrogen  at  0°,  and  a 
pressure  of  76  c.  m.  mercury,  weighs  0.089578  grm.     Hydrogen 
is  the  standard  of  specific  gravity  for  gases,  as  water  is  for  liquids 
and  solids ;  its  specific  gravity  is  therefore  unity. 

52.  The  exceeding  lightness  of  hydrogen  can  be  illustrated  in 
various  ways.     From  an  inverted  bottle,  even  though  it  be  open 
below,  hydrogen  will  escape  but  slowly.     But  if  a  bottle  of  hy- 
drogen be  opened  in  the  air,  with  the  mouth  upward,  the  gas 
will  quickly  escape.     Hence  it  can  readily  be  poured  or  decanted 
upwards  from  one  vessel  to  another. 

Exp.  20.  —  Lift  from  the  water-pan  a  thick,  strong,  wide-mouthed 
bottle,  of  200  to  300  c.  c.  capacity,  full  of  hydrogen,  taking  care  to 
hold  it  in  a  perpendicular  position,  with  the  mouth  downward.  With  the 
other  hand  place  another  bottle  of  equal  size  and  strength,  but  containing 
only  air,  beside  the  hydrogen  bottle,  so  that  the  mouths  of  the  bottles  shall 
touch  at  one  edge.  Gradually  turn  down  the  hydrogen  bottle,  and  at  the 
same  time  push  its  mouth  beneath  that  of  the  air-bottle  in  such  manner 
that  the  bottle  which  originally  contained  the  hydrogen  shall  at  last 
stand  upright  beneath  the  inverted  bottle.  During  this  operation,  the 
lighter  hydrogen  flows  up  into  the  upper  bottle,  while  the  heavier  air 
sinks  into  the  lower.  If  a  burning  match  be  now  thrust  into  the  upper 
bottle,  the  hydrogen  within  it  will  take  fire ;  but  upon  applying  the 
match  to  the  lower  bottle,  originally  full  of  hydrogen,  there  will  be 
found  in  it  nothing  but  air. 

In  like  manner,  hydrogen  may  be  collected  by  displacement ;  an  up- 
right delivery-tube  being  earned  from  the  bottle  in  which  the  gas  is 
generated,  to  the  top  of  an  inverted  recipient.  The  student  will  do 
well  to  remember  as  a  general  rule,  that  in  manipulating  with  hydro- 
gen we  must  operate  in  a  manner  precisely  opposite  to  that  which 


40  DIFFUSIVE    POWER    OF    HYDROGEN. 

would  be  adopted  if  we  were  at  work  with  water.     Where   water 
would  flow  down,  hydrogen  will  flow  up. 

Owing  to  its  lightness,  hydrogen  is  well  adapted  for  filling  bal- 
loons, and  it  is  still  sometimes  employed  for  this  purpose  in  military 
operations,  being  prepared  by  means  of  hot  iron,  as  in  Exp.  15. 
For  purposes  of  illustration,  soap-bubbles  filled  with  hydrogen 
will  serve  as  well  as  balloons  of  more  costly  construction. 

Exp.  21. —  By  means  of  a  caoutchouc  tube,  attach  an  ordinary  to- 
bacco pipe  to  a  gas-holder  containing  hydrogen.  (See  Appendix,  §  11, 
Fig.  xvii.)  Dip  the  pipe  in  a  solution  of  soap  for  a  moment,  then  slowly 
turn  the  stop-cock  of  the  gas-holder  so  that  hydrogen  may  flow  out  and 
inflate  the  film  of  water  upon  the  mouth  of  the  pipe.  The  bubble  will 
soon  break  away  from  the  pipe  and  rise  rapidly  through  the  air.  If  a 
burning  match  be  touched  to  the  bubble  the  hydrogen  within  it  will  of 
course  burst  into  flame. 

53.    There  is  another  noticeable  peculiarity  of  hydrogen  which 
is  directly  connected  with  its  extreme  lightness.,    It  possesses  in 
a  high  degree  the  power  of  diffusion.     This  diffusive-power  is  a 
FIG.  16.  physical  property  common  to  all  gases  and  vapors  ;  in  the 
f=*    case  of  hydrogen,  it  is  only  the  intensity  of  the  diffusive 
power  which  is  remarkable. 

When  different  gases,  which  have  no  chemical  action 
upon  each  other,  are  brought  into  contact,  they  will  not 
remain  separate,  but   will  commingle.     This   tendency  of 
the  gases  to  intermingle  is  so  strong  that  it  will  not  only 
overcome  the  greatest  differences  of  specific  gravity,  but 
even  cause  the  spread  of  gases  directly  against  powerful 
currents  of  air  or  vapor.     If  a  bottle  of  oxygen,  standing 
upright,  be  connected  with  an  inverted  bottle  full  of  hydro- 
gen, by  means  of  a  tube  a  metre  in  length,  and  no  more 
than  8  or  10  m.  m.  in  diameter,  both  the  bottles  will  be 
found  to  be  filled  with  a  uniform  mixture  of  the  two  gases 
after  the  lapse  of  a  very  few  hours.     Upon  now  touching 
a  lighted  match  to  the  open  mouth  of  either  ftottle,  the  gas- 
I  eous  mixture  will  explode.     As  a  precautionary  measure  it 
^~^  is  best  in  this  experiment  to  employ  the  thick,  strong,  bottles 
in  which  soda-water  is  kept ;  or,  in  lack  of    these,  strong,  wide- 
mouthed  bottles  enveloped  in  thick  towels. 


DIFFUSION    OF    GASES.  41 

The  velocities  with  which  gases  diffuse  are  in  the  inverse 
ratio  of  the  square  roots  of  their  specific  gravities.  Hence  it 
happens  that  hydrogen,  being  the  most  attenuated  of  all  gases, 
diffuses  with  the  greatest  rapidity.  Compared  with  that  of  oxy- 
gen, its  rate  of  diffusion  is  as  4  to  1  ;  that  is  to  say,  the  relative 
rates  of  diffusion  of  the  two  gases  are  inversely  as  the  square 
roots  of  the  numbers  1  and  16,  which  represent  the  specific 
gravities  of  hydrogen  and  oxygen  respectively. 

Exp.  22.  —  A  glass  tube,  3  or  4  c.  m.  in  diameter,  and  30  or 
40  c.  m.  long,  is  closed  at  one  end  with  a  plug  of  FIG.  17. 
plaster  of  Paris  1  or  2  c.  m.  thick.  The  tube  is  then 
set  aside  for  a  day  or  two,  in  order  that  the  plaster 
may  become  dry.  When  the  plug  is  dry,  fill  the 
tube  with  hydrogen  by  displacement,  and  set  it  upright 
in  a  glass  of  water.  Water  will  rise  rapidly  in  the  tube, 
since  hydrogen  escapes  through  the  plaster  more  rapidly 
than  air  can  enter  the  tube  through  this  porous  plug. 
That  some  air  does  enter,  however,  can  be  shown  by 
exploding  the  contents  of  the  tube  by  applying  a 
lighted  match,  after  the  lapse  of  some  time.  Of  course 
if  the  tube  be  left  to  itself,  air  will  slowly  enter  through 
the  plaster,  so  that  the  water  within  the  tube  will  in  due 
time  sink  to  the  level  of  the  outside  liquid. 

On  account  of  its  high  diffusive  power,  hydrogen  can  be  kept 
only  in  perfectly  tight  vessels.  It  has  been  found  that  it 
will  leak  rapidly  under  a  pressure  of  27  or  28  atmospheres 
through  stop-cocks  that  are  perfectly  tight  for  nitrogen  at  a 
pressure  of  even  50  or  60  atmospheres ;  from  the  same  cause  it 
cannot  be  kept  for  any  length  of  time  in  bladders  or  rubber 
bags.  If  a  sheet  of  paper  be  held  a  short  distance  in  front 
of  the  opening  of  a  gas-holder  from  which  hydrogen  is  escaping, 
the  current  of  gas  will  pass  directly  through  the  paper,  and  can 
be  inflamed  upon  the  other  side  of  the  sheet.  This  high  diffusive 
power  of  hydrogen,  which  is  to  some  extent  shared  by  its  com- 
pounds also,  is  an  obstacle  to  be  overcome  before  ballooning  can 
be  made  practicable. 

Sound  is  propagated  in  hydrogen  but  little  better  than  in  a 
vacuum.  The  specific  heat  of  hydrogen  is  3.4046,  that  of  an 
equal  weight  of  water  being  1.000  ;  it  is  0.2356,  that  of  an 


42  HYDROGEN    INFLAMMABLE. 

i 

equal  volume  of  air  being  0.2377.     It  refracts  light  very  power- 
fully. 

54.  Hydrogen  is  exceedingly  inflammable,  as  has  been  already 
seen ;  that  is  to  say,  the  temperature  at  which  it  takes  fire  is 
comparatively  low.     But,  as  a  matter  of  course,  it  extinguishes 
any  burning  body  which  is  immersed  in   it,  since    oxygen   is 
necessary  for  the  support  of  combustion. 

Exp.  23.  —  Carefully  lift  from  the  water-pan  a  bottle  of  200  or  300 
c.  c.  capacity,  completely  full  of  hydrogen,  slowly  carry  the  bottle,  the 
mouth  of  which  is  of  course  held  downward,  to  a  burning  candle  or 
splinter  of  wood,  and  depress  the  bottle  over  this  flame.  The  hydro- 
gen will  take  fire  and  burn,  below,  at  the  mouth  of  the  bottle  where  it 
is  in  contact  with  the  oxygen  of  the  atmosphere ;  but  the  flame  of  the 
candle  will  be  extinguished  the  moment  it  becomes  completely  en- 
veloped by  the  hydrogen.  The  candle  can  easily  be  relit  by  slowly 
lifting  the  bottle  until  the  wick  is  brought  into  contact  with  the  air  and 
the  burning  hydrogen. 

Exp.  24.  —  Fill  a  bottle  of  the  capacity  of  400  or  500  c.  c.  with 
hydrogen,  close  the  mouth  with  a  cork  or  a  plate  of  glass,  stand  the 
bottle  upon  the  table  with  the  mouth  upward,  remove  the  stopper, 
inflame  the  hydrogen,  and  immediately  pour  into  the  bottle  a  large 
quantity  of  water.  The  flame  will  instantly  be  increased,  since  the 
water  will  force  the  gas  out  of  the  bottle  into  the  air.  Within  the 
bottle  the  hydrogen  can  only  burn  gradually,  since  it  takes  time  for  the 
outside  air  to  enter ;  but  if  the  gas  be  pushed  out  of  the  bottle  into  the 
air,  it  will  burn  at  once. 

In  the  familiar  instances  where  water  extinguishes  fire,  it  does  so  by 
reducing  the  temperature  of  the  combustible ;  that  is  to  say,  by  cooling 
it  to  below  the  temperature  at  which  it  will  take  fire.  It  would  act 
thus  in  this  case,  were  it  not  for  the  lightness  and  mobility  of  the 
hydrogen,  by  virtue  of  which  this  gas  immediately  escapes  from  contact 
with  the  water. 

55.  It  has  been  seen  that  the  hydrogen  flame  affords  only  an 
exceedingly  feeble  light,  but  it  would  be  a  grave  error  to  infer 
that  but  little  heat  is  developed  Jby  the  combustion. 

The  temperature  of  the  hydrogen  flame  is  in  reality  very 
high.  Indeed,  it  has  been  found  that  when  a  given  weight  of 
hydrogen  enters  into  chemical  union  with  oxygen,  more  heat  is 
developed  than  in  the  burning  of  the  same  weight  of  any  other 
substance. 


UNIT    OF   HEAT.  43 

In  order  to  determine  the  amount  of  heat  which  is  developed 
in  any  act  of  combination,  this  heat  can  be  transferred  to  water, 
and  there  estimated  either  by  the  quantity  of  water  heated,  or 
the  amount  of  steam  produced.     A  unit  of  heat  is  that  amount  i 
of  heat  which  will  raise  1  kilogramme  of  water  from  0°  centi-  J 
grade  to  1°. 

The  amount  of  heat  evolved  during  the  combustion  of  a  body 
is  as  constant  and  unvarying  as  any  other  direct  consequent  of 
its  properties,  and  the  quantity  of  heat  evolved  is  absolutely  the 
same,  no  matter  whether  the  combustion  occurs  in  air  or  pure 
oxygen,  'or  whether  it  be  slow  or  rapid.  The  actual  amount  of 
heat  developed  during  the  most  vivid  combustion  is  no  greater 
than  when  the  same  combustible  combines  with  oxygen,  by  gradual 
oxidation,  without  visibly  burning.  From  1  kilogramme  of  hydro- 
gen, as  it  unites  with  oxygen,  there  are  evolved  34462  units  of  heat. 

Although  the  same  amount  of  heat  is  developed,  in  the  aggre- 
gate, when  a  litre  of  hydrogen  burns  in  the  air,  as.  when  it  burns 
in  pure  oxygen,  it  is  none  the  less  true  that  a  far  hotter  flame  is 
obtained  by  burning  the  hydrogen  in  oxygen  than  in  air.  If 
the  combustion  be  complicated  by  the  presence  of  nitrogen  from 
the  air,  a  great  deal  of  heat  will  expend  its  energy  in  expanding 
this  useless  nitrogen.  Moreover,  since  the  nitrogen  occupies 
much  room,  it  will,  as  it  were,  keep  asunder  the  particles  of  hy- 
drogen and  oxygen;  the  flame  will  thus  be  made  longer  and 
more  dispersed,  or,  in  other  words,  the  heat  evolved  by  the  union 
of  the  hydrogen  and  oxygen  will  be  spread  over  more  space 
than  if  the  nitrogen  were  absent.  Where  nothing  but  hydrogen 
and  oxygen  are  present,  and  in  the  precise  proportions  in  which 
these  gases  unite  to  form  water,  the  flame  produced  by  their 
union  will  be  to  all  intents  and  purposes  solid,  and  the  heat  will 
be  concentrated  in  the  smallest  possible  space. 

Exp.  25.  —  Provide  two  gas-holders  (see  Appendix,  §  11),  one  full 
of  hydrogen,  the  other  full  of  oxygen,  also  a  metallic  jet  so  constructed 
that  the  tube  [see  Fig.  18,  following  page]  which  carries  the  oxygen 
shall  pass  through  the  centre  of  the  hydrogen  tube.  Screw  the  jet  on 
to  the  oxygen  gas-holder,  and  connect  the  other  opening  with  the 
hydrogen  gas-holder  by  means  of  a  caoutchouc  tube.  Open  the  cock 
of  the  hydrogen  gas-holder,  and  inflame  the  gas  at  the  point  of  the  iet, 


44  OXY-HYDROGEN    BLOWPIPE. 

then  slowly  open  the  cock  of  the  oxygen  gas-holder  until  the  flame  of 
the  burning  hydrogen  has  been  reduced  to  a  fine  pencil.  This  apparatus 
is  known  as  the  compound,  or  oxy-hydrogen,  blow-pipe.  In  order  to 
insure  a  steady  flame,  care  must  be  taken  that  a  constant  and  sufficient 
pressure  be  maintained  upon  the  contents  of  the  gas-holders. 

FIG.  18. 


Exp.  26.  —  In  the  flame  of  the  oxy-hydrogen  blow-pipe,  described  in 
the  foregoing  paragraph,  hold  the  end  of  a  piece  of  platinum  wire, 
about  10  c.  m.  long  and  less  than  1  m.  m.  in  diameter.  The  platinum 
will  melt  and  fall  down  in  drops. 

The  intense  heat  of  the  oxy-hydrogen  flame  is  thus  admirably  illus- 
trated, for  platinum  is  an  exceedingly  infusible  metal,  which  can 
scarcely  be  softened  in  the  hottest  furnace.  The  drops  of  platinum 
should  be  caught  upon  sand,  or  in  water.  In  case  the  melted  platinum 
falls  into  water,  a  portion  of  the  water  will  be  decomposed  into  hydro- 
gen and  oxygen,  bubbles  of  which  will  be  seen  issuing  from  the  water. 
Some  of  this  gas  can  easily  be  collected,  by  an  assistant,  in  an  inverted 
ignition-tube  filled  with  water,  and  its  explosive  character  demonstrated 
by  applying  a  lighted  match. 

Exp.  27. —  If  a  piece  of  chalk  or  lime,  scraped  to  a  fine  point,  be 
held  in  the  flame  of  the  oxy-hydrogen  blow-pipe,  itwill  quickly  be  come 
white-hot  and  evolve  light  of  great  brilliancy,  almost  comparable  with 
that  of  the  sun.  If  the  bit  of  lime  be  long  exposed  to  the  intense  heat 
it  will  undergo  incipient  fusion,  and  afford  less  light  than  at  first. 

Where  a  constant  light  is  desired,  cylinders  or  plates  of  chalk  are 
kept  continually  moving  before  the  flame  by  mechanical  power,  so  that 
fresh  portions  shall  continually  be  brought  into  the  flame.  This  is 
the  so-called  Drummond  or  Calcium  light,  often  employed  for  night 
signals  and  optical  experiments. 

56.  No  matter  in  what  way  hydrogen,  is  burned,  whether  in 
the  pure  state  or  in  combination  with  other  materials,  whether  in 
pure  oxygen  or  in  the  air,  the  product  of  the  combustion  is 
always  water.  At  the  high  temperature  of  the  flame,  this 
water  must  of  course  remain  in  the  condition  of  a  gas,  but  it 


FORM    OF    GAS-FLAMES.  45 

can  readily  be  brought  to  the  liquid  state  by  reducing  the  tem- 
perature. 

Exp.  28. —  Over  a  jet  of  burning  hydrogen,  best  obtained  from  a 
gas-holder,  hold  a  dry,  cold  bottle.  The  glass  soon  becomes  cov- 
ered with  a  film  of  dew,  as  the  water  generated  by  the  union  of 
hydrogen  and  oxygen  condenses  in  droplets  upon  the  cold  sides  of 
the  bottle. 

57.  As  the  burning^  jet  of  hydrogen  is  the  simplest  instance 
of  combustion  with  flame,  some  exact  knowledge  of  the  form 
and  quality  of  flames  may  here  be  gained. 

As  hydrogen,  or  any  other  gas,  issues  from  a  small  orifice  into 
the  air  by  force  of  pressure  from  behind,  the  escaping  gas 
assumes  a  certain  definite  shape  in  accordance  with  the  physical 
conditions  to  which  it  is  exposed,  just  as  a  fountain  of  water 
takes  form  in  accordance  with  the  size  and  shape  of  the  orifice 
from  which  the  water  is  expelled,  the  pressure  by  which  it  is 
expelled,  the  gravity  of  the  water,  the  resistance  of  the  air, 
and  the  force  and  direction  of  the  wind,  and  so  forth. 

If  a  lighted  match  be  brought  in  contact  with  the  column  or 
fountain  of  gas,  the  gas  will  take  fire  and  burn  ;  that  is  to  say,  the 
hydrogen  will  enter  into  combination  with  oxygen  as  fast  as  the 
latter  can  be  furnished  from  the  air.  But,  in  any  event,  the 
column  of  gas  will  burn  only  upon  the  outside,  for  there  alone 
can  the  oxygen  of  the  air  come  in  contact  with  the  hydrogen. 
Neither  the  interior  of  the  flame  nor  the  contents  of  the  reser- 
voir from  which  the  gas  is  flowing  can  burn,  for  they  consist  of 
pure  hydrogen,  which,  as  has  been  shown  in  Exp.  23,  will  by 
itself  immediately  extinguish  combustion. 

All  ordinary  gas  flames  are,  of  necessity,  hollow.  They  are 
visible  to  us  through  the  evolution  of  light  which  is  an  accom- 
paniment of  the  chemical  action.  But  even  if  no  combustion 
were  going  on  upon  its  surface,  the  escaping  column  of  gas 
could  still  be  made  visible  by  causing  it  to  pass  through  a  quan- 
tity of  dust  or  other  fine  powder  before  coming  into  the  air.  A 
portion  of  the  solid  matter  would  be  transported  by  the  current 
of  gas,  and  the  form  assumed  by  the  latter  would  be  made  mani- 
fest. 

The  shape  of  the  unignited  gas  column  would  of  course  be 


46         EXPLOSIVE    MIXTURE    OF    HYDROGEN    AND    OXYGEN. 

somewhat  different  from  that  of  the  burning  flarne  ;  in  the  latter, 
not  only  is  the  outer  edge  of  the  column  sharply  defined  by  the 
zone  of  combustion,  but  the  actual  form  of  the  cohmin  itself  is 
modified  through  the  expansion  of  the  gas  as  it  becomes  heated 
by  the  enveloping  fire. 

58.  If,  instead  of  burning  pure  hydrogen  as  it  flows  into  the 
air,  as  in  the  foregoing  experiments,  the  gas  be  first  mixed  with 
air  or  oxygen  and  then  ignited,  a  very  different  result  will  be 
obtained.     The  hydrogen  being  now  in  contact  with  oxygen  at 
all  points,  the  entire  mass  of  gas  will  burn  with  a  violent  explo- 
sion at  the  instant  when  a  light  is  touched  to  it. 

Exp.  29.  —  Introduce  2  volumes  of  hydrogen  and  5  volumes  of  air 
into  a  strong  round-bottomed  bottle  such  as  is  used  for  soda-water. 
Close  the  mouth  of  the  bottle  with  a  cork,  and  shake  violently  in  order 
that  the  gases  shall  be  mixed.  A  small  quantity  of  water  should  be 
left  in  the  bottle  to  act  as  a  stirrer.  Grasp  the  bottle  firmly  in  one 
hand,  remove  the  cork  with  the  other,  and  apply  the  open  mouth  of  the 
bottle  to  a  lighted  candle.  An  explosion  will  immediately  ensue. 

Exp.  30.  —  Into  a  gas-holder,  bladder,  or  rubber  bag,  introduce  a 
mixture  of  2  vols.  hydrogen  and  1  vol.  oxygen.  Connect  therewith, 
by  means  of  caoutchouc  tubing,  a  tobacco-pipe,  or  bit  of  glass  tube. 
Press  the  gas  through  the  pipa  into  a  dish  of  soap-suds,  in  such  manner 
that  there  shall  be  formed  upon  the  surface  of  the  suds  a  mass  of  foam 
as  large  as  an  egg.  Close  the  gas-holder  and  remove  it  from  the 
vicinity  of  the  suds.  On  now  touching  the  foam  with  a  long,  lighted 
stick  an  exceedingly  violent  explosion  will  occur. 

It  is  well  to  avoid  the  formation  of  a  large  quantity  of  foam  in  this 
experiment,  since  the  concussion  is  in  any  event  deafening.  In  this 
experiment  the  explosive  mixture  is  purposely  confined  in  an  exceed- 
ingly flimsy  envelope,  in  order  that  no  harm  may  be  done  by  the 
fragments. 

Care  should  be  taken  to  throw  away  any  remnant  of  the  mixture  of 
hydrogen  and  oxygen  which  may  have  been  left  in  the  gas-holder  at 
the  close  of  the  experiment,  and  upon  no  account  should  fire  ever  be 
brought  into  its  vicinity. 

59.  The  cause  of  these  loud  explosions  is  two-fold.     By  the 
act  of  combination  water  is  formed,  and  at  the  same  time  intense 
heat  is  emitted  ;    the  water,  or  rather  steam,  is  thereby  enor- 
mously expanded,  so  that  for  a  moment  there  is  violent  motion 


OXYGEN  BURNS  IN  HYDROGEN.  47 

outwards  in  all  directions.  This  outward  motion  would  scatter 
about  in  a  most  dangerous  manner  the  fragments  of  any  vessel 
in  which,  through  carelessness,  a  mixture  of  hydrogen  and  oxy- 
gen might  be  ignited ;  unless,  indeed,  the  vessel  were  very  strong, 
small,  and  of  large  aperture.  But  in  the  next  instant,  as  the 
steam  condenses,  there  is  an  even  more  violent  motion  inwards. 

The  original  mixture  of  hydrogen  and  oxygen  occupies  about 
2000  times  as  much  space  as  the  liquid  water  which  results  from 
the  combination  of  these  gases.  Hence  a  partial  vaccuum  is 
formed,  into  which  air  rushes  from  all  sides  ;  and  it  is  the  heavy 
and  sudden  undulations  thus  communicated  to  the  air  which  occa- 
sion the  noise.  The  outward  and  inward  shocks  follow  one 
another  so  quickly  that  the  ear  cannot  distinguish  between  them. 

Mixtures  of  hydrogen  and  air  produce  less  violent  explosions 
than  mixtures  of  hydrogen  and  oxygen,  because  of  the  inert 
nitrogen  in  the  air,  which  acts  as  an  elastic  pad,  or  cushion,  to 
break  the  force  of  the  shock. 

60.  Since  air  is  everywhere  about  us,  and  since  all  ordinary 
combustions  occur  in  it,  it  has  become  customary,  to  speak  of  it 
and  of  oxygen  as  supporters  of  combustion,  in  contradistinction  to 
the  so-called  combustibles,  such  as  hydrogen.  These  terms  are 
often  convenient,  but,  as  will  appear  from  the  following  experi- 
ment, they  have  only  a  relative,  and  no  absolute  significance. 

Exp.  31.  —  Provide  a  tube  of  thin  glass,  the  neck  of  a  broken  retort 
for  example,  30  or  40  c.  m.  long,  and  3  or  4  c.  m.  in  diameter,  fix  it  in 
a  vertical  position,  so  that  the  lower  opening  shall  be  20  or  30  c.  m. 
above  the  table,  and  connect  the  upper  opening  with  a  gas-holder 
filled  with  hydrogen. 

To  a  second  gas-holder,  containing  oxygen,  attach  a  caoutchouc 
tube,  and  to  the  end  of  this  fit  a  piece  of  glass  tubing,  No.  7,  25  or  30 
c.  rn.  long,  bent  at  a  right  angle  at  5  or  10  c.  m.  from  the  end  which  is 
attached  to  the  caoutchouc  tube,  and  drawn  out  to  a  fine  open  point  at 
the  other.  The  caoutchouc  tube  must  be  long  enough  to  reach  as  far 
as  the  lower  mouth  of  the  wide,  vertical  glass  tube  above  mentioned. 

Open  now  the  stop-cock  of  the  hydrogen  gas-holder  so  that  the 
vertical  tube  shall  be  filled  with  the  gas,  then  apply  a  lighted  match  to 
the  mouth  of  this  tube,  and  regulate  the  flow  of  gas  so  that  the  latter 
may  continue  to  burn  slowly  at  the  lower  edge  of  the  tube.  Finally, 
open  the  stop-cock  of  the  oxygen  gas-holder,  so  that  a  current  of  thi 


48  PEROXIDE    OF    HYDROGEN. 

gas  shall  flow  through  the  pointed  delivery-tube,  and  thrust  this  tube 
up  into  the  middle  of  the  wide,  vertical  tube  which  has  been  filled  with 
hydrogen. 

As  the  stream  of  oxygen  passes  through  the  burning  hydrogen  at  the 
bottom  of  the  vertical  tube,  it  takes  fire,  and  afterwards  continues  to 
burn  in  the  atmosphere  of  hydrogen  within  the  tube. 


CHAPTER    VI. 

OTHER    COMPOUNDS    OF    THE    ELEMENTS    ALREADY    STUDIED. 

61.  Oxygen  and  hydrogen  do  not  unite  directly  in  any  other 
proportion  than  that  in  which  they  form  water,  but  by  indirect 
means,  too  complex  for  profitable  study  at  this  stage,  a  molecule 
of  water  can  be  made  to  combine  with  an  atom  of  oxygen,  form- 
ing a  new  substance  known  by  the  name  of  peroxide  of  hydrogen. 
Its  formula  is, -in  accordance  with  this  statement,  H2O2,  and  it 
would  yield,  if  completely  decomposed  into  its  elements,  equal 
volumes  of  hydrogen  and  oxygen ;  its  composition  by  weight 
must  be  2  parts  of  hydrogen  to  32  of  oxygen  ;  it  contains  just 
twice  as  large  a  proportion  of  oxygen  as  water.  The  best 
method  of  preparing  this  substance  does  not  yield  the  pure  thing 
itself,  but  only  a  concentrated  solution  of  it  in  water ;  the  specific 
gravity  of  this  solution  is  1.45  ;  it  is  colorless,  transparent,  and 
somewhat  syrupy,  has  a  metallic  taste,  corrodes  the  skin,  and 
bleaches  vegetable  colors. 

How  different  a  substance  this  is  from  water,  appears  at  once 
from  this  enumeration  of  some  of  its  properties.  It  is  very  un- 
stable, being  readily  decomposed  by  heat,  and  by  contact  with 
various  substances  at  the  ordinary  temperature,  into  oxygen  and 
water.  When  the  solution  of  peroxide  of  hydrogen  is  decomposed 
by  bringing  some  other  substance  into  contact  with  it,  either  one 
of  three  very  different  effects  may  be  produced :  —  first,  the  oxy- 
gen lost  by  the  peroxide  of  hydrogen  may  immediately  combine 
with  the  other  substance ;  secondly,  the  oxygen  parting  from  the 
peroxide  of  hydrogen  may  pass  the  substance  which  has  caused 


OXIDIZING   AND    REDUCING   AGENTS.  49 

the  decomposition  by,  and  escape  into  the  air ;  thirdly,  the  de- 
composition of  the  peroxide  into  hydrogen  and  water  may  in- 
duce a  precisely  similar  decomposition  of  the  substance  whose 
presence  incited  the  chemical  action,  and  this  foreign  body  may 
lose  oxygen  simultaneously  with  the  peroxide  solution. 

62.  When  a  substance  habitually  and  readily  imparts  oxygen 
to  other  bodies  with  which  it  is  brought  in  contact,  it  is  called  an 
oxidizing  agent;    and  on  the  other  hand,  a   substance  which 
habitually  and  readily  takes  oxygen  out  of  other  substances  with 
which  it  is  brought  in  contact,  is  called  a  reducing  agent.     In  the 
first  of  the  three  cases  above  described,  the  peroxide  of  hydrogen 
is  an  oxidizing  agent ;  in  the  last,  it  has  a  reducing  effect ;  in  the 
second  it  has  no  chemical  action  whatever  on  the  body  which  de- 
termines  its   decomposition.      Its   instability  and    the    intense 
chemical  activity  of  which  it  is  capable,  emphatically  distinguish 
this,  as  yet  obscure,  body  from  the  neutral,  stable,  inactive  com- 
pound of  hydrogen  and  oxygen,  common  water. 

But  though  the  peroxide  of  hydrogen  is  not  water,  it  is  never- 
theless a  true  chemical  compound  of  the  same  elements  which 
are  united  in  water ;  it  is  definite  and  constant  in  composition, 
and  its  properties  are  as  unlike  those  of  its  elementary  constitu- 
ents, as  are  those  of  water.  A  new  fact,  of  great  significance 
here  comes  plainly  into  view.  Two  of  the  elements  are  evi- 
dently capable  of  combining  in  two  definite  proportions  to  form 
two  chemical  compounds,  each  differing  from  the  other  and  from 
its  primary  constituents.  The  study  of  the  compounds  of  nitro- 
gen and  oxygen  will  bring  into  clearer  view  the  general  principle 
of  which  this  fact  is  a  single  illustration.  These  compounds 
form  a  series  of  five  members,  all  derived,  more  or  less  directly, 
from  common  nitric  acid. 

63.  Nitric  acid.     Two  abundant  sources  of  this  material  are 
found  hi    nature,  and   are   familiar   as   articles   of    commerce. 
Saltpetre  or  nitre,  a  whitish,  saline,  crystallized  substance,  now 
mainly  brought  from  India,  is  one  of  these  sources ;  a  similar 
substance,  known  as  Chili-saltpetre,  or  soda-nitre,  is  collected  on 
a  desert  tract  in  Chili  and  Peru,  and  forms  a  valuable  article  of 
export  from  those  countries.     These  two  substances  only  differ 
from  each  other  in  this,  —  that  the  first  contains  the  metal  potas- 

4 


50  PREPARATION    OF  NITRIC    ACID. 

slum,  the  second  the  very  similar  metal  sodium,  in  either  case 
combined  with  definite  proportions  of  the  elements  nitrogen  and 
oxygen.  By  the  reaction  of  sulphuric  acid  (oil  of  vitriol)  on 
either  of  these  two  substances,  nitric  acid  is  obtained.  On  a 
small  scale,  for  laboratory  purposes,  saltpetre,  or,  as  it  is  called 
in  chemistry,  nitrate  of  potassium,  is  generally  employed ;  on  a 
manufacturing  scale,  soda-saltpetre  or  nitrate  of  sodium  is  used, 
because  this  salt  costs  less  than  nitrate  of  potassium,  and  also, 
contains  a  larger  proportion  of  nitric  acid,  which  it  yields  up 
with  greater  facility. 

Exp.  32. —  Into  a  tubulated,  glass-stoppered  retort  of  250  c.  c. 
capacity  put  40  grammes  of  powdered  nitrate  of  potassium,  or,  better, 
34  grammes  of  powdered  nitrate  of  sodium  if  it  can  be  obtained,  and 
through  the  tubulature  pour  50  grammes  of  strong  sulphuric  acid. 
Imbed  the  bottom  of  the  retort  in  sand,  contained  in  a  small  iron  pan 
placed  over  the  gas-lamp  on  a  ring  of  the  iron-stand.  Thrust  the  neck 
of  the  retort  into  a  receiver  with  two  tubulatures ;  the  retort-neck 
should  fit  the  tubulature  of  the  receiver  with  tolerable  accuracy.  The 
second  tubulature  of  the  receiver  should  be  left  open,  or  loosely  cov- 
ered with  a  bit  of  glass,  in  order  to  avoid  the  possibility  of  any  pressure 
being  created  within  the  retort  during  the  operation.  Place  the  re- 
ceiver in  a  pan  of  cold  water,  and  cover  it  with  cloth  or  bibulous  paper, 
which  must  be  kept  constantly  wet  during  the  distillation.  (See  fig.  12, 
p.  31.)  Apply  a  moderate  heat  to  the  sand-bath  ;  reddish  vapors  will 
appear  for  a  moment,  then  disappear,  and  a  yellowish  fuming  liquid  will 
begin  to  condense  in  the  receiver.  Towards  the  end  of  the  operation 
the  red  vapors  reappear ;  when  this  happens,  and  very  little  liquid  passes 
over,  while  the  saline  matter  in  the  retort  is  in  a  state  of  tranquil  fusion, 
the  lamp  may  be  put  out,  for  the  process  is  finished. 

The  very  acid,  corrosive,  and  poisonous  liquid  in  the  receiver  is 
nitric  acid;  its  J'aint  color  is  not  its  own,  but  is  due  to  the  presence  of 
a  compound  of  nitrogen  and  oxygen  shortly  to  be  described.  Transfer 
the  liquid  to  a  glass-stoppered  bottle,  and  keep  it  for  future  use.  In 
all  manipulations  with  nitric  acid  it  is  necessary  to  avoid  getting  it 
upon  the  skin,  since  it  produces  rather  permanent  yellow  stains. 

As  the  retort  cools,  the  residue  solidifies  into  a  white,  saline  mass, 
which  must  be  dissolved  out  of  the  vessel  by  heating  it  with  water.  It 
will  be  observed  that  the  liquid  sulphuric  acid  which  was  used  has  dis- 
appeared, though  the  saline  residue  is  still  intensely  acid. 

64.  Pure  nitric  acid  is  colorless,  and  is  about  half  as  heavy 


ACID    AND    ALKALINE.  51 

again  as  water.  It  may  be  mixed  with  water  in  all  propor- 
tions. 

Exp.  33. —  To  one-third  of  the  nitric  acid  obtained  in  the  last  ex- 
periment add  an  equal  bulk  of  water.  The  solution  thus  obtained 
will  be  still  intensely  acid,  as  may  be  proved  by  its  action  on  vegetable 
colors.  Litmus  is  a  blue  coloring  matter,  prepared  from  various 
lichens,  and  used  in  dyeing.  Unsized  paper,  colored  with  a  solution 
of  litmus  in  water,  is  a  convenient  test  for  many  acids,  which,  as  a 
rule,  change  the  color  of  the  paper  from  blue  to  red.  If  the  acidity  of 
this  diluted  nitric  acid  be  now  destroyed  or  neutralized  by  the  addition 
of  some  other  substance  of  opposite  quality,  the  point  at  which  the 
liquid  ceases  to  be  acid  may  be  determined  by  observing  when  the  blue 
paper  remains  blue  on  immersion  in  the  liquid. 

To  the  diluted  nitric  acid,  placed  in  an  evaporating-dish,  add  cau- 
tiously ammonia- water  (the  Liquor  Ammonias  of  the  apothecaries), 
which  has  been  previously  diluted  with  its  own  bulk  of  water,  until  the 
liquid  no  longer  turns  the  litmus-paper  red.  The  ammonia-water  must 
be  added,  slowly  at  first,  and  at  last  drop  by  drop,  and  the  mixture 
must  be  constantly  stirred  with  a  glass  rod.  The  ammonia-water  has 
the  property  of  turning  litmus-paper,  which  has  been  reddened  by  an 
acid,  back  again  to  blue,  as  direct  experiment  may  prove ;  this  property 
is  possessed  by  a  class  of  bodies  called  alkalies,  and  this  reaction  with 
litmus  is  termed  the  alkaline  reaction,  in  contradistinction  to  the  change 
from  blue  to  red,  which  is  the  reaction  characteristic  of  acids.  When 
mixed  with  nitric  acid,  ammonia-water  produces  a  compound  which, 
when  fresh  and  pure,  has  no  action  on  vegetable  colors,  and  being 
soluble  in  water  is  not  visible  at  this  stage  of  the  experiment.  Place  the 
evaporating-dish  on  the  wire-gauze  over  the  gas-lamp,  and  evaporate 
the  liquid,  without  ebullition,  till  a  crust  begins  to  form  on  its  surface. 
Extinguish  the  lamp,  let  the  dish  become  perfectly  cold,  separate  the 
semi-transparent  crystals  which  have  formed  during  the  cooling  from 
the  fluid,  if  any,  which  remains  in  the  dish,  allow  them  to  drain,  dry 
them  by  gentle  pressure  between  folds  of  bibulous  paper,  and  reserve 
them  for  use  in  the  next  experiment.  Besides  water,  these  crystals  are 
the  sole  product  of  the  reaction ;  they  must  therefore  contain  both  all 
of  the  nitric  acid  and  all  of  the  ammonia  which  is  not  water.  The 
chemical  name  of  the  substance  is  nitrate  of  ammonium. 

65.  We  find  in  this  experiment  a  striking  illustration  of  what 
is  meant  by  chemical  combination.  From  two  fuming  liquids, 
of  very  intense  but  opposite  properties,  there  has  come  forth  a 
neutral  solid,  as  unlike  its  constituents  in  taste,  smell,  and  all 


52 


MAKING   NITROUS    OXIDE. 


physical  attributes,  as  could  well  be  imagined.  The  idea  of 
neutralization,  well  exemplified  in  this  experiment,  has  in  the 
history  of  chemistry  been  very  fruitful  both  of  names  and  hy- 
potheses. Bearing  in  mind  the  fiery  constituents  of  the  cooling, 
harmless  salt,  which  we  have  thus  synthetically  prepared,  we 
proceed  to  learn  by  experiment  whatever  its  decomposition  may 
teach. 

Exp.  34.  —  Introduce  into  a  Florence  oil-flask,  or  other  suitable  flask 
of  thin  glass,  and 'of  about  300  c.  c.  capacity,  the  nitrate  of  ammonium 
obtained  in  the  last  experiment.  From  the  mouth  of  the  flask,  placed 
upon  the  wire-gauze  on  the  iron-stand,  carry  a  gas  delivery-tube,  No.  6, 
beneath  the  saucer  in  the  water-pan;  but  interrupt  the  tube,  at  some 
convenient  point  to  interpose,  by  means  of  a  cork  or  caoutchouc  stopper 
with  two  holes,  a  small  bottle  which  can  be  kept  cool  with  water,  as 
shown  in  the  figure. 

FIG.  19. 


Heat  the  flask  moderately  and  cautiously,  to  avoid  breaking  it.  The 
nitrate  of  ammonium  will  melt,  and  little  bubbles  will  soon  begin  to 
escape  from  the  fused  mass.  The  heat  must  now  be  so  controlled  that 
the  evolution  of  the  gas  shall  not  be  tumultuous.  The  gas  is  to  be  col- 
lected in  bottles  of  300  to  400  c.  c.  capacity.  In  the  cooled  bottle, 
through  which  the  gas  passes,  a  clear  and  colorless  liquid  will  condense, 
which  will  be  found  on  examination,  if  the  process  has  been  success- 
fully conducted,  to  be  neither  acid  nor  alkaline,  to  have  neither  taste 
nor  smell,  and  to  be  wholly  volatile  on  platinum  foil,  —  in  short,  to 
possess  all  the  properties  of  water,  and  none  other.  When  two  bottles 
of  gas  have  been  filled,  and  enough  water  for  testing  condensed,  the 
delivery-tube  may  be  withdrawn  from  the  water  and  the  lamp  extin- 


COMPOSITION    OF    NITROUS    OXIDE.  53 

guished,  although  the  nitrate  of  ammonium  be  not  all  decomposed. 
The  nitrate  of  ammonium  may  be  entirely  resolved  into  water  and  the 
gas  which  now  awaits  examination,  but  it  is  difficult  to  push  the  de- 
composition to  actual  completion,  without  breaking  the  flask  in  which 
the  operation  is  performed.  That  the  nitrate  leaves  no  residue  behind, 
when  sufficiently  heated,  may  be  proved  by  heating  a  crystal  of  it  on 
platinum  foil  over  the  gas-lamp. 

66.  It  is  obvious  that  the  colorless  and  transparent  gas,  which 
is  the  most  voluminous  product  of  the  decomposition  of  nitrate 
of  ammonium  just  accomplished,  must  contain  all  the  elements, 
besides  those  of  water,  which  enter  into  the  composition  of 
nitrate  of  ammonium,  and  therefore  of  its  constituents,  nitric  acid 
and  ammonia-water;  much  interest,  therefore,  attaches  to  the 
determination  of  the  composition  of  this  gas. 

Exp.  35.  —  Insert  a  glowing  splinter  of  wood  into  a  bottle  of  the  gas. 
It  will  re-inflame  with  almost  as  much  energy  as  in  oxygen. 

If  oxygen  be  really  a  constituent  of  this  gas,  it  may  be  possible  to 
mix  the  gas  with  hydrogen,  and  effect  the  chemical  combination  into 
water  of  the  oxygen  in  the  gas  and  the  added  hydrogen,  by  heating  the 
mixture,  or  passing  through  it  an  electric  spark  (see  pp.  24,  26).  If 
just  enough  hydrogen  can  be  added  to  exactly  convert  all  the  oxygen 
contained  in  a  given  volume  of  the  gas  into  water,  the  constituents 
other  than  oxygen  will  be  left  behind  for  separate  examination,  and 
we  shall  have  determined  how  much  oxygen  a  given  volume  of  the  gas 
contains  by  observing  how  much  hydrogen  has  been  necessary  to  con- 
vert it  into  water,  the  volumetric  composition  of  water  being  already 
known.  Now  it  has  been  found  that  when  any  volume  of  this  gas  is 
mixed  with  an  equal  volume  of  hydrogen,  in  a  strong  tube,  provided 
with  platinum  points,  like  those  of  the  U  tube  already  used  (p.  25), 
and  the  mixture  is  fired  by  the  electric  spark,  a  violent  explosion  takes 
place,  a  dew  of  water  condenses  upon  the  walls  of  the  tube,  and  there 
remains  a  volume  of  colorless  gas,  precisely  equal  to  that  with  which 
the  hydrogen  was  originally  mix^d.  On  studying  the  properties  of 
this  residual  gas,  it  is  found  to  be  tasteless,  odorless,  a  little  lighter  than 
air,  and  to  be  neither  inflammable,  nor  yet  a  supporter  of  combustion  ; 
it  is  recognized  as  the  pure  element,  nitrogen. 

It  follows  from  this  experiment,  and  the  knowledge  of  the 
composition  of  water  previously  gained,  that  any  measure  of  the 
gas  obtained  from- nitrate  of  ammonium  contains  its  own  measure 
of  nitrogen  and  half  that  measure  of  oxygen.  The  constitution 


54  ATOMIC    WEIGHT    OF   NITROGEN. 

of  this  gas  is  strictly  analogous  to  that  of  steam ;  as  two  vol- 
umes of  hydrogen  and  one  volume  of  oxygen  are  compacted  into 
two  volumes  of  steam,  so  two  volumes  of  nitrogen  and  one  vol- 
ume of  oxygen  are  condensed  into  two  volumes  of  this  transpa- 
rent gas.  As  the  chemical  formula  or  symbol  of  water  is  H2O, 
so  the  formula  of  this  new  gas  is  N2O,  and  its  volumetric  com- 
position may  be  represented  by  a 
diagram  similar  to  that  by  which 
we  conveyed  to  the  eye  the  com- 


O 

position  of  water. 


67.  As  has  been  already  said, 
a  combination  of  oxygen  with 
another  element  is  called  an  oxide ;  the  name  of  the  second 
element  is  given  either  by  an  adjective  which  precedes  the  word 
oxide,  as  in  the  case  of  this  gas  N2O,  whose  name  is  nitrous 
oxide,  or  by  connecting  the  name  of  the  second  element  with 
the  word  oxide  by  the  preposition  of,  as  in  case  of  oxide  of  iron. 

From  the  above  composition  by  volume,  and  from  the  known 
specific  gravities  of  nitrogen  and  oxygen,  the  composition  of 
nitrous  oxide  by  weight  is  readily  deduced.  The  specific  gravity 
of  nitrogen,  referred  to  hydrogen,  is  14;  that  of  oxygen  16; 
since  there  are  two  volumes  of  nitrogen  for  each  volume  of 
oxygen,  the  two  elements  must,  in  any  given  weight  of  the  gas, 
be  combined  in  the  proportion  of  28  parts  by  weight  of  nitrogen 
to  16  of  oxygen.  The  molecule  of  nitrous  oxide,  N2O,  must 
be  composed,  like  any  other  quantity  of  the  gas,  of  28  parts  by 
weight  of  nitrogen  and  16  of  oxygen;  but  precisely  as  in  the 
case  of  water,  we  conceive  of  the  molecule  as  made  up  of  two 
atoms  of  nitrogen  and  one  atom  of  oxygen,  and  we  have  already 
learned  that  if  the  atomic  weight  of  hydrogen  be  represented  by 
1,  that  of  oxygen  must  be  16.  It  follows,  from  the  constitution 
of  nitrous  oxide,  that  if  16  represent  the  smallest  proportional 
weight  of  oxygen  which  exists  in  combination,  14  must  be  the 
corresponding  smallest  weight  of  nitrogen  when  thus  united  with 
oxygen.  Nitrous  oxide  contains  16-44ths,  or  36.36  per  cent,  of 
oxygen. 

68.  Nitrous  oxide  is  almost  without  odor,  but  has  a  distinctly 
sweet  taste ;  its  specific  gravity  referred  to  hydrogen  is  22  ;  it  is 


PROPERTIES    OF    NITROUS    OXIDE.  55 

quite  soluble  in  water,  which  at  0°  dissolves  more  than  its  own 
volume  of  the  gas,  and  more  than  half  its  volume  at  24°. 
Owing  to  this  solubility  there  is  a  trifling  loss  incurred  by  col- 
lecting it  in  the  usual  manner  over  water ;  this  loss  may  be  par- 
tially avoided  by  using  warm  water  in  the  pan.  Nitrous  oxide 
may  be  obtained  in  the  liquid  state  by  submitting  it  to  a  mechan- 
ical pressure  of  about  30  atmospheres  in  an  apparatus  cooled  to 
0°.  The  liquid  is  very  mobile,  boils  at  — 88°,  and  crystallizes 
at  about  — 100°  when  allowed  to  evaporate  spontaneously  under 
the  exhausted  receiver  of  an  air-pump.  It  is  noteworthy  that 
the  permanent  gases,  such  as  nitrogen,  oxygen,  and  hydrogen,  are  i 
but  slightly  soluble  in  water,  while  all  the  soluble  gases  are] 
liquifiable,  and  often  the  more  readily  liquifiable  in  proportion  to 
their  solubility.  A  drop  of  liquid  nitrous  oxide  blisters  the  skin 
like  a  hot  iron.  By  mixing  the  solid,  snow-like  nitrous  oxide 
with  the  volatile  liquid  called  bi-sulphide  of  carbon,  and  evapo- 
rating the  mixture  in  a  vacuum,  the  lowest  temperature  which 
has  hitherto  been  attained  is  produced ;  it  is  estimated  at  — 140°. 

Small  animals  immersed  in  gaseous  nitrous  oxide  die  after 
some  time,  but  it  may  be  respired  for  a  few  minutes  with  entire 
impunity  by  the  healthy  human  being.  The  physiological  effects 
of  this  gas,  when  respired,  vary  somewhat,  according  to  the 
quality  of  the  gas  and  the  mode  of  administration ;  sometimes  it 
produces  a  lively  intoxication,  attended  with  a  disposition  to 
muscular  exertion  and  violent  laughter,  whence  its  trivial  name 
of  laughing-gas  ;  sometimes,  on  the  contrary,  it  produces  a  com- 
plete insensibility,  during  which  surgical  operations  may  be  per- 
formed without  pain.  When  intended  for  respiration  great  at- 
tention should  be  paid  to  the  purity  of  the  gas  ;  carefully  pre- 
pared and  judiciously  administered,  it  is  advantageously  used  as 
an  anaesthetic  agent,  especially  for  operations  lasting  but  a  few 
seconds. 

As  the  gas  contains  nearly  twice  as  much  oxygen  as  atmos- 
pheric air,  it  does  not  seem  strange  that  it  should  make  common 
combustibles  burn  with  great  intensity ;  it  forms  explosive  mix- 
tures with  many  inflammable  gases ;  it  causes  glowing  charcoal 
to  burst  into  flame,  and  sulphur  and  phosphorus  burn  in  the  gas 
with  great  brilliancy,  if  well  on  fire  when  immersed  in  it. 


56 


PREPARATION    OF   NITRIC    OXIDE. 


Exp.  36. —  Place  a  bit  of  sulphur  in  a  deflagrating  spoon,  and 
ignite  it  with  the  least  possible  application  of  heat ;  then  thrust  it  into 
a  bottle  of  nitrous  oxide.  It  will  be  extinguished.  Yet  it  would  con- 
tinue to  burn  in  the  air.  Heat  the  sulphur  much  hotter,  and  again 
introduce  it  into  the  bottle  of  nitrous  oxide.  It  will  now  burn  far 
more  brilliantly  than  in  the  air. 

The  air  is  not  a  chemical  compound,  but  only  a  mechanical  mixture 
of  nitrogen  and  oxygen ;  so  that  a  body  burning  in  the  air  has  only  to 
take  oxygen,  which  is  perfectly  free  to  join  it.  The  case  is  entirely 
different  when  the  substance  at  whose  expense  the  oxygen  is  furnished 
is  a  chemical  compound ;  to  dismember  the  compound  will  require  a 
force  superior  to  that  which  binds  its  elements  together.  Before  the 
sulphur,  in  this  experiment,  can  unite  with  oxygen,  it  must  detach 
the  oxygen  from  the  nitrogen  with  which  it  is  combined.  To  accom- 
plish this,  the  sulphur  must  be  hotter  than  it  need  be  for  simple  burning 
in  the  air.  We  shall  soon  learn  that  there  are  many  chemical  com- 
pounds, much  richer  in  oxygen  than  either  the  air  or  nitrous  oxide, 
which  nevertheless  cannot  support  combustion  at  all,  in  the  ordinary 
sense  of  the  term,  and  this  simply  because  the  common  combustibles 
are  quite  unable  to  detach  the  oxygen  from  the  elements  with  which  it 
is  already  combined. 

69.  The  nitrous  oxide  which  we  have  thus  studied,  is  a  deriva- 
tive of  nitric  acid,  or  more  exactly,  of  the  compound  of  nitric 
acid  with  ammonia-water,  but  it  is  only  one  of  several  such  de- 
rivatives. We  proceed  to  investigate  another  substance  still 
more  directly  obtained  from  nitric  acid. 

PIG.  20.  Exp.  37.  —  Place  some  copper 

turnings  or  filings  in  a  bottle  ar- 
ranged precisely  as  for  generat- 
ing hydrogen  (see  Exp.  19),  and 
pour  upon  them  one  half  of  the 
nitric  acid  still  remaining  from 
Exp.  32,  previously  diluting  this 
portion  of  acid  with  twice  its 
bulk  of  water.  Brisk  action  will 
immediately  occur.  The  bottle 
becomes  filled  with  red  fumes, 
but  when  the  gas  disengaged  is 
collected  over  water,  it  is  found 
to  be  colorless.  Collect  four  bot- 
tles, of  300  to  400  c.  c.  capacity,  of  this  gas. 


NITRIC    OXIDE.  57 

Exp.  38.  —  Dip  a  lighted  candle  into  a  bottle  of  the  gas.  It  goes 
out.  Into  the  same  bottle  thrust  a  glowing  splinter  of  wood.  It  will 
not  inflame. 

Exp.  39.  — Lift  a  bottle  of  the  gas  from  the  water  so  that  air  may 
enter  the  bottle,  and  the  gas  may  escape  into  the  air.  Red  fumes,  of  very 
disagreeable  smell,  and  very  irritating  when  inhaled,  are  abundantly 
produced.  Bring  in  contact  with  these  fumes,  a  piece  of  moistened 
litmus-paper.  It  will  be  immediately  reddened. 

Exp.  40.  —  Thoroughly  ignite  a  bit  of  sulphur  in  a  deflagrating- 
spoon,  and  introduce  it  into  a  bottle  of  the  gas.  It  will  not  burn. 

Exp.  41.  —  Into  the  same  bottle  thrust  a  piece  of  phosphorus  as  big 
as  a  pea,  burning  actively.  The  combustion  will  be  continued  with 
great  brilliancy. 

70.  The  new  gas  is  transparent  and  colorless,  and  that  it  is 
sparingly  soluble  in  water  may  be  inferred  from  the  fact  that 
bottles  of  it  may  stand  indefinitely  over  water  without  apprecia- 
ble loss.  It  differs  from  nitrous  oxide,  and  from  all  the  other 
.gases  thus  far  studied,  in  its  relation  to  combustibles.  The  com- 
monest combustibles  will  not  burn  in  it  at  all ;  phosphorus  may 
be  melted  in  the  gas  without  inflaming,  but  when  its  combustion 
is  once  started,  phosphorus  burns  with  a  vividness  which  recalls 
its  burning  in  oxygen. 

When  the  gas  touches  the  air,  a  new  compound,  red,  acid,  and 
irritating,  is  immediately  produced ;  the  question  arises,  —  is  it 
the  nitrogen  or  the  oxyge'h  of  the  air  which  gives  rise  to  this 
new  combination  ?  Experiment  would  answer  this  question  in 
favor  of  oxygen.  If  into  a  bottle  of  this  new  gas  nitrogen  were 
introduced,  the  result  would  be  simply  negative ;  no  visible 
change  and  no  chemical  combination  would  take  place.  The 
introduction  of  oxygen  into  a  bottle  of  the  gas  would,  on  the 
other  hand,  produce  the  red  vapors  in  question,  only  more 
vividly  than  air,  because  dilution  with  the  inert  nitrogen  of  the 
air  would  have  been  avoided.  So  visible  and  trustworthy  is  this 
reaction,  that  the  gas  we  are  studying  may  be  used  to  exhibit  the 
presence  of  free  oxygen  in  gaseous  mixtures.  For  example, 
both  oxygen  and  nitrous  oxide  reinflame  a  glowing  splinter,  and 
we  cannot  distinguish  between  these  two  gases  by  this  test ;  but 
the  gas  we  are  now  studying  supplies  us  with  a  means  of  dis- 
crimination, since  it  produces  no  red  fumes  with  nitrous  oxide. 


58  ANALYSIS    OF   NITRIC    OXIDE. 

If  a  sufficient  quantity  of  the  metal  potassium  be  heated  in  the  dry 
gas  till  it  burns,  and  the  experiment  be  so  executed  as  to  allow  the 
volume  of  gas  to  be  measured  both  before  and  after  the  combustion,  it 
will  be  found  that  one  half  of  the  volume  of  gas  used  has  disappeared, 
and  that  the  half  which  remains  possesses  few  or  none  of  the  qualities 
of  the  original  gas  ;  a  slight  examination  would  convince  us  that  we 
had  set  free  the  well-known  element  nitrogen.  The  other  half  of  the 
original  volume  of  gas  has  united  with  the  potassium  to  form  a  body 
which  we  shall  hereafter  be  familiar  with  under  the  name  of  oxide  of 
potassium.  The  other  half  of  the  gas  is  then  oxygen,  and  we  have 
found  the  elements  which  enter  into  the  composition  of  this  gas,  and 
the  volumetric  proportions  in  which  they  are  united.  One  volume  of 
nitrogen  is  combined  with  one  volume  oroxygen  to  form  two  volume? 
of  the  compound  gas. 

We  meet  here  the  first  case  of  chemical  combination  between 
two  gases  unattended  by  any  condensation  of  the  ingredients. 
The  molecule  of  the  gas  will  be  represented  by  the  formula  NO, 
and  its  elements  are  united  by  weight  in  the  proportion  of  14 
parts  of  nitrogen  to  16  of  oxygen,  because  equal  volumes  of 
nitrogen  and  oxygen  weigh  respectively  14  and  16  times  as 
much  as  the  same  volume  of  hydrogen.  The  gas  is  another  oxide 
of  nitrogen,  and  is  distinguished  by  the  name  nitric  oxide. 

When  there  are  twp  or  more  oxides  of  one  element,  the  ter- 
mination ous  implies  less  oxygen  than  the  termination  ic,  as  in 
this  case ;  nitrous  oxide  contains  hgflf  as  much  oxygen  for  its 
nitrogen  as  nitric  oxide. 

Nitric  oxide  is  one  of  the  permanent  gases  ;  it  has  never  been 
liquified.  It  is  a  very  stable  compound,  and  if  perfectly  dry  is 
not  decomposed  by  a  red  heat  or  by  the  action  of  electric  sparks. 
Owing  to  its  rapid  union  with  oxygen  and  formation  of  acid 
products,  its  taste,  smell,  and  respirability  cannot  be  ascertained. 
Bearing  in  mind  the  fact  that  certain  red  acid  fumes,  the  like  of 
which  we  remember  to  have  seen  in  making  nitric  acid,  are 
formed  by  adding  oxygen  to  nitric  oxide,  we  proceed  to  a  further 
.  study  of  still  other  oxides  of  nitrogen. 

71.  When  a  mixture  of  two  volumes  of  nitric  oxide  and  one 
volume  of  oxygen,  thoroughly  stirred  together  and  perfectly 
dried,  is  submitted  to  the  action  of  a  freezing  mixture  of  salt 
and  ice,  transparent,  colorless  crystals  are  condensed  from  the 


HYPONITRIC    ACID.  59 

mixed  gases,  but  if  the  least  trace  of  moisture  has  been  present, 
the  product  will  be  an  almost  colorless  liquid.  The  vapor  of  this 
new  substance  has  a  brownish  red  color ;  from  a  mixture  of  two 
measures  of  nitric  oxide  and  one  measure  of  oxygen,  two  meas- 
ures of  the  new  vapor  are  produced.  Since  two  measures  of 
nitric  oxide  contain  one  measure  of  nitrogen  and  one  of  oxygen, 
the  composition  of  the  new  substance  may  be  represented  by  the 
accompanying  diagram ;  the  molecule  will  be  represented  by 

the  formula  N  O2,  and  the  com- 
position of  the  substance  by 
weight  will  be  14  parts  of  nitro- 
gen to  32  of  oxygen.  The 
name  of  this  new  body  is  Hypo- 
nitric  acid,  a  name  derived  from 
nitric  acid  by  prefixing  the  Greek  vao,  "  below."  The  term  is 
used  to  indicate  that  the  substance  to  which  it  is  applied  contains 
less  oxygen  than  the  other  substance  from  which  the  name  is 
derived.  Thus  hyponitric  acid  contains  less  oxygen  than  nitric 
acid,  hyposulphurous  acid  less  than  sulphurous,  and  so  forth.  It 
remains  to  justify  this  assertion  respecting  the  comparative  oxy- 
gen-contents of  hyponitric  and  nitric  acids. 

Exp.  42.  —  Add  to  the  nitric  acid  which  remains  from  Exp.  32,  pre- 
viously diluted  with  twice  its  bulk  of  water  and  warmed  over  the 
lamp,  finely  powdered  litharge  in  small  portions  so  long  as  it  readily 
dissolves.  The  operation  may  be  best  performed  in  an  evaporating- 
dish,  which  should  be  only  very  moderately  heated.  The  substance 
sold  under  the  name  of  litharge  is  a  simple  combination  of  the  two 
elements,  lead  and  oxygen ;  the  formula  of  its  molecule  is  PbO,  in 
which  Pb  represents  the  least  proportional  weight  of  metallic  lead 
(plumbum)  ;  its  composition  by  weight  is  accurately  known.  When 
the  litharge  no  longer  dissolves  with  promptness,  nb  more  should  be 
added,  and  the  liquid  in  the  dish  should  be  evaporated  to  dryness,  at 
first  on  the  wire-gauze  over  the  lamp,  but  towards  the  end  of  the  oper- 
ation on  a  water-bath. 

During  this  evaporation  there  escape  into  the  air  unchanged,  water 
and  the  excess  of  nitric  acid  which  was  not  neutralized  by  the  oxide 
of  lead.  There  remains  a  white,  saline  substance,  which  has  resulted 
from  the  combination  of  the  oxide  of  lead  with  a  portion  of  the  nitric 
acid ;  it  is  called  nitrate  of  lead.  Experience  has  proved  that  it  is  a 


60  StITROUS    ACID. 

perfectly  dry  substance,  containing  no  water  whatever ;  it  is  the  raw 
material,  so  to  speak,  of  important  experiments  shortly  to  be  described. 
If  dry  nitrate  of  lead  be  heated  strongly  in  a  small  glass  retort,  it  is 
decomposed ;  deep  red  fumes  are  produced,  and  oxide  of  lead  (litharge) 
is  left  behind.  If  these  red  vapors  be  carried  through  a  U-tube  im- 
mersed in  a  mixture  of  ice  and  salt,  a  portion  of  the  vapors*  condense 
into  a  liquid,  while  an  uncondensable  gas  passes  through  the  cold  tube, 
and  may  be  collected  by  a  suitable  arrangement  beyond  the  U-tube. 
The  condensed  liquid  is  hyponitric  acid ;  the  gas  is  oxygen.  If  the 
complete  absence  of  moisture  has  been  secured,  crystals  of  hyponitric 
acid  may  often  be  obtained  by  replacing  the  first  U-tube  by  a  second 
towards  the  end  of  the  distillation.  In  fact,  this  is  by  far  the  most  con- 
venient method  of  preparing  hyponitric  acid. 

The  name  of  hyponitric  acid  is  now  justified,  for  it  is  apparent 
that  when  the  litharge  was  dissolved  in  nitric  acid,  it  found  there, 
and  combined  with,  an  oxide  of  nitrogen  containing  more  oxygen 
than  hyponitric  acid  does,  since  when  this  oxide  of  nitrogen  is 
decomposed  by  heat,  as  in  the  experiment  just  described,  hypo- 
nitric  acid  and  oxygen  are  evolved,  the  litharge  remaining 
behind  unaltered.  Reserving  the  farther  discussion  of  the  com- 
position of  nitric  acid,  we  may  here  speak  of  the  properties  and 
products  of  hyponitric  acid. 

72.  Hyponitric  acid  occurs  in  the  solid,  liquid,  and  gaseous 
states ;  at  — 9°  it  crystallizes,  above  that  temperature  and  below 
22°  it  is  a  mobile  liquid  of  sp.  gr.  1.451,  and  of  Tarious  colors 
at  various  temperatures ;  it  boils  at  22°.  The  liquid  acid  gives 
off  red,  acid,  irrespirable  vapors  at  the  ordinary  temperature. 
If  to  liquid  hyponitric  acid,  cooled  by  ice  and  salt,  a  proportion- 
ally small  quantity  of  ice-water  be  added,  two  layers  of  liquid 
are  formed,  the  upper  and  least-colored  of  which  consists  prin- 
cipally of  nitric  acid,  the  lower  and  darker  of  a  fluid  which 
yields,  on  cautious  distillation  at  a  low  temperature,  a  very  vola- 
tile blue  liquid.  This  blue  liquid  is  so  unstable  that  its  composi- 
tion and  qualities  are  not  certainly  known ;  but  it  is  supposed  to 
be,  or  to  contain,  an  oxide  of  nitrogen  differing  from  all  those 
heretofore  studied,  —  an  oxide  capable  of  direct  derivation  from 
„  nitric  oxide  by  adding  to  any  volume  of  this  last,  one-fourth  that 
volume  of  pure  oxygen,  and  therefore  answering  to  the  formula 
N0O3,  and  being  composed  of  28  parts  by  weight  of  nitrogen 


ANALYSIS    OP   NITRIC    ACID.  61 

and  48  parts  of  oxygen.  This  obscure  substance  is  known  by 
the  name  of  nitrous  acid  ;  though  itself  but  imperfectly  known, 
some  of  its  compounds  are  not  unfamiliar  bodies. 

73.  We  have  already  learned  that  the  white,  saline  substance 
called  nitrate  of  lead  can  be  made  from  oxide  of  lead  and  nitric 
acid ;  that  it  contains  no  water  in  its  composition ;  and  that  when 
heated  it  is  decomposed  into  oxygen,  an  oxide  of  nitrogen,  and 
the  original  oxide  of  lead.  On  the  basis  of  these  facts,  a  method 
has  been  constructed  of  determining  the  composition  of  nitric 
acid  by  weight. 

To  10  grammes  of  oxide  of  lead  add  something  more  than  enough 
nitric  acid  to  transform  it  completely  into  nitrate  of  lead ;  evaporate 
the  excess  of  acid,  dry  the  residual  nitrate  of  lead  completely,  and 
weigh  it.  If  W  represent  this  weight  in  grammes,  W — 10  is  the 
weight  of  the  unknown  oxide  of  nitrogen  which  has  combined  with  10 
grammes  of  oxide  of  lead  to  form  W  grammes  of  nitrate  of  lead.  From 
these  data  the  percentage  composition  of  nitrate  of  lead  can  be  cal- 
culated ;  1 0  grammes  of  pure  nitrate  of  lead  invariably  contain 

Oxide  of  lead,  ....     6.738  grammes. 
Oxide  of  nitrogen,     .     .     3.262        " 

If  now  10  grammes  of  pure  nitrate  of  lead  be  decomposed  by  heat 
under  such  conditions  that  the  vapors  which  it  yields  shall  pass  over 
some  substance  capable  of  abstracting  all  the  oxygen  from  the  vapors 
without  aifecting  the  nitrogen,  it  will  be  possible  to  collect  all  the 
nitrogen  contained  in  3.262  grammes  of  the  unknown  oxide  of  nitro- 
gen, and  so  to  determine  its  volume  and  thence  its  weight. 

This  determination  is  actually  made  by  heating  the  nitrate  of  lead  in 
a  long  glass  tube,  in  such  a  manner  that  all  the  vapors  evolved  from  it 
pass  over  a  large  surface  of  red-hot  copper.  The  copper  absorbs  the 
oxygen,  the  nitrogen  passes  over  it  unaltered  and  is  collected  in  a  suit- 
able vessel  and  accurately  measured.  As  the  specific  gravity  of  nitro- 
gen at  any  given  temperature  and  pressure  is  known  with  precision, 
the  exact  weight  of  the  nitrogen  can  be  deduced  from  this  volume. 
Such  experiments  often  repeated  have  led  to  the  conclusion  that  the 
oxide  of  nitrogen,  which  with  oxide  of  lead  makes  up  nitrate  of  lead, 
contains 

Nitrogen, 25.93  per  cent. 

Oxygen, 74.07        " 

or  by  measure,  as  calculated  from  these  numbers  and  the  specific 


62  COMPOSITION    OF    NITRIC    ACID. 

gravity  of  these  two  permanent  gases,  two  volumes  of  nitrogen  and 
five  volumes  of  oxygen. 

The  molecule  of  this  compound  will  be  represented  by  the  formula 
N2O5,  and  the  weight  of  this  molecule  will  be  108,  of  which  28  parts 
will  be  nitrogen  and  80  oxygen ;  a  composition  precisely  corresponding, 
of  course,  to  the  percentage  composition  above  given.  This  new  oxide 
of  nitrogen  may  be  called  for  the  present  nitric  acid,  though  we  shall 
soon  learn  to  distinguish  between  this  body  and  common  nitric  acid. 

74.  The  combining  proportions  of  nitrogen  and  oxygen  in  this 
oxide  have  been  experimentally  determined  by  weight,  and  not 
by  volume,  as  was  the  case  with  all  the  preceding  oxides-^of 
nitrogen.     The  reason  of  this  different  treatment  is  to  be  found 
in  the  fact  that  nitric  acid  cannot  be  converted  into  vapor  with- 
out suffering  decomposition  ;  not  indeed  into  its   elements,  but 
into  oxygen  and  a  lower  oxide  of  nitrogen.     It  is,  therefore,  in 
the  present  state  of  science,  impossible  to  obtain  a  volume  of 
nitric  acid  gas  capable  of  experimental  resolution  into  nitrogen 
and  oxygen. 

Since  a  large  majority  of  the  elementary  bodies  are  non- 
volatile under  any  treatment  in  our  power  to  employ,  and  since 
the  greater  number  of  chemical  compounds  are  either  non- 
volatile, or  are,  like  nitric  acid,  decomposed  by  a  temperature 
high  enough  to  volatilize  them,  the  proportions  in  which  the 
elements  unite  by  weight  are  of  much  more  general  value  than 
the  proportions  in  which  they  unite  by  volume ;  and  the  methods 
of  determining  the  atomic  weights  of  the  elements  have  been  stud- 
ied with  a  thoroughness,  and  brought  to  a  perfection  commen- 
surate with  the  fundamental  importance  of  these  proportional 
numbers. 

75.  Nitric  acid  completes  the  series  of  oxides  of  nitrogen  ;  no 
higher  oxide  is  known.     We  are  now  prepared  to  exhibit  this 
series  in  a  diagram  which  shall  present  to  the  eye  at  once  the 
volumetric  composition  of  the  oxides,  the  resultant  volumes  after 
the   condensation  of  the  ingredients,  the  atomic  weights  of  the 
elements,  and  the  combining  weights  of  the  compounds.     Since 
the  atomic  weights  of  the  gaseous  elements  are  at  the  same  time 
their  specific  gravities  referred  to  hydrogen,  it  will  be  easy  to 
deduce  the  specific  gravities,  or  equal  volume  weights,  of  the 


OXIDES    OF    NITROGEN. 


63 


compound  gases,  N2O,  NO,  and  ^iOg,  from  their  combining 
weights  by  dividing  these  weights,  by  two.  As  the  resultant 
volumes  after  condensation  are  not  known  for  the  two  members 
of  this  series  N203  and  N2O5,  we  abstain  from  figuring  hypo- 
thetical volumes  which  analogy  may  point  to  as  probable,  but 
which  experiment  has  never  demonstrated  as  fact. 

THE  OXIDES  OF  NITROGEN. 
Nitrous  Oxide.  Nitric  Oxide. 


Hyponitric  Acid. 


NO 

30 

108 


7G.  These  five  bodies  are  all  chemical  compounds ;  they  are 
definite  and  constant  in  composition,  and  all  differ  essentially 
from  their  elementary  constituents  and  from  each  other,  as  the 
experiments  we  have  performed  with  them  have  abundantly 
demonstrated.  It  is,  therefore,  obvious  that  two  of  the  elements 
are  capable  of  combining  in  several  proportions  to  form  definite 
chemical  compounds  ;  and  what  is  here  proved  of  two  of  the 
elements  we  shall  hereafter  find  to  be  true  of  all,  though  not  of 


64  LAW    OP    MULTIPLE    PROPORTIONS. 

every  couple ;  so  that  the  series  of  oxides  of  nitrogen  is  but  one 
illustration  of  a  most  comprehensive  law.  The  difference  be- 
tween a  mechanical  mixture  and  a  chemical  compound  does  not 
on  this  account  become  less  marked.  The  possible  mixtures  of 
nitrogen  with  oxygen  are  innumerable  ;  the  known  combinations 
of  these  two  elements  "are  only  five;  two  volumes  of  nitrogen 
combining  chemically  either  with  one,  two,  three,  four,  or  five 
volumes  of  oxygen,  and  with  no  other  proportions  whatsoever. 
As  for  volumes,  so  for  weights ;  the  proportional  weight  of  oxy- 
gen in  these  oxides  rises  by  definite  leaps  from  the  first  member 
of  the  series  to  the  last. 

This  definite,  step  by  step  mode  of  forming  chemical  com- 
pounds is  one  of  the  most  characteristic,  as  it  is  one  of  the  most 
general  facts  of  chemistry ;  no  other  science  offers  a  parallel  to 
it,  but  long  experience  and  patient  labor  with  the  balance  and 
measuring-glass  have  established  it  as  the  habitual  mode  in  which 
the  force  called  chemical  ordinarily  acts.  The  abstract  results 
of  observation  and  experiment  may  be  expressed  in  the  following 
proposition,  often  called  the  Law  of  Multiple  Proportions  :  If 
two  bodies  combine  in  more  than  one  proportion,  the  ratios  in  which 
they  combine  in  the  second,  third,  and  subsequent  compounds  are 
definite  multiples  of  those  in  which  they  combine  to  form  the  first. 

While  the  mode  of  action  of  the  chemical  force  set  forth  in 
this  proposition  is  that  which  has  long  been  uppermost  in  the 
minds  of  chemists,  most  prominent  in  chemical  treatises,  and 
perhaps  most  important  to  the  progress  of  the  science  in  the 
direction  in  which  it  has  thus  far  been  cultivated,  it  should  be 
remembered  that  in  the  phenomena  of  solution,  in  the  formation 
of  metallic  alloys  by  fusion,  and  in  the  crystallization  of  minerals 
and  other  substances  with  constant  forms  but  variable  composi- 
tion, the  chemical  force  has  a  part  to  play  which,  if  more  obscure 
than  its  ordinary  manifestations,  is  not  le^s  real. 

77.  Air  a  mixture.  —  All  that  has  been  said  of  the  distinc- 
tion between  a  mechanical  mixture  and  a  chemical  combination 
finds  perfect  illustration  in  the  differences  between  air  and  the 
definite  oxides  of  nitrogen  which  have  just  been  studied.  Air  is 
not,  like  these,  a  chemical  combination.  The  evidence  that  it  is 
a  mechanical  mixture  merely  may  here  be  appropriately  presented 


AIR    A    MIXTURE.  bO 

The  statement  that  air  contains  1  volume  of  oxygen  to  4  vol- 
umes of  nitrogen  is  not  absolutely  true.  These  proportions  are 
never  actually  found  ;  the  gases  are  not  combined  in  any  simple 
ratio,  either  by  volume  or  weight ;  they  are  always  mixed  in  the 
proportion  of  20.81  measures  of  oxygen  to  79.19  measures  of 
nitrogen,  or  23.10  parts  by  weight  of  oxygen  to  76.90  parts  of 
nitrogen.  The  experimental  processes  by  which  these  numbers 
have  been  fixed  are  so  perfect,  that  it  is  impossible  to  entertain 
the  idea  that  the  gases  are  really  mixed  in  the  ratio  of  20  meas- 
ures to  80,  or  1  volume  to  4  volumes,  or  in  the  proportion  of  20 
parts  by  weight  of  oxygen  to  70  of  nitrogen,  as  the  formula 
N4O  would  require.  When  20.81  parts  of  oxygen  are  mixed 
with  79.19  of  nitrogen,  there  is  no  development  either  of  light, 
heat,  or  electricity,  such  as  usually  attends  the  formation  of  a 
chemical  compound ;  and  the  specific  gravity,  magnetism,  and 
refractive  power  of  the  mixture  are  such  as  calculation  would 
directly  deduce  from  the  numbers  expressing  these  properties  for 
the  two  constituents. 

We  have  seen  that  when  nitric  oxide  is  added  to  nitrous  oxide 
no  red  fumes  are  produced  (§  70),  but  that  when  the  nitric  oxide 
is  brought  in  contact  with  air  these  suffocating  fumes  are  abun- 
dantly formed,  though  the  air  contains  only  half  as  much  oxygen 
as  the  nitrous  oxide.  These  experiments  go  to  show  that  while  in 
nitrous  oxide  the  oxygen  is  held  in  chemical  combination,  in  air  it 
is  free. 

Strong  positive  evidence  that  air  is  a  mere  mixture  is  afforded 
by  its  behavior  towards  water.  All  gases  are  soluble  in  water 
to  a  greater  or  less  extent,  each  one  dissolving  in  a  certain  fixed 
and  definite  proportion  at  any  given  temperature  and  pressure. 
Thus  at  15°  and  a  pressure  of  76  c.  m.  of  mercury,  1  volume  of 
water  dissolves  0.0193  volumes  of  hydrogen  and  0.7778  volumes 
of  nitrous  oxide.  As  a  general  rule,  the  pressure  to  which  the 
liquid  is  exposed  being  constant,  the  quantity  of  gas  dissolved  by 
water  is  less  in  proportion  as  the  temperature  of  the  water  is 
high  ;  in  many  cases  boiling  water  is  altogether  incapable  of  re- 
taining gases  in  solution,  particularly  those  which  are  only  spar- 
ingly soluble  at  any  temperature. 

Hence,  by  prolonged  boiling,  many  gases  can  be  completely 
5 


66  AIR    A    MIXTURE. 

expelled  from  the  water  which  held  them  in  solution.  For  ex- 
ample, if  a  solution  of  nitrous  oxide  be  boiled,  and  the  gas  be 
collected  as  it  escapes,  this  gas  will  be  found  to  exhibit  the  char- 
acteristic properties  of  nitrous  oxide.  The  nitrous  oxide  has  not 
been  altered  by  the  act  of  solution.  It  still  remains  a  definite 
chemical  compound  as  before.  But  a  result  very  different 
from  this  is  obtained  upon  boiling  water  which  has  become 
charged  with  the  ingredients  of  atmospheric  air.  The  gas 
collected  in  this  case  is  composed  of  oxygen  and  nitrogen,  it  is 
true,  but  not  in  those  proportions  in  which  the  elements  are 
united  in  air. 

Water  does  not  dissolve  air  directly  as  such,  as  it  should  do 
were  air  a  chemical  compound ;  but  it  dissolves  out  from  it  a 
quantity  of  oxygen,  just  as  if  no  nitrogen  were  present ;  at  the 
same  time  it  dissolves  nitrogen  in  accordance  with  the  solubility 
of  this  element,  and  to  precisely  the  same  extent  that  it  would 
absorb  it  if  there  were  no  oxygen  in  the  air.  Oxygen  is  dis- 
solved by  water  in  larger  proportion  than  nitrogen,  —  1  volume 
of  water  at  15°  and  under  a  pressure  of  76  c.  m.  of  mercury 
dissolves  0.02989  volumes  of  oxygen,  but  only  0.0148  volumes 
of  nitrogen. 

Exp.  43. —  By  means  of  a  sound  perforated  cork  or  caoutchouc 
stopper,  adapt  to  a  flask  of  the  capacity  of  1  or  2  litres  a  gas-delivery 
tube,  No.  6,  long  enough  to  reach  to  the  water-pan  in  the  usual  way. 
Upon  the  outer  end  of  the  delivery-tube  tie  a  short  piece  of  caoutchouc 
tubing,  to  which  a  stopper,  made  of  a  bit  of  glass  rod  or  a  wooden  plug, 
has  been  fitted.  Fill  the  flask  completely  with  ordinary  well  or  river 
water;  fill  also  the  delivery-tube  with  water,  and  close  it  by  putting 
the  stopper  in  (the  caoutchouc  tube.  Carefully  place  the  cork  of  the 
delivery-tube  in  the  neck  of  the  flask  in  such  manner  that  no  air  shall 
be  entangled  by  the  cork  ;  at  the  same  moment  remove  the  plug  from 
the  delivery-tube,  and  finally  press  the  cork  firmly  into  the  flask.  Both 
flask  and  tube  will  now  be  completely  full  of  water.  Place  the  dried 
flask  upon  a  ring  of  the  iron  stand,  and  invert  a  bottle  filled  with 
water  over  the  end  of  the  deli  very -tube.  Now  slowly  bring  the  con- 
tents of  the  flask  to  boiling. 

As  the  water  gradually  becomes  warm,  numerous  little  bubbles  of 
gas  will  be  seen  to  separate  from  the  liquid  and  to  collect  upon  the 
Asides  of  the  flask  ;  these  subsequently  coalesce  to  larger  bubbles,  which 


NITRIC    ACID.  67 

collect  in  tlie*neck  of  the  flask.  As  soon  as  the  water  actually  boils, 
the  steam  will  force  this  air  out  of  the  flask,  and  it  will  collect  in  the 
inverted  bottle  at  the  end  of  the  delivery-tube,  the  steam  being  mean- 
while condensed  as  fast  as  it  comes  in  contact  with  the  cold  water  in 
the  pan.  By  continuing  to  boil  moderately  during  ten  or/fifteen  min- 
utes, nearly  all  the  air  can  be  swept  out  from  the  flask  by  means  of 
the  escaping  steam.  The  gas  delivery-tube  may  then  be  lifted  from  the 
water-pan  and  the  lamp  extinguished. 

From  a  litre  of  ordinary  water  about  50  c.  c.  of  gas  can 
usually  be  obtained.  This  contains,  of  course,  besides  oxygen 
and  nitrogen,  a  certain  amount  of  carbonic  acid ;  but  careful 
analyses  have  shown  that  it  is  much  richer  in  oxygen  than  ordi- 
nary air,  the  proportion  of  oxygen  to  nitrogen  in  the  gas  from 
water  being  as  32  to  68  in  100  volumes,  instead  of  20.81  to 
79.19  as  in  air. 

78.  Nitric  acid.  The  nitric  acid  above  referred  to,  whose 
composition  is  represented  by  the  formula  N2O5,  is  an  unstable 
solid  which  melts  at  30°  ;  the  liquid  produced  boils  at  47°  and  is 
decomposed  at  80°.  The  crystals  of  this  substance  are  trans- 
parent and  colorless  ;  they  undergo  spontaneous  decomposition 
into  hyponitric  acid  and  oxygen,  even  when  preserved  in  closed 
tubes.  It  is  obvious  that  this  is  not  the  common  nitric  acid  with 
which  we  are  already  familiar.  It  remains  to  demonstrate  that 
commercial  nitric  acid  contains  water  and  the  oxide  of  nitrogen 
N2O5.  A  single  experiment  may  be  made  use  of  to  demonstrate 
the  presence  of  water  in  common  nitric  acid,  and  at  the  same 
time  to  determine  the  proportion  in  which  it  enters  into  the  com- 
position of  the  sample  of  acid  examined. 

By  adding  to  a  known  weight  of  common  nitric  acid,  10  grammes 
for  example,  a  weighed  quantity  of  the  oxide  of  lead  much  larger  than 
the  acid  is  able  to  dissolve,  —  100  grammes  for  instance, —  and  then 
heating  the  mixture,  with  suitable  precautions  against  over-heating,  in 
a  weighed  flask,  the  vapor  of  water,  and  nothing  else,  is  given  off; 
this  vapor  may  of  course  be  condensed,  and  proved  to  be  common 
water.  Since  the  oxide  of  lead  contains  no  water,  and  the  nitrate  of 
lead  is  also  anhydrous,  it  follows  that  whatever  water  escapes  from  the 
flask  during  the  heating  must  be  derived  from  the  10  grammes  of  nitric 
acid  ;  and  further,  that  if  the  heating  be  long  enough  maintained,  all 
the  water  which  the  acid  contained  will  be  expelled.  By  weighing  the 


68  HYDRATED    NITRIC    ACID. 

flask  and  its  contents  after  all  water  has  been  thus  driven  out,  the  .loss 
of  weight  will  be  the  quantity  of  water  contained  in  the  10  grammes  of 
acid. 

It  has  been  already  proved  (Exp.  42)  that  the  nitrate  of  lead, 
which,  with  a  quantity  of  unused  litharge,  constitutes  the 
residue  in  the  tiask,  yields  the  oxide  of  nitrogen  N.>O5.  If 
this  oxide  of  nitrogen  be  called  nitric  acid,  then  is  the  com- 
mon acid  hydrated  nitric  acid ;  but  if  the  shorter  name,  nitric 
acid,  be  applied  to  the  commoner  substance,  the  commercial 
acid,  then  the  oxide  X2O5  must  be  distinguished  as  anhydrous 
nitric  acid. 

79.  Anhydrous  nitric  acid  unites  with  water  in  at  least  two 
definite  proportions ;  its  molecule  combines  with  one  molecule  of 
water,  or  \viihfour  to  form  the  two  hydrates  represented  by  the 
formula?  H3O,  N2O5,  and  4  H2O,  X.jO5  respectively,  wherein  the 
symbols  of  one  and  four  molecules  of  water  are  simply  placed 
beside  the  formula  of  the  molecule  of  anhydrous  nitric  acid. 
Ponderal  analysis  has  given  us  this  knowledge  of  the  compo- 
sition of  these  two  hydrates,  by  actually  weighing  the  propor- 
tion of  water  combined  with  the  oxide  of  nitrogen  in  the  two 
cases. 

When  pure  nitric  acid  is  spoken  of,  the  acid  containing  one 
combining  proportion  of  water  to  one  of  the  oxide  of  nitrogen  is 
generally  referred  to.  This  acid  is  often  called  the  monohydrated 
acid,  an  adjective  which  may  be  applied  to  any  substance  which 
is  coupled  with  a  single  molecule  of  water.  The  monohydrated 
acid  is  a  colorless,  transparent,  mobile  liquid  of  specific  gravity 
1.52,  which  boils  at  86°  and  freezes  at  about  — 50°.  Light 
slowly  decomposes  it,  and  a  very  moderate  heat  resolves  it,  not 
indeed  into  its  elements,  but  into  less  complex  compounds.  It 
exerts  a  highly  corrosive  action  on  organic  bodies,  and  stains 
tissues  containing  nitrogen  of  a  bright  orange  color.  It  absorbs 
water  from  the  air.  When  mixed  with  water,  heat  is  developed 
from  the  mixture,  and  the  second  definite  hydrate  is  formed 
4ELO,  N2O57  —  a  colorless,  strongly  acid  liquid,  having  a  specific 
gravity  of  1.42,  and  containing  60  per  cent,  of  anhydrous  nitric 
acid.  To  this  last  hydrate  all  weaker  and  stronger  acid-  are 
alike  converted  by  boiling.  The  nitric  acid  of  commerce  has  a 


1-l.S    OF    NITRIC    ACID.  69 

specific  gravity  of  either  1.40   or  1.42,  and  therefore   contains 
either  56  or  60  per  cent,  of  anhydrous  nitric  acid. 

80.  Nitric  acid,  especially  when  hot,  gives  tip  a  part  of  its 
oxygen  with  great  facility  to  substances  capable  of  combining 
with  oxygen.     When  concentrated  it  acts  with  more  energy  than 
when  diluted  with  water,  and  when  mixed  with  strong  sulphuric 
acid   (a  substance  which  tends  to  take  water  from  other  com- 
pounds) it  becomes  an  oxidizing  agent  of  intense  power.     We 
have  seen  liquid  nitric  acid  yield  a  part  of  its  oxygen  to  copper 
with   evolution  of  nitric  oxide,  a  lower  member  of  the  series 
(Exp.  37),  and  we  have  also  learned  that  the  vapor  of  anhydrous 
nitric  acid  will  give  all  its  oxygen  to  red-hot  copper,  nitrogen 
being  set  free  (§  73).    Most  of  the  metals  are  dissolved  by  nitric 
acid,  with  evolution  of  one  or  other  of  the  lower  oxides  of  nitro- 
gen ;  and  sulphur,  phosphorus,  arsenic,  carbon,  and  many  other 
Ic--;  familiar  elements  are  converted  by  it  into  oxides.     Organic 
substances  are  oxidized  by  nitric  acid  to  very  various  degrees 
and  with  very  various  products,  according  to  the  strength  and 
temperature  of  the  acid  employed. 

The  industrial  uses  of  nitric  acid  depend  upon  these  modes  of 
action.  Alone,  or  mixed  with  muriatic  acid,  it  is  the  commonest 
solvent  of  metals,  and  the  manufactures  of  aniline  dyes  and  gun- 
cotton  are  direct  applications  of  its  oxidizing  power. 

81.  The  atomic  weight  of  oxygen,  or  the  weight  of  the  least 
proportional  quantity  of  oxygen  which  enters  into  combination, 
is  the  same  when  it  unites  with  nitrogen  as  with  hydrogen.    It  is 
a  gt-neral  fact,  that  each  element   has   but   one  least  combining 
weight  with  each  and  all  of  the  other  elements.     The  atomic  hy- 
pothesis  is  based  upon  this  important  fact.     This  hypothesis  at- 
tributes to  the  imagined  atom  of  each  element  a  constant  propor- 
tional weight,  expressed  by  the  >ame  number  which  experiment 
proves  to  be  the  combining  proportion  by  weight  of  the  element 
taken  in  tinite,  ponderable  quantities.     In  this  sense  the  atom  of 
oxygen  is  said  to  be  1 6  times  as  heavy  as  the  atom  of  hydrogen,  and 
the  atom  of  nitrogen  14  times  as  heavy  as  the  atom  of  hydrogen. 
The  combining  weight  of  a  chemical  compound  is  always  equal 
to  the  sum  of  the  atomic  weights  of  the  elementary  atoms  con- 
tained in  its  molecule.     Thus  the  combining  weight  of  water, 


70  COMBINING    WEIGHTS. 

H2O,  is  18  =  2  -f  16  ;  of  anhydrous  nitric  acid,  N2O5,  is  108 
=  2  X  14  +  5  X  16;  of  monohydrated  nitric  acid,  H2O,  N2O5, 
is  126  =  2  -j-  2  X  14  +  6  X  16. 

It  must  never  be  forgotten  that  these  combining  weights  are 
not  absolute  weights,  but  simply  express  the  proportions  by 
weight  in  which  the  elements  and  their  compounds  invariably 
unite.  The  combining  weight  of  a  compound  is  directly  deduced 
from  the  composition  of  the  molecule,  or  least  quantity  of  the 
compound  which  can  exist  by  itself  uncombined,  or  take  part  in 
any  chemical  process.  To  correctly  determine  this  least  propor- 
tional quantity  often  requires  a  large  accumulation  of  facts,  and 
a  just  collation  of  these  facts,  such  as  is  possible  only  when  a 
wide  experience  is  added  to  a  keen  insight  into  the  principles  of 
chemical  philosophy. 

The  discussion  of  the  true  molecular  formulae  of  chemical 
compounds  presents  difficulties  which  render  it  entirely  unsuita- 
ble for  our  present  stage  of  progress  ;  nitric  acid,  however,  will 
enable  us  to  illustrate  one  of  the  difficulties  appertaining  to  the 
subject.  When  nitric  acid  dissolves  oxide  of  lead  (Exp.  42)  the 
reaction  which  occurs  may  be  thus  symbolized  : 

PbO    +      H20,N205    =     PbO,N205     +     H20 
Litharge.         Nitric  Acid.      Nitrate  of  Lead.          Water. 

The  resulting  molecular  formulae  of  nitrate  of  lead  is  not  divisible 
by  any  number  but  unity,  and  therefore  cannot  be  made  simpler 
and  still  express  the  same  proportional  combination  of  its  ele- 
ments. 

If  instead  of  the  oxide  of  lead  we  employ  the  oxide  of  silver, 
we  shall  find  iit  possible  to  express  the  resulting  molecule  of 
nitrate  of  silver  by  two  formulae,  either  of  which  will  represent 
correctly  the  proportions  by  weight  in  which  the  elements  have 
combined :  — 

Ag2,O     +      H2ON2O5    =     Ag2O,  N2O5    +     H2O ;  or 
Oxide  of  Silver.  Nitric  Acid.     Nitrate  of  Silver.      Water. 
Ag20     +     H20,N205     =      2AgN03      +     H2O. 

The  use  of  the  common  algebraic  signs  in  these  formulae  requires 
no  explanation ;  the  sign  of  equality  denotes  the  equality  of  the 
sum  of  the  atomic  weights  on  either  side  of  it ;  a  numeral  on 


COMBINING   WEIGHT    OF    NITRIC    ACID.  71 

the  left  of  a  group  of  symbols,  is  intended  to  multiply  the  whole 
group,  unless  a  comma  divides  the  group,  in  which  case  the  nu- 
meral multiplies  that  part  of  the  group  on  the  left  of  the  comma ; 
brackets  are  sometimes  used,  as  in  algebra,  to  mark  the  extent  of 
the  multiplication.  The  formula  of  nitric  acid  itself  admits  of 
simpler  expression : 

H20,  N20,:  =  H2N206  =  2HN03 

The  first  formula  reminds  us  that  the  nitric  acid,  of  which  we 
speak,  may,  by  indirect  means,  be  made  to  yield  anhydrous  nitric 
acid  and  water  ;  from  the  second  we  easily  learn  the  proportions 
in  which  the  three  elements  are  united  by  weight,  but  the  third, 
HN03,  expresses  these  same  proportions  with  precision,  and  is 
the  most  concise  of  the  three.  Now,  although  the  two  formula 
H2N2O6  and  HNO3  express  precisely  the  same  compound  of  the 
same  three  elements  in  the  same  fixed  proportions  by  weight,  the 
combining  weight  of  nitric  acid  is  63,  if  the  last  formula  be  cor- 
rect, and  126  if  the  first  represents  the  real  molecule  of  the  acid. 
In  the  great  majority  of  chemical  processes  in  which  nitric  acid 
is  involved,  that  proportional  weight  of  nitric  acid  is  necessary 
which  is  implied  by  the  molecular  formula  H2N2O(5 ,  but  there 
are  not  a  few  cases  in  which  the  proportional  weight  represented 
by  the  simpler  formula  HNO3  completely  accomplishes  the  actual 
reaction,  and  is  capable  of  representation  in  the  algebraic  form. 
To  illustrate  the  first  class  of  cases  we  have  the  formula,  just 
given,  of  the  reaction  which  occurs  when  oxide  of  lead  is  dis- 
solved in  nitric  acid  ;  a  less  proportional  weight  of  nitric  acid 
than  H2N2O6  or  2HN03  will  not  answer  the  conditions  of  the  re- 
action. Let  us  draw  from  the  preceding  sections  some  further 
illustrations  of  this  class  of  nitric  acid  reactions.  When  copper 
(whose  symbol  is  Cu,  from  cuprum)  is  used  to  set  free  nitric 
oxide  from  nitric  acid  (see  Exp.  37),  the  reaction  is  symbolized 
as  follows :  — 

3Cu     +     4H2N2Ofl    —     2NO     +      3CuN206    +      4H2O. 
Copper.       Nitric  acid.    Nitric  oxide.  Nitrate  of  Copper.    Water. 

When  nitrate  of  lead  is  decomposed  by  heat  into  litharge,  hypo- 
nitric  acid,  and  oxygen,  the  following  equation  represents  the 
chemical  change : 


72  NITRIC    ACID    REACTIONS. 


PbN2OG    —     PbO     +     2NO2     +     O. 

In  fhese  two  reactions  the  nitrates  of  copper  and  lead  contain 
two  combining  weights  of  nitrogen  and  six  of  oxygen  for  each 
one  of  the  metal. 

To  illustrate  the  second  class  of  nitric  acid  reactions,  we  have 
only  to  explain  more  fully  the  nature  of  some  of  the  substances 
with  which  we  have  already  experimented.  As  the  nitrate  of 
lead  may  be  made  by  bringing  together  oxide  of  lead  and  nitric 
acid,  water  being  eliminated,  so  the  nitrates  of  potassium  and 
sodium,  from  which  we  originally  prepared  nitric  acid  (see  Exp. 
32),  may  be  formed  by  an  analogous,  though  not  identical,  reac- 
tion. The  composition  of  common  caustic  potash,  or  caustic 
soda,  may  be  expressed  in  two  ways,  —  some  chemists  represent 
these  substances  as  consisting  of  oxide  of  potassium,  or  sodium, 
and  water,  and  therefore  prefer  the  formula  K20,H2O,  while 
other  chemists  divide  this  molecular  formula  by  two,  and  repre- 
sent caustic  potash  by  the  briefer  formula  KHO,  and  caustic 
soda  by  the  corresponding  symbol  NaHO.  In  these  formula?  K 
stands  for  Kalium,  the  Latin  name  of  potassium,  and  Na  for  Na- 
trium, the  Latin  name  of  sodium.  If  we  adopt  for  the  moment 
these  shorter  formula,  which  of  course  express  precisely  the 
same  proportional  composition  by  weight  as  the  longer,  the  reac- 
tion by  which  nitrate  of  potassium,  or  sodium,  may  be  pre- 
pared will  be  written  as  follows  :  — 

KHO       +        HN03  KN03        +         H2O 

Caustic  potash.     Nitric  acid.  Nitrate  of  Potassium.        Water. 

NaHO       +       HNO3      =        NaNO,        +         H2O 
Caustic  soda.         Nitric  acid.       Nitrate  of  sodium.          Water. 
From  this  nitrate  of  potassium,  or  sodium,  nitric  acid  is  pre- 
pared (see   Exp.  32)   by  treating  it  with  sulphuric  acid,  a  sub- 
stance whose  composition  by  weight,  as  we  shall  hereafter  learn, 
may  be  correctly  expressed  by  the  formula  H2S04.     This  reac- 
tion may  be  thus  symbolized  :  — 

KXO8      +       H2SO4      =      KHSO4      +      HNO3 
Nitrate  of  potas.  Sulpk.  acid.  Acid  sulph.  of  potas.  Nitric  acid. 

By  substituting  Na  for  K  the  reaction  with  nitrate  of  sodium 
would  be  represented.  It  thus  appears  that  the  molecule  of 


EMPIRICAL    AND    RATIONAL    FORMULA.  73 

nitric  acid,  which  will  represent  in  the  simplest  way  its  reactions 
with  caustic  potash,  will  not  represent  at  all  its  reactions  with  oxide 
of  lead,  unless  two  molecules  are  assumed  to  enter  into  every 
reaction  with  this  latter  substance.  The  formula  H2N206  is  the 
more  comprehensive,  because  nitrate  of  potassium  can  be  repre- 
sented by  the  formula  K2N206  as  accurately,  if  not  as  simply,  as 
by  the  formula  KNO3 . 

It  is  noteworthy  that  in  the  reactions  above  formulated,  one 
atom,  or  combining  proportional  weight  of  potassium,  or  sodium, 
changes  place  with  one  atom  of  hydrogen,  while  one  atom  of 
lead  or  copper  replaces  two  atoms  of  hydrogen.  These  differ- 
ent capacities  of  the  other  elements  to  replace  hydrogen  are  of 
great  importance  in  chemical  philosophy,  and  will  be  more  fully 
treated  of  hereafter. 

82.  A  formula  which  simply  represents  the  number  of  atoms 
of  each  element  in  one  molecule  of  any  substance,  as  determined 
by  its  analysis,  is  called  an  empirical  formula.  The  truth  of  such  a 
formula  depends  solely  upon  the  correct  performance  of  the  ana- 
lytical process,  and  upon  the  accuracy  with  which  the  atomic 
weights  have  been  determined.  Concerning  such  formulas,  there 
is  little  room  for  difference  of  opinion ;  they  express  all  that  we 
actually  know  of  the  elementary  composition  of  any  compound 
body.  But  chemists  have  endeavored  to  contrive  formulas  which 
should  express  something  more  than  the  mere  elementary  composi- 
tion by  weight ;  which  should  recall  the  materials  from  which  the 
formulated  substance  was  made,  and  prophesy  the  products  of 
its  decomposition ;  which  should  not  only  name  and  number  the 
atoms  of  the  substance,  but  should  also  suggest  such  a  grouping  or 
arrangement  of  those  atoms  as  might  serve  to  interpret  its 
known  reactions.  Such  formulae  are  called  rational  formulas. 
Thus  NaHO  is  the  empirical  formula  of  caustic  soda,  while 
Na2O,H2O  is  a  rational  formula  of  the  same  substance,  which 
recalls  the  facts  that  it  may  be  made  from  anhydrous  oxide  of 
sodium  and  water,  and  that  it  enters  into  many  reactions  in 
which  the  Na2O  goes  one  way  and  the  H2O  another. 

Another  rational  formula  of  the  same  substance  is  ^  >•  0, 
which  suggests  many  reactions  in  which  caustic  soda  is  involved 


74  DUAL1STIC    FORMULAE. 

either  as  product  or  ingredient  ;  for  example,  the  decomposition 
of  water  by  metallic  sodium  (see  Exp.  14),  which  may  be  thus 
written  :  — 


We  shall  hereafter  meet  with  many  such  reactions  in  which  an 
atom  of  sodium  replaces  an  atom  "of  hydrogen  ;    the   formula 


Na  ) 
H    f 


suggests  this  large  class  of  reactions  by  implying  that 

caustic  soda  is  itself  constituted  as  water  in  which  an  atom  of 
sodium  has  replaced  an  atom  of  hydrogen.  Such  formulae  as 
H2O,N2O5,  PbO,N2O5,  Na2O,N2O5,  andNa2O,H2O,  are  called 
dualistic,  because  they  represent  these  bodies  as  of  a  dual 
nature,  —  as  being  made  up  of  two  oxides  which  were  distinct 
before  they  were  brought  together  to  form  the  compound, 
and  will  be  distinct  when  separately  extracted  from  it  ;  in 
a  dualistic,  formula  these  two  distinct  parts  are  convention- 
ally represented  as  having  some  separate  existence  within 
the  compound  itself.  The  supposition  is  not  unnatural  ;  thus 
for  example,  common  plaster  of  Paris  is  a  substance  con- 
taining the  metal  calcium  and  the  elements  sulphur  and  oxygen 
in  the  proportions  by  weight  which  are  correctly  expressed  by 
the  formula  CaSO4  ;  but  this  substance  may  be  made  by  methods 
which  suggest  another  formula.  If  we  put  together  quicklime 
CaO,  and  anhydrous  sulphuric  acid  SO3  in  due  proportions,  under 
suitable  conditions,  plaster  of  Paris,  or,  as  its  chemical  name  is, 
sulphate  of  calcium,  results  :  — 

CaO  +  SO3=CaO,SO3; 

or  if  we  mix  slaked  lime  CaO,H2O  with  strong  sulphuric  acid 
H2O,SO3,  in  proper  proportions,  at  a  suitable  temperature,  we 
shall  again  obtain  sulphate  of  calcium,  and  water  will  be  elim- 
inated :  — 

CaO,H2O  +  H2O,SO3  =  CaO,SO3  +  2  H20. 
Accordingly  we  find  that  the  great  majority  of  chemists  have 
hitherto  written  the  formulae  of  sulphate  of  calcium,  hydrated 
oxide  of  calcium,  and  hydrated  sulphuric  acid,  in  conformity  with 
the  suggestion  of  these  reactions,  CaO,SO3,  CaO,H2O,  and 
H2O,S03  respectively.  Of  positive  knowledge  concerning  the  ac- 


TYPICAL    FORMULAE.  75 

tual  grouping  of  the  imaginary  atoms  which  are  supposed  to  make 
up  the  hypothetical  molecule  of  a  compound  body,  we  have  ab- 
solutely none,  and  it  must  never  be  forgotten  that  a  rational  for- 
mula is  merely  a  suggestion  of  some  of  the  chemical  processes 
in  which  the  substance  formulated  is  capable  of  taking  part.  In 
some  cases  the  rational  formula  may  point  to  the  majority  of  all 
known  transformations  of  the  substance,  but  generally  it  suggests 
only  a  few  of  the  possible  changes.  Since  a  rational  formula 
never  represents  a  fact,  but  only  an  hypothesis  or  opinion,  it  is  to 
be  expected  that  a  great  diversity  of  rational  formulas  should  be 
in  use  among  chemists,  and  this  is  really  the  case. 

The  dualistic  view,  above  illustrated,  has  long  been,  and  still 
is,  the  prevailing  view  of  the  proximate  composition  of  inorganic 
compounds  ;  but  in  the  chemistry  of  the  very  numerous  com- 
pounds which  \he  element  carbon  forms  with  oxygen,  hydrogen, 
and  nitrogen,  a  different  view,  called  the  doctrine  of  types,  widely 
obtains,  and  has  been  adopted  by  not  a  few  chemists  as  affording 
the  best  theoretical  representation  of  all  chemical  combinations, 
whether  in  the  inorganic  or  organic  kingdoms.  According  to 
this  doctrine  every  possible  chemical  combination  may  be  imag- 
ined to  be  built  upon  the  plan,  or  framed  upon  the  type  or  model, 
of  some  one  of  the  four  substances,  chlorhydric  acid,  water,  am- 
monia, and  marsh-gas.  These  will  all  shortly  be  to  us  well-known 
substances,  but  the  most  important  of  these  types  is  water,  a 
body  with  whose  composition  we  are  already  familiar  ;  reserving 
for  a  subsequent  section  the  full  exposition  of  this  fruitful  theo- 
retical conception,  let  us  write  upon  the  type  of  water  the  for- 
mulae of  several  substances  with  which  we  have  already  dealt, 
and  which  are  classified  under  this  type  :  — 

TT  ")  TT   ~) 

Water  =  TJ  [•  0  =  one  molecule  ;  T  2  [•  O2  —  two  molecules. 

Caustic  Soda  =         I  O  =  one  molecule. 


Hydrated  Oxide  of  Calcium,  slaked  lime,  =  j  V*  [•  O2  =  1  molecule. 
Monohydrated  Nitric  acid  =   TJ     j-  O  =  one  molecule. 
Nitrate  of  Potassium    T^  2  !•  O  =  one  molecule. 


76  RATIONAL    FORMULAE. 

2  (NO  ^ ) 
Nitrate  of  Lead  =     Vp^  *'  >•  O2  =  one  molecule. 

SO  ) 
Sulphuric  acid  =    „*  r  O2  =  one  molecule. 

SO  ) 
Sulphate  of  Calcium   -,   2  >•  02  =  one  molecule. 

In  these  formulae  it  is  to  be  observed  that  K,  Na,  and  NO2  re- 
place one  atom  of  hydrogen  in  one  molecule  of  water,  while  Ca, 
Pb,  and  SO2  replace  two  atoms  of  hydrogen  in  two  molecules  of 
water.  Facts  of  this  class  will  accumulate  as  we  advance,  and 
will  be  the  subject  of  future  discussion.  The  typical  notation  is 
doubtless  capable  of  expressing,  in  a  logical  and  consistent  system, 
the  greater  part  of  the  reactions  of  inorganic  as  well  as  of  organic 
chemistry,  but  at  present  it  finds  its  best  application  in  the  chemis- 
try of  the  compounds  of  carbon,  and  has  gained  but  little  foot-hold 
in  the  great  departments  of  mineral  and  industrial  chemistry. 

The  need  of  rational  formulae  is  much  more  urgently  felt  in 
that  department  of  chemistry,  called  organic,  which  treats 
of  tfye  chemistry  of  carbon,  than  in  the  wider  field  of  mineral 
and  inorganic  chemistry.  Among  the  very  numerous  compounds 
of  carbon  there  are  many  cases  in  which  one  empirical  formula 
represents  not  one  compound,  but  several ;  hence  it  becomes  of 
consequence  to  determine,  or  to  guess,  how  the  atoms  of  a  com- 
pound are  arranged,  as  well  as  to  know  what  and  how  many  the 
atoms  are.  The  diversity  of  opinion  concerning  this  arrange- 
ment of  atoms  is  so  great,  and  the  possible  modes  of  grouping 
the  numerous  atoms  which  often  enter  into  organic  compounds 
are  so  many,  that  the  number  of  rational  formulae  proposed  for 
any  organic 'substance  is  commonly  large  in  proportion  to  the 
thoroughness  with  which  the  substance  has  been  studied.  For 
acetic  acid,  for  example,  one  of  the  best  known  of  the  compounds 
of  carbon  with  oxygen  and  hydrogen,  no  less  than  nineteen  dif- 
ferent rational  formulae  have  been  proposed. 

Remembering  that  a  rational  formula  is  never  to  be  regarded 
as  the  expression  of  an  absolute  truth,  but  only  as  a  guide  in 
classification,  an  aid  to  the  memory,  and  a  help  in  instruction, 
and  holding  fast  to  the  empirical  formula  as  containing  all  the 
results  of  actual  observation  and  experiment,  we  shall  endeavor 


USES    OF    FORMULAE.  77 

to  familiarize  the  student  with  both  the  dualistic  and  typical 
guesses  at  the  hidden  mysteries  of  chemical  processes  and  the 
unknowable  structure  of  chemical  compounds,  giving  the  prefer- 
ence rather  to  the  dualistic  view,  as  being  that  which  at  the 
present  moment  prevails  in  the  great  bulk  of  chemical  literature, 
and  has  become  incorporated  into  the  language  of  the  chemical  arts. 

Lest  any  doubt  should  suggest  itself  to  the  student's  mind  as 
to  the  value  of  symbolic  formulae,  let  it  be  observed  that  they 
express  the  elementary  composition  of  a  compound  much  more 
tersely  than  words  can  ;  that  they  are  written  and  "read  more  rap- 
idly than  the  sentences  of  the  same  signification  would  be,  and 
that  by  their  brevity,  clearness,  and  precision  they  greatly 
facilitate  the  comparative  study  and  comprehensive  classification 
of  chemical  compounds.  Again,  the  chemical  equations,  of  whose 
construction  we  have  already  had  several  examples,  enable  us  to 
set  forth  with  precision  the  changes  which  accompany  compli- 
cated, as  well  as  simple,  reactions.  Thus  the  somewhat  complex 
decomposition  of  nitric  acid  by  copper  takes  definite  form  in  the 
appropriate  equation  which  has  been  given  above  (p.  71),  and  the 
very  simple  reaction  by  which  nitric  oxide  yields  red  fumes  of 
hyponitric  acid  in  contact  with  air  or  oxygen  is  concisely  stated 
by  the  simple  equation  NO  -J-  O  =  NO2. 

The  chemistry  of  the  analysis  of  nitric  oxide  by  potassium 
(§  70)  is  all  condensed  into  the  equation  NO  -f-  K2  =  K2O  -|-  N. 
When  a  little  ice-cold  water  is  added  to  liquid  hyponitric  acid 
(Exp.  42),  the  reaction  which  occurs  is  very  concisely  set  forth 
in  the  equation  :  — 
Empirical:  2NO2  +  H2O  =  HN02  +  HNO3 

**-• 


Dualistic:  4NO2  +  2H2O  =  H2O,  N2O3  +  H2O,  N2O6. 
But  besides  having  all  the  advantages  of  a  short  hand,  chemical 
symbols  are  susceptible  of  another  application  of  hardly  less  im- 
portance ;  they  often  direct  the  chemist  beforehand  to  the  most 
perfect  experiment  among  many  similar,  or  point  out  in  anticipa- 
tion the  possibility  of  certain  methods  of  research,  and  the  in- 
evitable fruitlessness  of  others.  Thus  the  equation 
N2O5  +  5Cu  =  N2  +  5CuO 


78 


AMMONIA. 


actually  directs  the  chemist  to  the  due  proportion  of  copper  for 
the  exact  decomposition  of  anhydrous  nitric  acid  (§  73)  ;  neither 
four,  nor  six,  nor  any  other  number  than  five,  parts  of  copper, 
would  give  a  perfect  reaction  without  excess  of  either  ingredient. 
Practically,  an  excess  of  copper  does  no  harm,  and  is  always 
used  to  make  sure  of  the  decomposition. 

The  student  should  endeavor,  from  the  beginning,  to  familiarize 
himself  with  the  use  of  chemical  symbols  and  equations,  and  to 
this  end  he  should  invariably  write  the  formula  of  every  reaction 
described,  or  actually  witnessed  in  the  execution  of  an  experiment. 

83.  Nitrogen  and  Hydrogen.  Ammonia- water,  such  as  we 
made  use  of  in  Exp.  33,  when  gently  heated,  evolves  a  very 
pungent,  colorless  gas,  which  now  claims  our  attention. 

Exp.  44.  —  Fill  a  flask  of  250  to  500  c.  c.  capacity  about  half  full  of 
the  strongest  ammonia-water  to  be  had  at  the  druggists.  Close  the 
flask  by  a  cork  provided  with  a  funnel-tube  and  an  exit-tube ;  carry 
the  delivery-tube  to  the  bottom  of  a  tall  bottle,  having  a  capacity  of  at 
^east  a  litre ;  this  bottle  is  to  be  filled  with  fragments  of  quick-lime 

which     will     absorb     all 
FIG.  21.  . 

moisture  from  the  gas  as 

it  comes  from  the  flask  ; 
the  gas,  issuing  dry  from 
this  bottle,  may  either  be 
collected  over  mercury, 
or  by  displacement,  as 
shown  in  the  figure  (Fig. 
21).  The  gas  is  so  ex- 
tremely soluble  in  water 
that  it  cannot  be  collected 
over  the  ordinary  water- 
pan  ;  as  it  has  little  more 
than  half  the  density  of 
atmospheric  air,  it  can  be 
readily  collected  by  dis- 
placement. When  thus 
collected,  the  gas  should  be  allowed  to  pass  into  the  very  loosely  corked 
bottle,  until  a  piece  of  turmeric  paper,  held  at  the  mouth,  is  immedi- 
ately turned  brown ;  the  delivery-tube  is  then  withdrawn,  and  the 
mouth  of  the  bottle  is  tightly  closed  with  a  caoutchouc  or  glass  stopper. 
The  gas  thus  obtained  is  transparent  and  colorless,  possesses 


PROPERTIES    OF    AMMONIA. 


79 


an  extraordinarily  pungent  odor  which  provokes  tears,  and  has 
an  acrid,  alkaline  taste.  It  will  be  found  to  be  uninflammable,  and 
is,  of  course,  irrespirable.  It  turns  red  litmus  to  blue  most  ener- 
getically. Its  specific  gravity  as  deduced  from  actual  experiment 
is  8.62  ;  a  litre  of  the  gas  weighs  0.7625  grm.  One  measure 
of  water  at  0°  dissolves  1049  measures  of  the  gas. 

Exp.  45. —  Fill  a  stout  glass  tube,  an  ignition-tube  for  example,  over 
mercury  with  the  gas ;  grasp  the  tube  by  the  top,  and,  holding  it  upright, 
dip  its  mouth  into  a  vessel  of  water.  The  water  will  rush  up  the  tube, 
if  the  gas  be  pure,  with  a  force  which  might  break  the  tube,  if  too  thin. 

84.  The  solution  of  ammonia  exposed  to  the  air,  or  placed  in 
a  vacuum,  or  simply  boiled,  loses  all  its  gas.     As  its  ready  solu- 
bility in  water  suggests  (compare  §  68),  the   liquefaction  of  the 
gas  it  not  only  possible  but  easy ;  the  gas  becomes  a  colorless, 
transparent,  mobile  liquid  at  0°,  under  a  pressure  of  4J  atmos- 
pheres, or  at  — 40°  at  the  ordinary  pressure.     This  liquid  freezes 
at  about  — 80°.     An  excellent  freezer,  applicable  on  both  the 
large  and  small  scale,  is  now  constructed,  in   which  the  cold  is 
produced  by  the  rapid  evaporation  of  liquefied  ammonia-gas.     It 
is  the  low  pressure  at  which  ammonia  becomes  a  liquid  which 
renders  this  machine  possible. 

85.  But  of  what  is  this  gas,  whose  properties  are  so  strikingly 
unlike  those  of  any  gas  previously  studied,  composed  ? 

Exp.   46.  —  With    the  FlG-  22t 

exit -tube  of  the  drying 
bottle  of  the  apparatus  al- 
ready used  to  generate 
ammonia  -  gas  connect,  as 
shown  in  the  figure  (Fig. 
22),  a  tube  of  hard  glass, 
in  which  a  bulb  has  been 
blown  (see  Appendix,  §4). 
Thrust  into  this  bulb  a 
piece  of  the  metal  potassi- 
um ;  cause  ammonia  gas  to 
flow  through  the  bulb  by 
heating  the  contents  of  the 
flask,  and  then  warm  the 
glass  bulb.  As  soon  as  the  potassium  melts,  it  becomes  covered  with  a 


80  ANALYSIS    OF    AMMONIA. 

brownish-green  film,  and  a  gas  begins  to  escape  which  we  recognize  as 
hydrogen  by  lighting  it  at  the  mouth  of  the  tube.  The  burning  gas  is 
certainly  not  ammonia,  for  ammonia  is  not  inflammable,  but,  as  the 
odor  proves,  it  is  mixed  with  some  ammonia  which  has  escaped  decom- 
position by  the  potassium. 

To  separate  this  ammonia,  and  collect  the  pure  hydrogen,  fill  a  test- 
tube,  about  14  c.  m.  long,  three-quarters  full  of  mercury;  pour  water 
upon  the  mercury  till  the  tube  is  full,  close  the  tube  with  the  thumb, 
and  invert  it  into  a  cup  of  mercury;  with  the  outer  end  of  the  bulb- 
tube  (Fig.  22)  connect  a  suitable  delivery-tube  which  shall  dip  into  the 
cup  of  mercury  and  deliver  the  gas  into  the  test-tube,  whose  upper 
quarter  is  full  of  water.  The  gas  must  pass  through  this  water,  which 
frees  the  hydrogen  from  intermixed  ammonia. 

It  is  absolutely  necessary  to  use  mercury  in  this  experiment,  because 
if  the  delivery-tube  were  allowed  to  dip  into  water,  the  extreme  solu- 
bility of  ammonia  in  water  might  cause  the  water  to  suck  back  into  the 
bulb  containing  the  heated  metal ;  whereupon  an  explosion  would  in- 
evitably ensue. 

86.  Having  thus  set  free  hydrogen  from  ammonia  by  means  of 
potassium,  just  as  we  eliminated  the  same  hydrogen  from  water 
by  means  of  the  analogous  metal  sodium,  we  shall  be  inclined  to 
try  upon  ammonia  the  same  powerful  agent  by  which  we  resolved 
water  into  its  elements  —  the  galvanic  current. 

Exp.  47. —  A  glass  tube  (No.  1),  60  to  80  c.  m.  long,  open  at  one 
end  and  closed  rtt  the  other,  is  bent  into  the  form  of  a  V ;  the  closed 
limb  is  provided  with  a  platinum- wire  fused  into  the  glass,  through 
FJQ  23  which  the  wire  passes,  to  terminate  near  the  bend 

of  the  V,  in  a  slip  of  platinum  foil.  Fill  the 
whole  of  the  closed  limb  and  nearly  half  of  the 
open  limb  of  this  tube  with  ammonia-water,  to 
which  a  few  drops  of  sulphuric  acid  have  been 
added  to  increase  its  conducting  power ;  sup- 
port the  tube  as  shown  in  the  figure  (Fig.  23), 
and  connect  with  the  platinum-wire  in  the 
closed  limb  of  the  tube  the  negative  or  zinc 
pole  of  two  medium-sized  Bunsen  cells,  at  the 
same  time  inserting  a  platinum  wire  and  plate, 
attached  to  the  positive  or  carbon  pole,  in  the 
open  limb.  Gas  quickly  collects  in  the  closed 
limb.  Disconnect  the  battery-wires,  fill  the 
open  limb  with  water,  close  it  with  the  thumb,  and  by  inclining  the, 


SYNTHESIS    OF    AMMONIA.  81 

tube  transfer  the  gas  to  the  open  limb.  On  applying  a  match  to  the 
gas,  it  proves  to  be  inflammable,  and  we  recognize  it  without  difficulty 
as  hydrogen. 

The  experiment  is  now  repeated  with  the  electrodes  reversed ;  —  the 
positive  pole  is  connected  with  the  sealed,  and  the  negative  with  the 
open,  limb.  The  hydrogen,  which  is  disengaged  at  the  negative  pole, 
now  escapes  through  the  open  end  of  the  tube  into  the  air,  while  a 
transparent  and  colorless  gas,  previously  evolved  at  the  positive  pole  in 
the  open  limb  and  consequently  lost,  is  now  collected  in  the  sealed 
end  of  the  apparatus.  The  quantity  of  gas  evolved  at  the  positive 
pole  is  comparatively  small,  but  in  half  an  hour  enough  for  examination 
will  probably  have  been  collected.  -By  the  same  manipulation  as  before, 
transfer  this  gas  to  the  open  limb,  and  thrust  into  it  a  lighted  match. 
It  is  neither  inflammable  nor  will  it  support  combustion,  and  has  in 
itself  neither  taste  nor  smell,  —  it  is  the  inert  nitrogen. 

87.  These  experiments  have  conclusively  proved  hydrogen  and 
nitrogen  to  be  constituents  of  ammonia,  and  it  now  becomes 
desirable  to  prove  synthetically  that  these  two  gases  are  the  only 
constituents  of  ammonia ;  which  would  be  done,  if  ammonia 
could  be  experimentally  produced  by  the  direct  union  of  the  two 
gases.  Unfortunately,  no  process  has  been  discovered  whereby 
ammonia  can  be  directly  reproduced  from  free  hydrogen  and 
free  nitrogen.  The  following  experiments,  however,  will  demon- 
strate that  ammonia  is  actually  produced  from  materials  which 
are  known  to  generate  a  mixture  of  hydrogen  and  nitrogen ;  or, 
more  strictly,  which  are  known  to  be  capable  of  generating  both 
hydrogen  and  nitrogen. 

Exp.  48.  —  Place  in  an  ignition-tube  an  intimate  mixture  of  3 
grammes  of  fine  iron  filings  with  0.2  gramme  of  caustic  potash ;  adapt 
a  delivery-tube  (No.  7)  to  the  ignition-tube,  heat  the  contents  of  the 
tube  over  the  gas-lamp,  and  collect  the  gas  which  escapes  in  a  test-tube 
over  the  water-pan.  Examine  this  gas,  which  will  prove  to  be  the  in- 
flammable hydrogen.  Caustic  potash,  as  we  have  already  learned 
(p.  72),  consists  of  potassium,  hydrogen,  and  oxygen ;  at  a  high  tem- 
perature, metallic  iron  is  able  to  seize  upon  a  portion  of  the  oxygen  in 
this  compound,  setting  free  hydrogen,  which  finds  no  place  in  the  new 
combinations. 

Exp.  49. —  Heat  in  a  second  ignition-tube,  similarly  disposed,  a 
mixture  of  3  grammes  of  fine  iron  filings  and  0.2  gramme  of  nitrate  of 
potassium,  and  collect  the  gas,  as  before,  over  water.  This  gas  has 


82  THE    NASCENT    STATE. 

neither  taste  nor  smell,  and  when  tested  with  a  lighted  splinter,  it  is 
found  to  be  uninflammable,  and  in  fact  to  extinguish  the  taper.  It  is 
nitrogen.  Nitrate  of  potassium  contains,  as  has  been  already  stated 
(p.  72),  potassium,  nitrogen,  and  oxygen  ;  at  the  high  temperature  em- 
ployed the  salt  is  partially  decomposed,  the  metallic  iron  combines  with 
the  oxygen  of  the  nitrous  vapors  formed,  and  their  nitrogen  is  set 
free. 

Exp.  50.  —  In  a  third  ignition-tube,  heat  the  same  quantities  of  the 
same  materials  which  have  been  used  in  the  last  two  experiments,  at 
once  and  together.  A  delivery-tube  is  not  necessary  in  this  case ;  the 
tube  may  be  held  in  the  wooden  nippers  by  the  open  end.  Neither 
hydrogen  nor  nitrogen  will  be  evolved  as  before,  but  instead  of  them 
we  have  ammonia,  whose  presence,  revealed  by  its  pungent  odor,  may 
be  further  manifested  by  holding  a  bit  of  reddened  litmus-paper 
at  the  mouth  of  the  tube.  The  intense  alkaline  reaction  of  the 
gas,  and  its  odor,  sufficiently  distinguish  it  from  both  hydrogen  and 
nitrogen. 

88.  It  is  to  be  observed  that  the  hydrogen  and  nitrogen,  which 
refuse  to  unite  when  once  actually  in  the  free  state,  will  form  a 
chemical  compound,  as  in  the  last  experiment,  at  the  precise  in- 
stant when  they  issue  from  other  compounds  of  which  they  formed 
part.  This  fact  is  expressed  by  saying  that  these  two  gases  will 
enter  into  combination  when  in  the  nascent  state,  that  is,  at  the 
moment  of  birth. 

There  are  numerous  cases  in  which  bodies,  which  do  not  unite 
under  ordinary  conditions,  are  capable  of  chemical  combination 
at  the  instant  when  they  are  disengaged  from  other  compounds, 
and  the  phrase  "  in  the  nascent  state  "  is  one  of  some  convenience, 
though  it  must  not  be  supposed  to  explain,  or  in  any  way  to 
account  for,  the  phenomena  with  reference  to  which  it  is  used. 
FIG.  24.  89.  The  exact  quantitative  analysis  of  ammonia 
gas  will  afford  proof  that  hydrogen  and  nitrogen  are 
the  sole  constituents  of  this  gas,  and  will  further  show 
the  proportions  in  which  they  are  combined.  Ammo- 
nia is  completely  decomposed  into  its  elements  by  heat, 
—  the  heat  of  a  furnace  or  the  heat  produced  by  a 
continuous  discharge  of  electric  sparks.  When,  there- 
fore, 50  c.  c.  of  the  gas  are  placed  in  a  eudiometer 
(gas-measurer)  (Fig.  24),  and  sparks  are  passed  between  the 


COMPOSITION    OF   AMMONIA. 


83 


platinum  points,  by  means  of  a  Ruhmkorff  apparatus,  the  gas  is 
finally  resolved  into  its  elements ;  its  volume  increases  until  it 
reaches  100  c.  c.,  or  double  its  original  bulk,  when  it  remains 
constant.  We  know  that  a  part,  at  least,  of  these  100  c.  c.  of  gas 
is  hydrogen,  and  that  this  hydrogen  can  be  eliminated  from  the 
gaseous  mixture  by  introducing  oxygen  in  sufficient  quantity  to 
convert  the  hydrogen  into  water,  and  then  exploding  the  mix- 
ture. Were  the  100  c.  c.  of  gas  all  hydrogen,  50  c.  c.  of  oxy- 
gen would  convert  it  into  water.  That  we  may  be  sure  of 
having  enough  oxygen,  let  us  introduce  into  the  eudiometer  50  c.  c. 
of  oxygen,  and  then  pass  a  spark  to  explode  the  mixture. 

After  the  explosion,  only  37.5  c.  c.  of  gas  will  remain;  112.5 
c.  c.  have  disappeared,  having  been  converted  into  water.  But 
of  these  lost  112.5  c.  c.  we  know  that  75  c.  c.  must  have 
been  hydrogen  and  37.5  c.  c.  oxygen,  in  accordance  with  the 
known  volumetric  composition  of  water ;  consequently,  the  100 
c.  c.  of  gas,  into  which  the  original  50  c.  c.  of  ammonia  were  di- 
lated, contained  75  c.  c.  of  hydrogen.  After  the  explosion,  there 
remained  37.5  c.  c.  of  gas,  and  since  we  used  only  37.5  c.  c.  of 
oxygen  out  of  50  c.  c.  added,  we  may  infer  that  12.5  c.  c.  of 
oxygen  still  remain  in  the  residue  from  the  explosion.  On  intro- 
ducing into  the  eudiometer  a  little  pyrogallic  acid  (an  acid  used 
in  photography)  dissolved  in  water,  and  a  few  drops  of  a  solu- 
tion of  caustic  potash,  these  12.5  c.  c.  of  oxygen  will  all  be  ab- 
sorbed by  these  liquids,  and  there  will  remain  25  c.  c.  of  a  color- 
less gas,  which  may  readily  be  recognized  as  pure  nitrogen.  The 
original  50  c.  c.  of  ammonia  have  therefore  yielded  75  c.  c.  of  hy- 
drogen and  25  c.  c.  of  nitrogen,  and  the  composition  of  ammonia, 
both  by  weight  and  measure,  may  be  fully  expressed  by  the 


diagram : 


NH3 

17 

84  THE    GROUP   AMMONIUM. 

This  composition  of  the  gas  is  verified  by  its  specific  gravity  as 
determined  by  experiment,  namely,  8.62. 

Three  volumes  of  H  weigh    .         .         .         .3. 

One  volume  of  N  weighs  .         .         .          14. 

Two  volumes  of  NH3  should  weigh       .         .17. 

One  volume  of  NH8  should  weigh    .         .  8.5 

a  number  sufficiently  near  the  result  of  direct  experiment. 

90.  The  knowledge,  thus  acquired,  of  the  composition  of  am- 
monia will  enable  us  to  recur  with  advantage  to  some  of  the  ex- 
periments performed  in  the  first  part  of  this  chapter.  Ammonia 
gas,  when  dissolved  in  water  as  in  the  Liquor  Ammoniae,  must  be 
considered  to  be  in  combination  with  one  molecule  of  water,  in 
the  form  of  the  compound  NH8,H2O  or  NH5O.  This  compound 
may  be  supposed  to  be  dissolved  in  the  water  present  in 
excess  of  what  is  necessary  to  form  the  compound.  When 
this  water  of  ammonia  combines  with  nitric  acid,  as  in  Exp.  33, 
to  form  the  compound  we  have  called  nitrate  of  ammonium,  a 
reaction  occurs  precisely  similar  to  that  which  takes  place  where 
caustic  soda  combines  with  nitric  acid  (p.  72)  ;  but  in  order  to 
bring  out  the  resemblance,  the  elements  of  the  compound  of 
ammonia  and  water  must  be  so  arranged  as  to  exhibit  its 
analogy  with  caustic  soda,  whose  formula  is  NaHO.  For 
that  purpose  its'  formula  must  be  written  (NH4)HO,  so  that 
the  group  of  elements  NH4  shall  stand  in  the  formula  of^  water 
of  ammonia  where  the  element  sodium  stands  in  the  formula  of 
caustic  soda*.  The  combination  of  water  of  ammonia  with  nitric 
acid  may  then  be  represented  by  the  equation, 

(NH4)HO     -f     HNO8    =      (NH4)NOS      -f      H2O 
Ammonia  water.     Nitric  acid.  Nitrate  of  ammonium.      Water. 

just  like 

NaHO  +  HNO8  =  NaNO3  +  H2O 
The  student  may  write  both  of  these  reactions  in  the  typical 

TT  "\ 

manner ;  the  simple  type  water,  ^  >-  O,  is  the  only  one  needed  to 

represent  all  these  substances,  whether  before  or  after  the  reac- 
tions which  take  place  between  them. 


SALTS    OF    AMMONIUM.  85 

91.  Ammonia-water  combines  with  nearly  all  the  acids  with 
which  soda  is  capable  of  combining,  forming  a  series  of  compounds 
in  which  the  group  of  atoms  NH4  plays  the  same  part  which  the 
single  atom  Na  plays  in  the  corresponding  compounds  of  sodium. 
For  this  reason  it  has  been  found  convenient  to  give  to  this 
group  of  atoms  a  name  bearing  some  resemblance  to  the  names 
of  metals,  and  it  has  therefore  been  called  ammonium.     Ammo- 
nium is  known  only  in  its  compounds  ;  many  attempts  have  been 
made  to  obtain  it  in  a  free  state,  but  hitherto  in  vain  ;  as  soon  as 
the  group  of  atoms  escapes  from  combination,  it  is  resolved  into 
ammonia  and  hydrogen. 

The  important  compounds  into  which  ammonium  enters,  com- 
monly called  the  salts  of  ammonium,  will  be  studied  hereafter  in 
immediate  connection  with  the  analogous  salts  of  sodium  and 
potassium.  Already,  however,  it  will  be  possible  to  verify  the 
statement  of  §  66,  to  the  effect  that  nitrous  oxide  contains  "  all 
the  elements,  beside  those  of  water,  which  enter  into  the  compo- 
sition of  nitrate  of  ammonium,  and  therefore  of  its  constituents, 
nitric  acid  and  ammonia-water."  The  reaction  last  given  shows 
that  a  molecule  of  nitrate  of  ammonium  contains  all  the  elements 
of  a  molecule  of  nitric  acid  and  a  molecule  of  ammonia-water, 
less  one  molecule  of  water.  If  the  formulae  of  ammonia-gas 
and  nitrate  of  ammonium  have  been  correctly  determined,  it  will 
be  easy  to  exhibit  in  an  equation  the  actual  result  of  Exp.  34,  as 
follows :  — 

(NH4)NO3  =  N2O  +  2H2O. 

We  thus  link  together  several  distinct  experiments,  and  confirm 
the  determinations  previously  made  of  the  composition  of  nitric 
acid,  nitrous  oxide,  ammonia,  and  water,  by  showing  that  the 
representative  formulae  of  'these  substances  exhibit  with  perfect 
precision  reactions  already  well  known  to  us,  but  other  than  those 
from  which  the  formulae  in  question  were  originally  derived. 

92.  Ammonia  exists  in  very  minute  quantity  in   the  atmos- 
phere, and  hence  in  rain-water,  fog,  and  dew.     The  proportion 
of  ammonia  in  rain-water  has  been  variously  given  by  different 
observers  from  3.49  parts  to  0.744  parts  of  ammonia  in  1,000,000 
parts  of  water.     The  water  of  fog  and  dew  contains  a  larger 


86  SOURCES    OF    AMMONIA. 

proportional  quantity  of  ammonia  ;  on  account  of  the  high  solu- 
bility of  the  gas,  the  proportion  of  ammonia  in  water  derived 
from  the  atmosphere  is  greater,  the  smaller  the  fall  of  water. 
Ammonia  fs  given  off  by  putrifying  animal  and  vegetable  sub- 
stances containing  nitrogen,  and  almost  every  process  of  slow 
oxidation  in  the  presence  of  air  and  moisture  is  attended  with  the 
formation  of  ammonia  or  ammonia  salts.  Moistened  iron-filings, 
if  exposed  to  the  air,  become  rusty ;  and  this  rust  is  found  to 
contain  a  small  quantity  of  ammonia.  When  some  of  the  metals, 
by  preference  tin,  zinc,  or  iron,  are  dissolved  in  dilute  nitric 
acid,  the  oxidation  of  the  metal  is  frequently  accompanied,  to  a 
greater  or  less  extent,  by  the  production  of  ammonia.  But  the 
chief  source  of  ammonia  and  its  compounds  is  the  decomposition, 
either  by  putrefaction  or  destructive  distillation,  of  nitrogenous 
organic  matter.  The  distillation  of  bones  and  animal  refuse,  for 
the  purpose  of  making  bone-black,  yields  a  large  amount  of  am- 
moniacal  liquor,  which  was  formerly  the  principal  source  of  the 
compounds  of  ammonia ;  the  horns  of  deer  used  to  be  thus  dis- 
tilled, whence  the  name  "  hartshorn."  At  present,  the  destruc- 
tive distillation  of  coal  in  gas-works  furnishes  the  great  bulk  of 
ammonia  compounds  used  in  the  arts.  The  ammoniacal  liquor 
of  the  gas-works  is  water  contaminated  with  tarry  matters,  and 
holding  in  solution  a  small  proportion  of  very  volatile  ammonium 
salts.  These  volatile  salts  are  distilled  off,  by  application  of  heat, 
into  dilute  sulphuric  or  muriatic  acid,  and  the  sulphate,  or  chloride, 
of  ammonium  thus  formed  is  obtained  from  the  .dilute  liquid  by 
evaporation  and  crystallization.  These  salts  will  be  studied  in 
detail  hereafter. 

93.  The  solution  of  ammonia-gas  in  water  is  a  re-agent  con- 
tinually required,  as  a  test,  in  the  laboratory,  and  much  used  in 
the  arts.  The  solution  is  colorless,  intensely  alkaline,  has  a 
caustic  taste,  and,  when  concentrated,  blisters  the  skin.  The  solu- 
tion is  lighter  than  water,  and  so  much  the  lighter  in  proportion 
to  the  amount  of  ammonia  it  contains  ;  for  this  reason  its  strength 
may  be  accurately  determined  by  its  specific  gravity.  Tables  of 
the  relation  of  strength  to  specific  gravity  may  be  found  in  chem- 
ical dictionaries.  The  solution  is  ordinarily  prepared  from  a 
mixture  of  chloride,  or  sulphate,  of  ammonium,  with  slaked  lime. 


MAKING   AMMONIA-WATER. 


87 


Exp.  51. —  Mix  50  grms.  of  chloride  of  ammonium,  a  substance 
generally  sold  under  the  name  of  sal  ammoniac,  with  about  the  same 
weight  of  cold,  freshly-slaked  lime.  Introduce  the  mixture  into  a  flask 
of  500  c.  c.  capacity,  and  place  the  flask  on  a  sand-bath  over  the  gas- 
lamp.  Close  the  mouth  of  the  flask  with  a  good  cork,  provided  with  a 
delivery-tube  so  bent  as  to  connect  conveniently,  by  means  of  a  caout- 
chouc connector,  with  the  first  of  the  series  of  three-necked  bottles 
(Woulfe's  bottles)  represented  in  Fig.  25. 

FIG.  25. 


The  first  of  this  series  of  bottles  is  smaller  than  the  rest,  and  is  not 
filled  so  full  of  water  as  the  others ;  it  should  be  kept  cool  by  immer- 
sion in  cold  water ;  the  delivery-tube  coming  from  the  flask  into  this 
bottle  must  not  dip  into  the  water  at  all,  so  that  it  will  be  impossible 
for  any  water  to  suck  back  into  the  flask,  should  the  gas  suddenly 
cease  to  come  off  from  the  dry  mixture.  The  construction  of  the 
apparatus  is  easily  to  be  understood  from  the  figure ;  the  open  tube 
which  dips  beneath  the  water  in  each  bottle  is  a  safety-tube,  which,  by 
admitting  air  into  any  bottle  in  which  a  partial  vacuum  may  happen  to 
be  created  by  rapid  absorption,  prevents  the  contents  of  the  succeed- 
ing bottle  from  flowing  back  into  it. 

The  ammonia-gas  cannot  avoid  four  separate  contacts  with  water  as 
it  passes  through  the  apparatus,  so  that  all  the  gas  is  sure  to  be  ab- 
sorbed ;  the  contents  of  the  first  bottle  will  not  be  as  pure  as  those  of 
the  succeeding.  This  apparatus,  or  modifications  of  it,  is  used  on  the 
large  scale  as  well  as  the  small,  in  operations  which  involve  the  absorp- 
tion of  a  gas  by  a  liquid  capable  of  dissolving  it.  When  a  large  quantity 
of  gas  is  continuously  delivered  from  the  generating  vessel,  the  absorp- 
tion can  be  made  equally  continuous  by  successively  removing  the 
bottle  nearest  the  flask  as  soon  as  the  liquid  in  it  is  saturated,  and  add- 
ing a  fresh  bottle  at  the  other  end  of  the  series.  On  heating  the 


88  CHLOROHYDRIC    ACID. 

flask,  the  ammonia-gas  will  be  set  free  from  the  mixture,  and  in  this 
experiment  will  be  mostly  absorbed  in  the  first  and  second  Woulfe- 
bottles. 

The  reaction  between  the  chloride  of  ammonium  and  the 
slaked  lime  is  represented  by  the  following  equations  :  — 

2NH4C1  +  CaH2O2    =     2NH3    +     CaCl2    +     2H2O 

Chloride  of    en  i    ^  T  A  •         Chloride  of        n7 

.    •      Slaked  lime.     Ammonia.        ^  7  .      J         Water, 
ammonium.  Calcium. 

Chloride  of  ammonium  is  a  compound  which  may  be  obtained 
by  bringing  together  dry  ammonia,  NH8 ,  and  dry  muriatic  acid 
gas  (see  Exp.  68),  HC1. 

NH8  +  HC1  =  NH8HC1  =  NH4C1 

It  may  obviously  be  regarded  as  a  compound  of  the  group  called 
ammonium,  NH4 ,  with  the  element  chlorine ;  from  this  view  is 
derived  the  name  chloride  of  ammonium.  Slaked  lime  is  pre- 
pared from  quick-lime,  which  is  chemically  the  oxide  of  the 
metal  calcium,  and  water. 

CaO  +  H2O  =  CaO,H2O  ==  CaH2O2 

94.  The  applications  of  ammonia-water  are  numerous  and 
various ;  it  is  an  ingredient  in  many  pharmaceutical  prepara- 
tions ;  applied  to  the  skin,  it  is  an  irritant,  and,  when  very  strong, 
even  a  caustic ;  its  pungent  odor  is  reviving  and  stimulating ;  in 
veterinary  practice  it  is  a  useful  medicament ;  on  account  of  its 
alkalinity  it  is  used  in  removing  grease  from  cloth,  and  in  re- 
storing colors  which  have  been  changed  by  acids. 


CHAPTER    VII. 

CHLORHYDRIC      ACID. 

95.  Muriatic  (sea-salt)  acid,  called  in  modern  nomenclature 
chlorhydric  acid,  is  a  liquid  which  has  been  known  for  centuries, 
and  is  to-day  an  article  of  commerce,  largely  employed  in  the 
useful  arts.  The  pure  acid  is  a  gas,  as  ammonia  is  ;  the  liquid 
muriatic  acid  of  commerce  is  only  an  aqueous  solution  of  this 


MAKING    CHLORHYDRIC    ACID    GAS. 


89 


FlG 


gas,  and  gives  it  up  when  heated,  precisely  as  ammonia-water 
yields  ammonia-gas. 

This  operation  may  be  conveniently  performed  in  the  apparatus 
shown  in  Fig.  26.  About 
250  c.  c.  of  the  commercial 
acid  is  poured  into  the  flask, 
which  is  then  moderately 
heated;  the  gas  disengaged 
is  charged  with  aqueous  va- 
por, which  needs  to  be  re- 
moved before  the  gas  is  col- 
lected. For  this  purpose 
the  delivery-tube  is  carried 
to  the  bottom  of  a  bottle 
filled  with  pieces  of  pumice- 
stone  saturated  with  strong 
sulphuric  acid  ;  the  moisture 
of  the  gas  is  greedily  ab- 
sorbed by  the  large  surface 
of  acid  with  which  the  gas 

comes  in  contact,  as  it  is  forced  upward  through  the  acid-soaked  stone. 
The  dry,  colorless,  transparent  gas  must  be  collected  over  mercury,  for 
it  is  extremely  soluble  in  water. 

96.  The  gas  is  strongly  acid  in  taste  and  reaction  on  vegetable 
colors,  provokes  violent  coughing,  and  is  wholly  irrespirable.  It 
is  neither  combustible  nor  will  it  support  combustion.  The  gas 
is  somewhat  heavier  than  air;  its  specific  gravity  referred  to 
hydrogen,  as  determined  by  experiment,  is  18.12,  its  theoretical 
density  being  18.25  ;  it  is  possible,  though  not  convenient,  to 
collect  it  by  downward  displacement.  It  forms  opaque,  white 
fumes  in  the  air,  owing  to  its  union  with,  and  condensation  of,. 
atmospheric  moisture.  Under  a  pressure  of  40  atmospheres,  at  a 
temperature  of  10°,  chlorhydric  acid  gas  is  condensed  into  a  color- 
less liquid.  Its  great  solubility  in  water  would  lead  us  to  expect 
that  it  could  be  readily  reduced  to  the  liquid  state  ;  but,  on  the 
contrary,  it  is  a  difficultly  condensable  gas.  At  0°,  one  volume 
of  water  dissolves  about  500  volumes  of  chlorhydric  acid  gas  ; 
at  common  temperatures,  something  more  than  400.  The  specific 
gravity  of  the  solution  is  greater  than  that  of  water,  and  the 


90 


PROPERTIES    OF    CHLORHYDRIC    ACID. 


more  concentrated  the  solution  the  higher  the  specific  gravity ; 
so  that  the  strength  of  any  sample  of  the  commercial  acid  may 
be  ascertained  by  taking  its  specific  gravity.  Tables  for  this  use 
will  be  found  in  chemical  dictionaries. 

The  avidity  of  water  for  chlorhydric  acid  gas  may  be  neatly 
shown  by  thrusting  a  bit  of  ice  into  a  small  cylinder  of  the  dry 
gas  standing  over  mercury  ;  the  ice  instantly  melts,  and  the  gas 
as  quickly  disappears.  A  solution  of  the  acid  containing  20.2 
per  cent,  of  the  gas,  and  having  a  specific  gravity  of  1.104,  distils 
unchanged  at  a  temperature  of  about  111°;  stronger  solutions 
than  this,  on  being  heated  under  the  ordinary  atmospheric 
pressure,  lose  gas  until  reduced  to  this  strength  ;  weaker  solutions 
lose  water  until  raised  to  this  degree  of  concentration.  This 
stable  solution,  which  distils  unchanged,  is  supposed  to  be  a  defi- 
nite 'compound  of  the  dry  gas  and  water,  whose  composition  the 
formula  HC1  -f-  8  H2O  would  correctly  represent. 

97.  We  propose  to  answer  the  question,  —  of  what  is  chlor- 
hydric acid  composed,  —  by  a  partial  analysis  and  a  complete 
synthesis. 

One  of  the  elements  of  this  gas  can  be  isolated  by  a  method  which 
we  have  already  applied  to  the  analysis  of  ammonia.  It  is  only  neces- 
sary to  remove  the  delivery-tube  from  the  apparatus  already  used  to 
generate  the  dry  gas  (Fig.  26),  and  to  fix  in  its  place  a  bulb-tube 

of  hard  glass,  containing  a 
piece  of  potassium.  As 
soon  as  the  acid  gas  reaches 
the  potassium,  the  metal 
becomes  covered  with  a 
white  incrustation,  and  if 
the  bulb  be  now  very  gent- 
ly heated  (Fig.  27),  the  po- 
tassium fuses,  and  taking 
fire,  burns  with  a  violet 
light.  During  the  reaction 
the  chlorhydric  acid  is  de- 
composed, an  inflammable 
gas,  easily  recognized  as 
hydrogen,  is  evolved,  and 
may  be  lighted  at  the  end 
of  the  tube. 


FIG.  27. 


ANALYSIS    OF    CHLORHYDRIC    ACID.  91 

The  metal  sodium  produces  similar  results,  but  at  a  much  higher 
temperature.  A  solution  of  sodium  in  mercury,  known  amongst 
chemists  as  sodium-amalgam,  will  however  bring  about  the  decomposi- 
tion of  the  acid  at  the  ordinary  temperature.  This  solution  is  best  pre- 
pared by  very  gently  heating  some  mercury  in  a  glass  flask,  and 
gradually  adding  the  sodium,  cut  into  fragments  not  bigger  than  a  grain 
of  wheat;  the  fragments  dissolve  with  evolution  of  light  and  heat. 
Why  the  sodium-amalgam  should  act  at  a  lower  temperature  than  the 
sodium  itself,  is  not  clear ;  unless  it  be  that  the  minute  subdivision  of 
the  sodium  in  the  mercury  gives  the  gas  a  freer  contact  with  the  metal. 

Hydrogen  is  then  one  ingredient  of  chlorhydric  acid ;  the  other,  or 
others,  have  combined  with  the  potassium  or  sodium-amalgam.  The 
isolation  of  these  unknown  ingredients  may  be  FlG  28 

accomplished  by  means  of  the  V  tube  already 
used  for  the  analysis  of  ammonia.  Into  this 
tube  liquid  chlorhydric  acid  of  specific  gravity 
1.1,  colored  with  indigo  solution,  is  introduced, 
so  as  to  fill  the  whole  length  of  the  sealed,  and 
about  half  the  length  of  the  open,  limb ;  the 
negative  pole  of  the  battery  is  connected  with 
the  wire  of  the  sealed  limb,  while  the  positive 
pole  is  inserted  into  the  open  limb.  Gas  rap- 
idly collects  at  the  negative  pole  in  the  closed 
limb,  but  at  the  positive  pole  the  disengage- 
ment of  gas  is  so  slight  that  it  would  hardly 
attract  attention  but  for  its  intensely  disagree- 
able odor  and  powerful  bleaching  action  upon  the  blue  liquid.  The 
gas  in  the  sealed  limb  has  no  such  bleaching  power.  When  enough 
gas  for  examination  has  collected  in  the  sealed  limb,  it  is  transferred  to 
the  open  limb  by  the  manipulation  previously  (Exp.  47)  described  ; 
the  gas  is  inflammable,  and  is,  in  short,  hydrogen. 

The  poles  are  now  reversed,  and  immediately  hydrogen  escapes  in 
abundance  from  the  open  mouth  of  the  tube,  while  the  liquid  in  the 
closed  limb  becomes  decolorized.  In  the  course  of  fifteen  minutes  the. 
bleached  liquid  in  the  sealed  limb  begins  to  assume  a  yellowisK-green 
color,  and  the  evolution  of  gas  becomes  gradually  more  and  more 
copious,  so  that  in  three  quarters  of  an  hour  the  greater  portion  of  the 
tube  is  filled  with  a  transparent,  yellowish-green  gas.  As  the  gas  is 
transferred  to  the  open  limb  of  the  tube  for  examination,  it  manifests 
its  powerful  bleaching  property  by  decolorizing,  as  it  passes,  the  portion 
of  the  acid  which  had  retained  the  blue  color  of  the  indigo.  The  tube  is 
no  sooner  opened  to  admit  a  burning  taper  than  the  suffocating  odor  of 


92  ANALYSIS    OP    CHLORHYDRIC    ACID. 

the  gas  becomes  offensively  perceptible  ;  the  gas  proves  to  be  uninflam- 
mable, and  it  supports  combustion  but  imperfectly,  as  is  evidenced  by 
the  sooty  cloud  which  is  produced. 

This  peculiar  gas,  so  different  in  properties  from  any  gas  heretofore 
studied,  is  an  element ;  it  has  been  named,  on  account  of  its  color, 
Chlorine,  from  the  Greek  word  for  yellowish-green.  This  element  is 
the  subject  of  the  next  chapter,  where  it  will  be  fully  studied.  As  will 
there  be  seen  (Exp.  54),  chlorhydric  acid,  when  heated  with  a  sub- 
stance called  black  oxide  of  manganese,  yields  chlorine  in  abundance, 
with  great  facility  ;  in  fact,  this  acid  is  the  source  of  chlorine  whenever 
large  quantities  of  this  gas  are  required.  Chlorine  is  soluble  in  about 
one-third  of  its  volume  of  cold  water,  —  a  property  which  explains  its 
apparently  slow  evolution  at  the  outset  of  the  foregoing  experiment, 
and  the  more  rapid  disengagement  of  the  gas  when  the  liquid  has 
become  saturated  therewith.  Chlorine  is  heavier  than  air,  and  conse- 
quently very  much  heavier  than  hydrogen ;  the  best  experimental  de- 
termination of  its  specific  gravity  has  given  the  number  35.66  :  but 
there  can  be  no  doubt  that  the  true  specific  gravity  of  the  gas  is  35.5, 
or  in  other  words  that  it  is  35£  times  heavier  than  hydrogen. 

98.  We  have  thus  learned  that  the  electric  current  sets  free 
from  chlorhydric  acid  two  essentially  different  gases,  —  hydrogen 
and  chlorine,  —  and* that  each  of  these  gases  may  be  separately 
evolved  from  muriatic  acid,  the  hydrogen  by  potassium,  and  the 
chlorine  by  oxide  of  manganese.  It  remains  to  prove  that  chlor- 
hydric acid  contains  no  other  than  these  two  constituents,  and 
to  demonstrate  the  proportions  in  which  they  are  united.  To 
FIG.  29.  ^is  end  the  first  step  shall  be  to  make  a  partial 

quantitative  analysis  of  chlorhydric  acid  gas. 

The  instrument  employed  is  a  glass  U  tube,  about 
50  c.  m.  long  by  1.5  in  diameter,  having  one  sealed 
and  one  open  limb  ;  communicating  with  the  latter  is 
a  small  outlet  tube  which  may  be  closed  by  a  spring- 
clip  on  a  piece  of  caoutchouc  tubing.  The  apparatus, 
mounted  on  a  convenient  stand,  is  represented  in 
Fig.  29.  The  U  tube  is  first  filled  with  mercury,  and 
then,  the  spring-clip  being  open,  the  delivery-tube  of 
the  apparatus  used  to  generate  dry  chlorhydric  acid 
gas  is  passed  down  the  open  limb  to  the  bend  of  the 
tube  in  such  a  manner  that  the  gas  bubbles  up  through 
the  mercury  into  the  sealed  limb,  from  which  the  mercury  escapes  as 
the  gas  enters.  When  the  closed  limb  is  two-thirds  full,  the  outlet-tube 


COMPOSITION    OF    CHLORHYDRIC    ACID.  93 

is  closed,  the  gas  delivery-tube  withdrawn,  and  mercury  is  poured  into 
the  apparatus  until  it  stands  at  the  same  level  in  both  limbs.  The 
space  occupied  by  gas  in  the  tube  is  then  marked  by  a  caoutchouc 
ring  slipped  over  the  tube.  That  portion  of  the  open  limb  which  is 
not  occupied  by  mercury  is  then  filled  with  sodium-amalgam.  By 
closing  the  orifice  of  the  tube  with  the  thumb,  and  inclining  the  tube, 
the  gas  may  be  transferred  from  the  sealed  limb  to  the  other,  there 
shaken  up  with  the  amalgam,  and  re-transferred  to  the  sealed  limb 
This  thorough  contact  with  the  sodium  decomposes  the  gas.  On  re- 
moving the  thumb  from  the  mouth  of  the  open  limb,  the  mercury 
therein  falls  a  little,  and  must  be  further  lowered  by  opening  the  spring- 
clip  until  the  mercury  stands  at  one  level  in  the  two  limbs.  When 
this  is  the  case,  it  will  be  observed  that  the  gas  is  reduced  to  half  its 
original  volume.  The  gas  which  remains,  is,  of  course,  hydrogen. 
The  experiment  proves  that  any  given  bulk  of  chlorhydric  acid  con- 
tains half  that  bulk  of  hydrogen. 

By  availing  ourselves  of  the  known  specific  gravities,  or  like- 
volume  weights,  of  chlorhydric  acid  and*  chlorine,  referred  to 
hydrogen,  we  may  establish  a  strong  presumption  in  favor  of  the 
supposition,  that  that  half  of  any  bulk  of  chlorhydric  acid,  which  is 
not  hydrogen,  is  chloVine  without  admixture  of  any  other  sub- 
stance. 

From  the  relative  weight  of  any  volume  of  chlorhydric  acid  gas  .  .  .  18.12 
Subtract  the  relative  weight  of  half  that  volume  of  hydrogen 50 

And  the  remainder 17.62 

is  very  nearly  equal  to  17.83,  the  relative  weight  of  half  the 
same  volume  of  chlorine,  according  to  the  best  experimental 
determinations.  If  we  assume,  for  the  moment,  that  any  volume 
of  chlorhydric  gas  is  really  composed  of  half  that  volume  of 
hydrogen  and  half  of  chlorine,  and  if  we  use  the  theoretical 
specific  gravities,  which  are  doubtless  the  true  ones,  instead  of 
the  above  approximate  determinations,  which  are  the  best  which 
experiment  has  hitherto  furnished,  the  numerical  statement  will 
be  as  follows  :  — 

From  the  relative  weight  of  any  volume  of  chlorhydric  acid  gas  .  .  18. 25 
Subtract  the  relative  weight  of  half  that  volume  of  hydrogen 50 

And  the  remainder 17 . 75 

is  the  relative  weight  of   half  the  same  volume  of  chlorine 


94  ATOMIC    WEIGHT    OF    CHLORINE. 

17.75  =  --^-->  The  very  close  coincidence  between  the  first 
numerical  statement,  which  is  based  wholly  upon  experiment, 
with  the  second,  which  is  based  on  the  theory  that  chlorhydric 
acid  is,  by  volume,  half  hydrogen,  half  chlorine,  is  evidence 
enough,  when  taken  in  connection  with  the  preceding  experi- 
mental isolation  of  chlorine  from  chlorhydric  acid,  to  convince 
us  that  in  any  two  volumes  of  chlorhydric  acid  gas,  one  volume 
of  hydrogen  and  one  volume  of  chlorine  are  united  without  con- 
densation. 

The  formula  of  the  molecule  of  the  acid  will  therefore  be  HC1, 
in  which  Cl  is  the  symbol  of  chlorine.  The  following  diagram 
represents  the  composition  of  this  important  compound,  both  by 
volume  and  weight :  — 


HC1  36.5 


99.  The  atomic  weight  of  the  new  element,  chlorine,  is  hereby 
determined.     Hydrogen  and  chlorine  unite  by  equal  volumes  to 
form  this  single    stable    compound,  chlorhydric  acid.;    and  the 
proportions    in   which    the  two  elements  unite  by  weight  are 
directly  deducible  from  the  proportions  in  which  they  unite  by 
volume  and  the  known  specific  gravities  of  the  two  gases.     It, 
indeed,  admits  of  direct  proof  by  appropriate  experiment,  also, 
that  3G.5   parts  by  weight  of   chlorhydric  acid    gas  invariably 
yield  35.5  parts  by  weight  of  chlorine  and  1  part  by  weight  of 
hydrogen  ;  and,  since  it  matters  not  what  the  absolute  weight  of 
these  parts  may  be,  millionths  or  millions  of  grammes,  the  mole- 
cule of  chlorhydric  acid,  the  least  proportional  weight  in  which 
it  is  conceived  to  exist  uncombined,  must  be  composed,  like  any 
other  quantity  of  the  acid,  of  35.5  parts  by  weight  of  chlorine  to 
1  of  hydrogen.     But  we  conceive  of  this  molecule  as  consisting 
of  one  atom  of  chlorine  and  one  atom  of  hydrogen ;  the  chlorine 
atom,  therefore,  weighs   35.5   times  as  much  as  the  hydrogen 
atom. 

100.  If  it  were  entirely  inconceivable  that  another  substance, 
not  identical  with  chlorine,  should  have  precisely  the  same  spe- 
cific gravity  as  chlorine,  the  reasoning  by  which  we  have  just 


SYNTHESIS    OF    CHLORYHYDRIC    ACID.  05 

arrived  at  the  composition  of  chlorhydric  acid,  would  be  not  only 
convincing,  it  would  be  entirely  conclusive  ;  it  would  do  more 
than  establish  a  very  strong  presumption,  it  would  furnish  a  com- 
plete demonstration.  But  it  is  not  inconceivable,  though  in  the 
highest  degree  improbable,  that  there  should  exist  a  body,  differ- 
ent from  chlorine,  yet  possessing  the  same  specific  gravity,  and 
not  to  be  detected  by  the  qualitative  tests  to  which  we  subjected 
the  acid ;  and  we  therefore  welcome  the  perfect  demonstration 
with  which  the  synthesis  of  chlorhydric  acid  supplies  us. 

This  syn diesis  is  readily  effected,  but  as  the  experiment  involves  the 
preparation  of  chlorine,  the  actual  performance  of  the  experiment  will 
be  best  postponed  until  chlorine  has  been  prepared  and  studied  in  the 
next  chapter.  The  method  is  as  follows  :  —  Two  cylinders,  or  bottles, 
of  equal  capacity,  whose  mouths  have  been  ground  together  till  they 
fit  exactly,  are  filled,  the  one  with  dry  hydrogen  and  the  other  with 
dry  chlorine,  and  closed  with  glass  plates.  The  cylinder  containing 
chlorine  is  then  inverted,  and  placed  upon  the  cylinder  containing 
hydrogen ;  the  glass  plates  by  which  the  cylinders  are  closed  are  then 
withdrawn,  and  the  cylinders  are  left  for  several  hours  in  diffused  and 
not  too  bright  daylight,  but  sheltered  from  the  direct  rays  of  the  sun 
which  would  cause  an  explosive  union  of  the  two  elements.  Under 
the  influence  of  the  light  the  two  gases  gradually  combine  ;  when  the 
yellowish  tint  of  the  mixture  has  nearly  disappeared,  the  cylinders 
may  be  exposed  to  the  direct  influence  of  the  solar  rays,  in  order  to 
complete  the  reaction.  Finally,  the  cylinders  are  separated  under 
mercury  ;  no  mercury  will  enter  the  vessels,  neither  will  any  com- 
pressed gas  escape.  The  chemical  union  of  one  volume  of  hydrogen 
with  one  volume  of  chlorine,  is  attended  neither  by  condensation  nor 
expansion. 

That  there  has  been  chemical  action,  resulting  in  the  disappearance 
of  the  properties  of  the  original  materials,  is  evident  from  the  fact  that 
the  contents  of  the  cylinders  will  no  longer  take  fire,  or  bleach  vege- 
table colors.  In  contact  with  air  the  new  gas  forms  white  clouds ;  blue 
litmus  paper  it  turns  red,  and  if  one  of  the  cylinders  be  dipped,  mouth 
downward,  into  water,  the  gas  is  rapidly  absorbed;  the  taste  and 
smell  also,  and,  in  short,  all  the  properties  of  this  gas  are  those  of 
chlorhydric  acid. 

By  the  synthetical  method  we  therefore  prove  that  chlorine 
and  hydrogen  are  the  only  constituents  of  chlorhydric  acid.  On 
this  fact  is  based  the  chemical  name  of  this  compound.  Sum- 


96  MANUFACTURE    OF    CHLORHYDRIC    ACID. 

ming  up  our  previous  and  present  results,  we  now  possess  a  com- 
plete demonstration  that  chlorhydric  acid  is  composed  solely  of 
hydrogen  and  chlorine,  united  in  equal  volumes  without  conden- 
sation. 

101.  The  muriatic  acid  of  commerce  is  made  from  the  most 
abundant  and  cheapest  of  all  the  natural  compounds  of  chlorine, 
common  salt,  whose  chemical  name  is  chloride  of  sodium,  and 
formula  NaCl.     This  substance  supplies  the  chlorine  ;  the  neces- 
sary hydrogen  is  obtained  from  common  sulphuric  acid  (oil  of 
vitriol),  whose  composition,  as  expressed  in  its  formula  H28O4, 
we  have  already  become  familiar  with. 

The  commercial  acid  is  obtained  by  heating,  in  iron  cylinders, 
common  salt  with  sulphuric  acid  and  absorbing  the  evolved  gas 
in  water  contained  in  a  series  of  stone-ware  Woulfe-bottles.  The 
reaction  is  somewhat  various  according  to  the  proportion  of  sul- 
phuric acid  employed  ;  it  may  be  either  of  the  reactions  ex- 
pressed in  the  following  equations,  or  may  lie  between  them  :  — 

NaCl       +       H2S04  HC1        +         NaHSO4 

Chloride  of          Sulphuric  Chlorhydric          Acid  sulphate 

sodium.  acid.  acid.  of  sodium. 

2  NaCl      +       H2S04        =       2  HC1         +     Na2SO4 

Sulphate  of  sodium. 

In  the  first  reaction,  only  one-half  of  the '  hydrogen  in  each 
molecule  of  sulphuric  acid  is  replaced  by  sodium ;  in  the  second, 
both  atoms  of  hydrogen  are  replaced.  The  first  reaction  requires 
more  sulphuric  acid  than  the  second,  in  proportion  to  the  amount 
of  the  product,  but  is  accomplished  with  less  wear  of  the  appa- 
ratus, because  a  less  heat  sufficesjfor  the  first  than  for  the  second 
reaction. 

102.  If  the  question  suggest  itself,  —  why  not  get  the  hydro- 
gen wanted  from  water,  H2O,  a  much  simpler  and  cheaper  sub- 
stance than  sulphuric  acid,  —  the  only  answer  is,  that  experience 
has  taught  that  water  has  no  action  upon  salt  except  to  dissolve 
it,  while  sulphuric  acid  has  power  to  part  the  two  elements  of 
salt,  and  giving  hydrogen  to  the  chlorine  of  the  salt,  to  accept 
the  detached  sodium  of  the  salt  in  the  place  of  its  own  lost 
hydrogen.     Of  the  nature  of  the  play  of  forces  by  which  this 


CHEMICAL    AFFINITY.  97 

new  adjustment  in  definite  proportions  of  the  atoms  of  five 
elements  is  brought  about,  we  have  no  distinct  conception.  All 
that  we  know  has  been  said,  when  it  is  stated  that  water  works 
no  chemical  change  on  salt,  while  sulphuric  acid  (and  a  few  other 
substances  of  analogous  composition)  does  bring  about  a  very 
essential  change. 

In  the  hope  of  rendering  these  and  similar  facts  more  intelli- 
gible, many  chemists  have  assumed  that  an  element  like  chlorine, 
or  a  group  of  elements  like  sulphuric  acid,  may  possess  a  supe- 
rior 'chemical  attraction,  or  a  greater  affinity,  for  some  elements 
or  groups  than  for  others.  They  would  explain  the  reaction 
between  salt  and  sulphuric  acid  by  saying  that  chlorine  has  a 
greater  affinity  for  hydrogen  than  for  sodium,  while  a  part  of  the 
sulphuric  acid  has  a  stronger  attraction  for  sodium  than  for 
hydrogen  ;  and  in  like  manner  they  would  account  for  the  absence 
of  action  between  water  and  salt  by  saying,  that  the  affinity  of 
oxygen  for  sodium  is  no  stronger  than  that  of  chlorine  for 
sodium.  If  the  second  of  the  equations  above  given  be  written 
after  the  dualistic  theory,  as  follows :  — 

2  NaCl  +  H2O,SO3  —  2  HC1  +  Na20,S03 

we  shall  perceive  the  basis  of  a  still  more  ample  explanation, 
often  given,  of  such  reactions.  The  reaction  above  written  is 
said  to  be  determined,  or  caused,  by  three  affinities: — 1.  The 
affinity  of  the  metal  for  oxygen  ;  2.  The  affinity  of  the  hydrogen 
for  chlorine  ;  3.  The  affinity  of  the  oxide  of  sodium  for  sulphuric 
acid.  It  will  be  at  once  perceived  that  the  contact  of  water  with 
salt  gives  opportunity  ror  the  play  of  the  first  two  affinities  ;  it  is, 
therefore,  the  third  affinity,  superadded  to  the  other  two,  which 
in  this  view  actually  determines  the  decomposition  of  salt  by 
sulphuric  acid. 

Such  speculations  as  these  have  not  been  altogether  fruitless 
in  the  development  of  chemistry,  and  to  some  minds  they  seem 
to  render  the  actual  phenomenon  more  intelligible ;  the  term 
affinity  is  also  sometimes  convenient  in  expressing  the  varying 
intensity  with  which  one  element  grapples  and  holds  other  ele- 
ments, or  groups  of  elements ;  the  student  must  not  fail  to  dis- 
tinguish, however,  between  the  matters  of  fact  and  the  matters 
7 


98 


PREPARATION    OF    CHLORHYDRIC    ACID. 


of  speculation,  in  whatever  stands  written  in  chemical  literature 
touching  affinities  and  their  play.  The  best  use  of  the  ill-chosen 
term  affinity,  is  as  a  synonyme  for  chemical  force  ;  phrases  in 
which  the  term  is  used  in  this  sense  may  contain  simple  state- 
ments of  fact,  but  very  frequently,  especially  when  the  word 
"  elective"  is  coupled  with  it,  the  term  is  used  in  connection  with 
unprofitable  hypotheses.  What  we  actually  know  of  the  re- 
action between  salt  and  sulphuric  acid  is, comprehended  in  the 
statement  that  the  hydrogen  of  the  acid  and  the  sodium  of  the 
salt  change  places  in  the  definite  proportions  by  weight 
which  are  expressed  in  the  atomic  weights  of  the  two  ele- 
ments. 

Commercial  chlorhydric  acid  is  not  pure ;  its  commonest 
impurities  are  sulphuric  acid,  which  gets  mechanically  mixed 
with  the  acid,  iron  derived  from  the  iron  vessels,  arsenic  supplied 
by  the  impure  sulphuric  acid  employed,  the  salts  contained  in  the 
water  which  dissolved  the  gas,  sulphurous  acid,  and,  not  unfre- 
quently,  free  chlorine. 

Exp.  52.  —  Place  60  grammes  of  pulverized  rook-salt  in  a  flask  of  a 
litre  capacity,  provided  with  a  delivery-tube  which  can  be  conveniently 
connected  with  a  series  of  Woulfe-bottles,  such  as  is  represented  in 

FIG.  30. 


Fig.  30,  by  a  caoutchouc  connector.  Pour  100  grammes  of  sulphuric  acid 
upon  the  salt,  and  immediately  cork  the  flask,  place  it  upon  a  sand-bath 
on  the  iron-stand,  and  connect  the  delivery-tube  with  the  Woulfe-bottles. 
The  tubes  by  which  the  gas  enters  the  bottles  should  barely  dip  be- 
neath the  water  contained  in  them,  inasmuch  as  the  solution  of  chlor- 
hydric acid  is  heavier  than  water ;  the  bottles  should  not  be  more  than 


USES    OF    CHLORHYDRIC    ACID.  99 

half  full,  for  the  water  becomes  hot  and  increases  considerably  in  bulk. 
As  hot  water  holds  less  gas  in  solution  than  cold  water,  it  is  not  amiss  to 
place  each  three-necked  bottle  in  a  vessel  of  cold  water.  The  first 
Woulfe-bottle  should  contain  but  a  small  quantity  of  water,  and  the  tube 
coming  from  the  flask  should  not  dip  into  this  water.  The  contents  of  the 
flask  must  be  very  gradually  and  moderately  heated,  else  a  violent 
frothing  is  liable  to  occur,  which  would  spoil  the  experiment.  The 
process  is  like  that  of  making  ammonia-water,  except  that  the  delivery- 
tube  passes  to  the  bottom  of  each  Woulfe-bottle  in  making  ammonia- 
water,  because  the  solution  of  ammonia-gas  is  lighter  than  water,  instead 
of  heavier,  as  is  the  case  with  the  solution  of  chlorhydric  acid  gas.  As 
with  the  ammonia  process,  the  solution  will  be  purer  in  the  second  bottle 
than  in  the  first,  in  the  third  than  in  the  second,  and  so  forth.  The  pure 
acid  thus  obtained  should  be  preserved  for  use  in  experiments  which 
cannot  be  performed  except  with  an  acid  purer  than  the  commercial 

article. 

i 
103.    The  uses  of  chlorhydric  acid  are  very  numerous  ;  it  is 

employed  in  making  chlorine,  chlorate  of  potassium,  and  chloride 
of  lime  (bleaching  powder),  in  preparing  chloride  of  ammonium 
and  chloride  of  tin  ;  in  the  manufacture  of  gelatine  ;  for  dis- 
solving metals,  either  by  itself  or  mixed  with  nitric  acid  ;  it  'is 
one  of  the  most  useful  reagents  in  the  chemical  laboratory. 

Chlorhydric  acid  dissolves  most  metallic  oxides,  and  appears 
to  combine  with  them,  but  on  evaporating  such  a  solution  a  com- 
pound is  obtained  which  contains  neither  hydrogen  nor  oxygen, 
but  only  chlorine  and  the  metal.  When  caustic  soda,  for  ex- 
ample, combines  with  chlorhydric  acid,  chloride  of  sodium  and 
water  are  the  products,  as  exhibited  by  the  equation, 

NaHO     +     HC1    =     NaCl     '+     H2O . 

When  the  black  oxide  of  copper  is  dissolved  in  chlorhydric  acid, 
the  green  liquid  produced  is  an  aqueous  solution  of  chloride 'of 
copper ; 

CuO     +     2HC1    =     CuCl2    +     H20. 

But  though  the  metal  may  exist  in  solution  in  the  form  of  chlo- 
ride, it  is  quite  possible  to  precipitate  it  as  oxide,  if  it  have  an 
insoluble  oxide,  by  adding  to  the  solution  of  the  chloride  a 
soluble  oxide  of  another  metal  capable  of  displacing  the  first. 
Thus,  if  to  a  boiling  solution  of  chloride  of  copper  a  hot  solution 


100  AQUA    REGIA. 

of  caustic  soda  be   added,  the  sodium    and   the   copper  change 
places,  and  the  insoluble  black   oxide   of  copper  is   precipitated. 

CuCl2  -f  2KHO  =  2KC1  +  H2O  +  CuO. 
Chlorhydric  acid  is,  in  fact,  the   chloride  of  hydrogen,  strictly 
analogous  in  composition  to  the  chloride  of  a  metal  like  sodium, 
arid  it  takes  part  in  double  decompositions  like  any  other  chloride. 

104.  Aqua  Regia  (Royal  Water).  This  name  was  given  by 
the  alchemists  to  a  mixture  of  chlorhydric  and  nitric  acids,  be- 
cause of  its  power  to  dissolve  gold,  the  "  king  of  metals." 

Exp.  53. —  Place  a  few  square  centimetres  of  genuine  gold-leaf  at 
the  bottom  of  a  test-tube,  and  pour  upon  the  gold  a  little  strong  chlor- 
hydric acid;  put  some  gold-leaf  in  a  second  test-tube,  and  pour  upon 
ft  a  few  drops  of  nitric  acid ;  neither  acid  attacks  the  gold,  which  re- 
mains undissolved.  If  the  contents  of  the  two  test-tubes  be  mixed 
together  in  either  tube,  the  gold-leaf  will  almost  immediately  dissolve. 

Platinum,  which  like  gold  resists  the  action  of  both  chlorhydric 
and  nitric  acids  singly  applied,  yields  at  once  to  the  mixture  of 
the  two  acids.  Both  these  precious  metals  are  converted  by 
aqua  regia  into  chlorides  soluble  in  water.  Strong  chlorhydric 
acid  is  oxidized  by  strong  nitric  acid,  chlorine,  water,  and  oxides 
of  nitrogen  being  the  products.  This  decomposition  may  be 
represented  by  the  equation 

HC1  +  HN°s  =  Cl  +  H2O  +  NO2. 
The  presence  of  nascent  (§  88)  chlorine  explains  the  energetic 
conversion  of  metals  into  chlorides  by  aqua  regia,  and  the 
strong  oxidizing  effect  of  the  liquid  is  accounted  for  by  the 
presence  of  such  an  unstable  oxide  of  nitrogen  as  hyponitric 
acid.  Aqua  regia  has  indeed  a  very  strong  oxidizing  power  ;  it 
can  change  sulphur  into  sulphuric  acid,  arsenic  into  arsenic  acid, 
and  effect  many  other  similar  oxidations.  Hyponitric  acid  is  so 
unstable  a  substance  that  it  probably  never  really  issues  from  the 
reaction  of  the  two  strong  acids  upon  each  other  ;  a  compound  in 
which  half  the  oxygen  in  hyponitric  acid  is  replaced  by  chlorine 
is,  however,  produced  in  the  later  stages  of  the  decomposition  of 
aqua  regia ;  it  is  a  deep  orange-colored  gas,  called  chloronitrous 
gas,  whose  composition  answers  to  the  formula  NOC1.  The  re- 
action by  which  this  substance  is  generated  may  be  represented 
by  the  equation 


CHL0RINE.   -        ,   .     ,     .  101r 

3HC1  +  HNO8  =  201  +  '  2H2O  +  NOC1. 
The  same  compound  results  from  the  direct  combination  of 
chlorine  and  nitric  oxide.  Still  another  compound  of  nitrogen, 
oxygen,  and  chlorine  is  obtained  by  passing  the  vapors  of  aqua 
regia,  previously  cooled  sufficiently  to  condense  the  aqueous 
vapor  they  contain,  through  a  tube  surrounded  by  ice  and  salt. 
A  heavy  red  liquid,  called  chloro-nitric  acid,  is  condensed,  and 
free  chlorine  escapes.  This  liquid  boils  at  — 7°,  and  has  a  com- 
position answering  to  the  formula  NOC12.  Its  formation  from 
aqua  regia  may  be  rendered  in  symbols  as  follows :  — 

3HC1    +     HN08    =     Cl    +     2H2O    +     NOC12. 
Wlien  brought  in  contact  with  water,  this  substance   is  at  once 
decomposed,  yielding  hyponitric  and  chlorhydric  acids. 

NOC12  +  H2O  =  NO2  +  2HC1. 
It  is  to  be  regretted  that  the  names  by  which  these  two  com- 
pounds of  nitrogen,  oxygen,  and  chlorine  are  generally  known, 
do  not  correspond  with  their  composition  as  expressed  in  their 
formulae.  Other  compounds  of  these  three  elements  exist,  but 
a  description  of  them  would  not  be  appropriately  connected  with 
this  statement  of  the  nature  and  properties  of  aqua  regia. 

Aqua  regia  is  made  by  simply  mixing  the  two  acids,  though 
in  various  proportions,  according  to  the  use  to  be  made  of  it ; 
the  commonest  mixture  is  composed  of  one  part  of  nitric  acid 
and  three  parts  of  chlorhydric  acid. 


CHAPTER    VIII. 

CHLORINE. 

105.  Chlorine  can  readily  be  prepared  from  chlorhydric  acid 
by  removing  the  hydrogen  of  that  acid  by  chemical  means. 

Exp.  54. —  In  a  flask  of  about  500  c.  c.  capacity,  furnished  with  a 
suitable  delivery-tube,  place  8  or  10  grms.  of  coarsely  powdered  black 
oxide  of  manganese ;  pour  upon  it  20  or '30  grms.  of  common  muriatic 
acid,  and  gently  heat  the  mixture.  Chlorine  will  soon  be  disengaged, 


102  .MAKING    CHLORINE. 

•>i.^  «  n  .,  f.  ^  ^  £  .:>     ,-,   ~^  -         ^    T^ ,,         "* 

and  may  be  recognized  by  its  peculiar  color.  Being  very  heavy  the 
gas  may  best  be  collected  by  displacement  in  dry  bottles,  placed  in  the 
open  air,  or  in  a  case  or  box  provided  with  an  efficient  draft.  It  may 
also  be  collected  over  warm  water  or  brine  in  the  water-pan.  It  can- 
not bo  well  collected  over  water  at  the  ordinary  temperature,  since  it 
is  rather  easily  soluble  therein ;  though  the  difficulty  may  be  obviated 
in  part  by  evolving  the  gas  rapidly,  or  by  passing  the  delivery-tube  to 
the  top  of  the  bottle  in  which  the  gas  is  collected.  It  must  not  be 
left  standing  over  water,  since  it  would  soon  be  entirely  absorbed.  In 
experimenting  with  chlorine  care  must  always  be  taken  not  to  inhale  it. 

The  reaction  which  occurs  in  this  experiment  may  be  thus 
formulated  :  — 

Mn02  +  4HC1  =  2H20  +  MnCl,  +  C12 

Black  oxide  of  manganese  is  a  substance  rich  in  oxygen,  which, 
under  certain  conditions,  it  readily  yields  up  to  other  elements. 
In  the  case  before  us,  the  oxygen  of  the  oxide  of  manganese 
unites  with  the  hydrogen  of  the  chlorhydric  acid  to  form  water. 
The  chlorine  of  the  chlorhydric  acid  unites  in  part  with  the 
manganese,  and  is  in  part  left  free. 

In  place  of  the  black  oxide  of  manganese  in  this  experiment, 
several  other  substances  which  readily  give  up  oxygen  may  be 
employed ;  and  instead  of  the  free  chlorhydric  acid  of  the  fore- 
going experiment,  the  mixture  of  common  salt  and  sulphuric 
acid,  which  generates  chlorhydric  acid  (Exp.  52),  is  often  used. 
This  last  is  the  method  commonly  adopted  in  manufacturing 
establishments  where  chlorine  is  generated  on  the  large  scale. 
It  has  the  advantage  of  eliminating  the  whole  of  the  chlorine 
from  the  chlorine  compound  used,  whereas,  in  the  decomposition 
of  the  oxide  qf  manganese  by  chlorhydric  acid  alone,  half  the 
chlorine  remains  combined  with  the  manganese.  Moreover, 
when  present  in  excess,  the  sulphuric  acid  has  the  effect  of  drying 
the  chlorine.  The  reaction  may  be  expressed  as  follows  :  — 

2NaCl  +  2H2SO4  +  MnO2  =  NaaSO4  +  MnS04  +  2H2O  +  2C1 
Another  method,  practised  with  economy  in  some  sulphuric  acid 
works,  is  to  heat  a  mixture  of  common  salt  and  nitrate  of  sodium 
with  an  excess  of  sulphuric  acid.  Chlorhydric  and  nitric  acids 
are  evolved,  and  reacting  upon  one  another,  generate  chlorine, 
hypo-nitric  acid,  and  water :  — 


PROPERTIES    OF    CHLORINE.  103 

HC1  +  HNO3  =  Cl  +  NO2  +  H2O. 

The  hyponitric  acid  is  absorbed  by  sulphuric  acid,  and  subse- 
quently employed  in  the  manufacture  of  sulphuric  acid,  while  the 
chlorine  is  collected  apart  and  employed  in  such  manner  as  may 
be  desired. 

106.  Chlorine  is  an  abundant  element,  and  very  widely  distrib- 
uted in  nature.     It  exists  chiefly  in  combination  with  sodium  as  a 
chloride  of  sodium,  which  is  called  rock-salt  or  sea-salt,  accord- 
ingly as  it  is  found  in  beds  in  the  earth,  or  dissolved  in  the  water 
of  the  ocean.     Every  litre  of  sea-water  will  yield  about  5  litres 
of  chlorine  gas.     Besides  chloride  of  sodium,  sea-water  contains 
small  quantities  of  the  chlorides  of  several  other  metals ;  there 
are  numerous  minerals,  also,  which  contain  chlorine. 

107.  At  the  ordinary  temperature  chlorine  is  a  gas  of  yellow- 
ish-green color,   2.5   times   heavier  than    atmospheric  air.     Its 
specific  gravity  and  atomic  weight  are  35.5.     It  is  excessively 
irritating  and   suffocating,   even   when  inhaled   in    exceedingly 
small   quantities.     Any   attempt   to   breathe   the   undiluted    gas 
would  undoubtedly  be  fatal.     Under  a  pressure  of  4  atmospheres 
at  15°  it  is  condensed  to  a  yellow  mobile  liquid,  having  a  sp.  gr. 
1.33  ;  this  liquid  has  never  yet  been  solidified.     It  is  soluble  to  a 
considerable  extent  in  water  at  the  ordinary  temperature,  1  vol- 
ume of  it  being  dissolved  by  half  a  volume  of  water  at  15°.    This 
solution,  which  exhibits  the   color,  odor,  and  general  chemical 
properties  of  the  gas,  is  called  chlorine-water.     At  low  tempera- 
tures, water  dissolves  a  still  greater  proportion  of  chlorine,  and  at 
0°  a  definite  hydrate  of  chlorine,  C1.5H2O,  crystallizes  out. 

Exp,  55. —  Fill  with  water  the  body  of  a  retort  of  the  capacity  ot 
500   c.  c.,    and   without   tubulature. 

T  i  •  rlG.  ol. 

Invert  the  retort  and  set  it  upon  a 
ring,  or  upon  a  bed  of  sand,  with 
the  neck  pointed  upwards  in  such 
manner  that  no  air  shall  enter  the 
body.  From  a  flask  in  which  chlo- 
rine is  being  generated  pass  a  long 
delivery-tube  down  the  neck  of  the 
retort  to  the  water,  so  that  the  chlo- 
rine may  slowly  bubble  through  the 
water.  The  absorption  of  the  gas  may  be  promoted  by  gently  shaking 


104  CHLORINE-WATER. 

the  retort  from  time  to  time.  As  soon  as  the  water  becomes  saturated 
with  chlorine,  so  much  gas  will  collect  in  the  retort  that  the  liquid  will 
be  pressed  out  of  the  body  and  will  flow  over  from  the  neck ;  when 
this  occurs  the  operation  may  be  stopped. 

At  the  beginning  of  the  experiment,  before  all  the  atmospheric  air 
has  been  expelled  from  the  flask  in  which  the  chlorine  is  generated,  it 
is  well  not  to  push  the  gas  delivery-tube  completely  to  the  bottom  of 
the  neck  of  the  retort,  but  to  simply  immerse  it  in  the  edge  of  the 
water,  so  that  none  of  the  escaping  bubbles  of  gas  shall  enter  the  body 
of  the  retort  until  it  has  become  evident  that  nothing  but  pure  chlorine 
is  coming  over ;  the  tube  may  then  be  immersed  more  deeply. 

The  water  saturated  with  chlorine  should  be  transferred  to  a  bottle 
and  preserved  for  future  use.  It  may  be  employed,  more  conveniently 
than  the  gas,  to  illustrate  many  of  the  properties  of  the  element. 

In  sunlight,  or  even  in  ordinary  daylight,  chlorine  water  suffers  de- 
composition (see  §  113),  but  in  the  dark  it  undergoes  no  change.  It 
should  be  kept,  therefore,  either  in  a  cellar  or  tight  closet,  or  in  a  stone- 
ware bottle,  or  in  a  bottle  of  black,  red,  or  yellow  glass,  or  in  one 
covered  with  black  paper. 

Through  the  blackened  glass  no  light  can  penetrate  to  the  chlorine- 
water,  and  through  red  or  yellow  glass  few,  if  any,  of  the  so-called 
chemical  or  actinic  rays  can  pass.  The  violet  rays  of  the  spectrum  are 
those  which  exhibit  actinic  power,  and  these  are  stopped  by  red  or 
yellow  glass,  which  is  red  or  yellow  because  it  permits  the  passage  of 
only  these  colored  rays. 

108.  Chlorine  is  a  powerful  chemical  agent.  It  combines 
with  hydrogen  with  explosive  violence  upon  being  heated,  or 
even  on  being  exposed  to  sunlight. 

Exp.  56.  —  In  a  soda-water  bottle,  which  must  be  screened  from 
strong  light  by  wrapping  it  in  a  towel,  unless  direct  and  reflected  sun- 
light be  excluded  from  the  room,  mix  equal  volumes  of  chlorine  and 
hydrogen,  then  remove  the  cork  and  hold  the  mouth  of  the  bottle  in 
the  flame  of  a  lamp.  A  sharp  explosion  will  ensue.  Or  the  mixture 
may  be  made  in  a  phial  of  white  glass  rolled  up  in  a  thick  towel  and 
filled  in  a  darkened  chamber.  The  explosion  can  then  be  brought 
about  by  carefully  rolling  the  phial  out  of  its  envelope  into  a  ray  of 
sunlight,  in  a  place  where  the  fragments  of  glass  can  do  no  harm.  In 
this  last  modification  of  the  experiment  the  phial  is,  of  course,  left 
corked.  The  operator  should  stand  behind  a  window-shutter  or  other 
suitable  screen. 

Still  another  method  is  to  place  the  bottle  in  a  shady  place,  and  by 


CHLORINE    UNITES    WITH    METALS.  105 

means  of  a  looking-glass  reflect  upon  it  a  ray  of  sunlight.     The  moment 
the  beam  touches  it,  the  bottle  will  explode. 

A  mixture  of  the  two  gases  may  be  kept  in  the  dark  for  any 
length  of  time  without  change  ;  in  diffused  daylight  they  usually 
unite  only  slowly  and  gradually,  but  in  direct  sunlight  the  union 
is  so  instantaneous  as  to  be  attended  with  explosion. 

109.  Chlorine  combines  also  very  readily  with  many  of  the 
metals,  the  combination  being  in  several  instances  attended  with 
evolution  of  light. 

Exp.  57. —  Fill  a  bottle  of  at  least  half  a  litre  capacity  with  dry 
chlorine  gas,  by  displacement ;  the  gas  should  be  dried  by  passing  it 
through  a  tube  filled  with  chloride  of  calcium,  as  described  in  the  Appen- 
dix, §  15.  Gradually  sift  a  gramme  or  two  of  very  finely  powdered 
metallic  antimony  into  the  bottle.  The  metal  will  instantly  take  fire 
and  fall  in  a  glowing  state  to  the  bottom  of  the  bottle.  This  fire 
attends  the  formation  of  a  compound  of  chlorine  and  antimony,  a 
portion  of  which  will  be  seen  pervading  the  bottle  as  a  white  smoke. 

This  experiment,  and  indeed  all  experiments  with  chlorine,  should 
be  performed  only  in  places  where  there  is  a  current  of  air  sufficiently 
powerful  to  carry  away  from  the  operator  the  volatile  products  of  the 
reaction,  together  with  any  chlorine  which  may  escape  from  the  bottle. 

As  in  the  case  of  the  union  of  sulphur  with  copper  (Exp.  1), 
so  here  it  will  be  seen  that  burning,  as  commonly  understood,  is 
in  no  wise  peculiar  to  the  union  of  oxygen  with  the  other  ele- 
ments. In  the  act  of  chemical  combination  heat  is  always 
evolved,  and,  of  course,  light  as  well,  if  particles  of  solid  matter 
be  present,  and  become  hot  enough  to  be  luminous. 

Since  oxygen  is  very  abundant,  we  are  more  accustomed  to 
witness  exhibitions  of  its  chemical  action  than  those  of  any  other 
element ;  but  we  must  not,  therefore,  lose  sight  of  the  fact  that 
among  the  elements  there  are  several  which  possess  chemical 
power  as  great  when  brought  into  play,  though  not  as  frequently 
exhibited  as  that  of  oxygen. 

Exp.  58.  —  Into  a  small  dry  bottle  throw  loosely  several  leaves  of  the 
so-called  Dutch  metal,  —  an  imitation  gold-leaf  made  from  an  alloy  of 
the  metals  copper  and  zinc,  —  and  invert  over  it  a  bottle  of  dry 
chlorine.  As  the  heavy  gas  falls  into  the  lower  bottle,  the  chlorine 
attacks  the  metal,  which  becomes  red-hot  for  a  moment,  shrivels  up, 
and  is  converted  into  a  mixture  of  chloride  of  copper  and  chloride  of 


106  CHLORINE  BURNS  IN  HYDROGEN. 

zinc.  Both  these  compounds  are  readily  soluble,  the  chloride  of  copper 
imparting  to  the  water  a  peculiar  green  tinge.  The  term  chloride  is 
used  to  denote  the  combination  of  chlorine  with  another  element,  just 
as  the  term  oxide  denotes  a  compound  of  oxygen. 

110.  A  burning  jet  of  hydrogen,  on  being  introduced  into  a 
jar  of  chlorine,  will  continue  to  burn  with  a  peculiar  green  light, 
the  two  gases  uniting  to  form  chlorhydric  acid. 

Exp.  59. —  From  a  gas-holder  containing  hydrogen,  carry  a  glass 
tube,  No.  6,  outwards  horizontally  a  few  centimetres,  then  downwards 
to  reach  the  bottom  of  a  wide-mouthed  litre  bottle  filled  with  dry 
chlorine,  and  then  bend  the  end,  previously  drawn  to  a  point,  sharply  up- 
ward, that  the  jet  of  hydrogen  may  stream  upwards  through  the  chlorine. 
Light  the  hydrogen  jet,  and  then  insert  it  into  the  bottle  of  chlorine. 

By  reversing  the  experiment,  chlorine  may  just  as  well  be 
burned  in  an  atmosphere  of  hydrogen. 

Exp.  60. —  In  a  small  flask  of  75  or  100  c.  c.  capacity,  provided 
with  a  small  chloride  of  calcium  tube  prolonged  into  an  upright  delivery- 
tube  which  is  drawn  out  to  a  fine  point  at  the  top,  generate  a  free  supply 
of  chlorine.  Inflame  ajar  of  hydrogen,  held  mouth  downwards,  and  press 
it  slowly  down  upon  the  chlorine  flask  so  that  the  orifice  from  which  the 
chlorine  is  issuing  may  be  at  the  centre  of  the  hydrogen  bottle,  in  the 
midst  of  the  gas.  In  passing  through  the  burning  hydrogen  at  the  bottom 
of  the  jar,  the  chlorine  will  be  heated  to  the  temperature  necessary  for 
its  own  inflammation,  and  it  will  continue  to  burn  in  the  hydrogen  in  the 
same  way  that  oxygen  burns  in  hydrogen  under  similar  circumstances. 

111.  The  heat  evolved  during  the  combustion  of  hydrogen  in 
chlorine  is   less   intense   than  that  produced  by  its  union  with 
oxygen.     When  one  gramme  of  hydrogen  is  burned  to  chlor- 
hydric acid,  there  is  disengaged  23783  units  of  heat,  while  34462 
units  of  heat  are  evolved  when  it  burns  to  water. 

112.  As  has  been   seen,  chlorine  is  both  combustible  and  a 
supporter  of  combustion  in  so  far  as  hydrogen  is  concerned,  and 
it  exhibits  a  strong  affinity  for  many  of  the  metals  ;  but  it  does 
not  unite  directly  with  either  oxygen  or  carbon. 

^Exp.  61. —  If  a  burning  taper,  or  a  bit  of  flaming  wood  or  paper,  be 
thrust  into  a  bottle  of  chlorine  gas,  the  flame  will  become  murky,  and 
after  struggling  for  a  moment  will  go  out.  Much  smoke  is  at  the  same 
time  given  off. 

Exp.  62. —  A  bit  of  paper,  attached  to  a  wire,  dipped  in  hot  oil  of 
turpentine  and  then  quickly  plunged  into  a*  bottle  of  chlorine,  will 


COMBUSTION    IN    CHLORINE.  107 

usually  take  fire  spontaneously,  and  burn  with  evolution  of  dense  black 
fumes.  On  account  of  the  volatility  and  ready  inflammability  of  oil  of 
turpentine,  it  should  be  carefully  heated  upon  a  water-bath  (Appendix, 
§  1 7)  in  a  porcelain  dish.  If  by  any  chance  the  turpentine  take  fire  in 
the  dish,  it  can  be  instantly  extinguished  by  covering  the  dish. 

Exp.  63.  —  In  a  tall  bottle  mix  a  quantity  of  ordinary  illuminating- 
gas  with  chlorine.  Place  the  jar  in  an  upright  position,  remove  the 
cover,  and  touch  a  lighted  match  to  the  gas ;  fire  will  be  propagated 
from  above  downwards,  while  clouds  of  smoke  are  evolved. 

The  wax,  wood,  paper,  turpentine,  and  gas  of  the  foregoing 
experiments,  and  indeed  most  of  the  substances  ordinarily  used 
as  combustibles,  contain  hydrogen  and  carbon.  The  hydrogen  of 
these  things  will  burn  in  chlorine,  will  unite  chemically  with  the 
chlorine  to  form  chlorhydric  acid,  but  the  carbon  will  not  thus 
unite  with  chlorine.  Hence  it  is  that  in  the  experiments  in  ques- 
tion, the  combustion  is  at  the  expense  of  the  hydrogen  ;  the 
hydrogen  of  the  candle,  turpentine  and  so  forth  alone  unites  with 
chlorine,  while  the  carbon  is  set  free  as  lamp-black  or  smoke. 

113.  Chlorine  can  even  decompose  water  under  certain  condi- 
tions, taking  away  its  hydrogen,  while  the  oxygen  is  left  free. 
This  occurs;  for  example,  when  a  mixture  of  chlorine  and  aqueous 
vapor  is  passed  through  a  red-hot  glass  or  porcelain  tube  filled 
with  fragments  of  the  same  material.  So,  too,  when  an  aqueous 
solution  of  chlorine  is  exposed  to  light,  the  water  is  gradually 
decomposed,  as  has  been  stated  in  §  107,  oxygen  being  set  free, 
and  chlorhydric  acid  formed. 

4C1  +  2H2O  =  4HC1  +  2O. 

.  32.  Exp.  64.  —  Fill  a  narrow-mouthed  bottle,  ol 

the  capacity  of  at  least  half  a  litre,  with  water 
which  has  been  saturated  with  chlorine  at  a 
comparatively  low  temperature,  —  such  as  is 
readily  obtained  by  immersing  the  receiver  in 
ice-water  during  the  absorption  of  the  gas. 
By  means  of  a  perforated  cork,  or  better, 
a  caoutchouc  stopper,  fit  tightly  to  the  bottle  a 
glass  tube,  No.  6,  bent  twice  at  right  angles, 
one  branch  of  which  shall  be  long  enough  to 
reach  to  the  bottom  of  the  bottle,  while  the  other  arm,  made  much 
shorter  than  the  first,  dips  into  an  open  beaker-glass  half  full  of 
water. 


108  USES    OF    CHLORINE. 

Place  the  apparatus  in  such  a  position  that  it  shall  be  exposed  to  as 
much  direct  sunlight  as  possible.  After  a  time  oxygen  gas  will  begin 
to  collect  at  the  top  of  the  bottle,  and  in  the  course  of  several  hours, 
or  days,  so  much  will  have  collected  that  it  can  be  tested  by  removing 
the  cork  from  the  bottle  and  thrusting  in  a  glowing  splinter.  The 
liquid  displaced  by  the  oxygen  flows  over,  through  the  tube,  into  the 
beaker-glass.  The  chlorhydric  acid  of  course  remains  dissolved  in 
the  water  of  the  bottle. 

114.  The  applications  of  chlorine  in  the  arts  depend  upon  that 
readiness  to  combine  with  hydrogen  which  has  just  been  exem- 
plified. By  virtue  of  this  affinity  for  hydrogen,  chlorine  acts  in- 
directly as  a  powerful  oxidizing  agent.  It  acts  as  a  purveyor  of 
nascent  oxygen,  and  is  hence  a  much  more  efficient  agent  than 
free  oxygen,  such  as  exists  in  tbe  air.  Its  chief  uses  are  for 
bleaching  cotton  goods,  paper  stock,  and  so  forth,  and  for  destroy- 
ing foul  and  unhealthy  emanations. 

Exp.  65.  —  Pour  into  a  test-glass  a  quantity  of  chlorine  water 
(Exp.  55),  drop  into  it  a  small  quantity  of  a  solution  of  indigo,  and 
stir  the  mixture  with  a  glass  rod.  The  blue  color  of  the  indigo  will  be 
immediately  destroyed. 

In  the  same  way  the  color  of  litmus,  cochineal,  aniline- 
purple,  or  of  flowers,  calico,  and  the  like,  can  be  readily 
destroyed  by  immersion  in  chlorine-water  or  in  moist  chlorine 
gas.  The  presence  of  water  is  essential ;  perfectly  dry  chlorine 
will  not  bleach. 

Exp.  66.  — Fill  a  glass  tube,  No.  1,  about  20  c.m.  long,  with  scraps 
of  colored  calico  and  bits  of  paper  which  have  been  written  upon  with 
ink.  Take  care  that  the  tube  and  its  contents  are  perfectly  dry,  and 
that  the  tube, is  closed  at  either  end  with  a  cork,  through  which  passes 
a  short  piece  of  tubing,  No.  6.  Place  the  tube  in  a  vertical  position, 
and  pass  into  it,  from  below,  chlorine  gas  which  has  been  thoroughly 
dried  by  means  of  chloride  of  calcium  (Appendix,  §  15).  The  color- 
ing matters  will  not  be  destroyed  so  long  as  they  remain  dry,  but  if, 
after  the  dry  chlorine  has  been  allowed  to  act  for  a  few  minutes,  a  little 
water  be  poured  in  at  the  top  of  the  tube,  so  that  its  contents  may  be 
wetted,  they  will  be  bleached  at  once. 

115.  Those  coloring  matters  which  are  of  vegetable  or  ani- 
mal origin  are  for  the  most  part  complex  compounds  of  carbon, 
hydrogen,  nitrogen,  and  oxygen.  When  moist  chlorine  is 


HOW    CHLORINE    BLEACHES    AND    DISINFECTS.  109 

brought  into  contact  with  them,  a  somewhat  complicated  reaction 
occurs  ;  a  portion  of  their  hydrogen  is  no  doubt  taken  out  by  the 
chlorine,  but  at  the  same  time  some  of  the  water  which  is  pres- 
ent is  decomposed,  and  its  oxygen  assists  the  disorganization  of 
the  compound  which  is  to  be  destroyed. 

Of  the  hydrogenized  or  carburetted  compounds  exposed  to 
the  action  of  the  nascent  oxygen,  those  which  are  most  complex, 
and  of  which  the  elements  are  held  together  least  firmly,  will  of 
course  be  first  acted  upon,  burned  up,  and  destroyed.  As  a  rule, 
the  coloring  matters  are  far  more  easily  oxidized  than  the  cotton 
cloth ;  hence  they  can  readily  be  removed  by  the  action  of  chlo- 
rine, without  injury  to  the  cloth.  But  if  the  action  of  the  chlorine 
were  to  be  continued  after  the  coloring  matter  had  been  destroyed, 
the  cloth  itself  would  gradually  be  burned  up. 

In  actual  practice,  where  the  duration  of  the  exposure  of  the 
cloth  to  the  chlorine  is  carefully  regulated,  and  the  portions  of 
bleaching  liquor  which  at  first  remain  adhering  to  the  cloth  are 
completely  removed  by  washing  and  by  chemical  treatment,  the 
process  is  perfectly  safe  and  trustworthy  as  regards  cotton  or 
even  linen ;  but  the  animal  fibres,  such  as  wool  and  silk,  are  of 
more  complex  composition  than  cotton  and  linen  ;  they  cannot 
be  bleached  by  chlorine,  since  this  gas  would  attack  and  disorgan- 
ize them. 

116.  In  destroying  noxious  effluvia,  chlorine  either  acts  upon 
them  as  upon  coloring  matters,  or  it  simply  takes  away  hydrogen, 
as  in  the  case  of  sulphuretted  hydrogen  hereafter  to  be  studied. 
Putrid  animal  matter  may  be  rendered  comparatively  odorless, 
by  sprinkling  it  copiously  with  chlorine-water;  hence  it  finds 
some  application  in  inquests  and  judicial  investigations. 

The  energy  with  which  chlorine  seizes  upon  hydrogen  may  be 
further  illustrated  by  causing  chlorine  to  act  upon  ammonia-water. 

Exp.  67.  —  Into  a  glass  tube,  No.  1 ,  about  a  metre  long,  pour  enough 
chlorine-water  to  fill  it  nine-tenths  full,  and  then  ammonia-water  enough 
to  fill  the  remaining  space.  Close  the  tube  with  the  thumb,  invert  it  and 
place  it  in  an  upright  position  upon  the  water-pan.  The  ammonia- 
water,  being  specifically  lighter  than  the  solution  of  chlorine,  will  flow 
upwards  and  become  mixed  with  the  latter  ;  a  reaction  will  immediately 
ensue,  some  of  the  chlorine  will  unite  with  the  hydrogen  of  a  portion 


110  ACTION    OF    CHLORINE    ON    AMMONIA. 

of  the  ammonia,  to  form  chlorhydric  acid,  and  nitrogen  will  be  set  free- 
Numberless  fittie  bubbles  of  this  gas  will  escape  from  the  liquor  and 
collect  at  the  top  of  the  tube,  and  may  be  subsequently  tested  with  a 
burning  match.  The  chlorhydric  acid  formed  unites  with  the  remain- 
der of  the  ammonia  to  form  chloride  of  ammonium. 

8NH3  +  6C1  =  2N  +  6NH4C1. 

By  modifying  the  apparatus  employed  in  the  foregoing  experiment, 
so  that  a  current  of  chlorine  can  be  passed  into  a  vessel  containing 
ammonia- water,  the  evolution  of  nitrogen  can  readily  be  made  con- 
tinuous, and  large  quantities  of  the  gas  may  be  collected.  It  would  be 
an  excellent  and  easy  method  of  preparing  nitrogen  for  use  in  the  labora- 
tory, were  it  not  that  care  must  be  taken  that  the  ammonia  shall  always  be 
present  in  considerable  excess.  If  this  precaution  were  neglected,  there 
might  be  formed,  by  the  action  of  the  chlorine  upon  the  chloride  of 
ammonium,  a  very  dangerous  compound  called  chloride  of  nitrogen. 
As  prepared  by  this  method,  the  nitrogen  is  always  contaminated  with 
a  certain  amount  of  oxygen. 

In  the  foregoing  experiment,  the  chloride  of  ammonium  which 
is  produced  remains  dissolved  in  the  water.  It  may  be  recov- 
ered  by  evaporating  the  water,  or  a  new  portion  of  it  may  be 
prepared  by  mixing  chlorhydric  acid  with  ammonia. 

Exp.  68.  —  Fill  one  half-litre  bottle  with  dry  ammonia-gas,  and  an- 
other with  dry  chlorhydric  acid  gas.  Invert  the  latter,  and  place  it 
over  the  former,  so  that  the  mouth  of  the  upper  bottle  shall  rest  upon 
that  of  the  lower.  The  gases  will  immediately  unite  to  form  solid  chlo- 
ride of  ammonium,  a  dense  white  cloud  of  which  will  fill  the  bottles. 
NH3  +  HC1  =  NH4C1. 

One  volume  of  ammonia  unites  with  one  volume  of  chlorhydric  acid, 
and  the  gases  are  completely  condensed  to  a  white  solid. 

117.  Chloride  of  nitrogen,  the  dangerous  compound  of  chlo- 
rine and  nitrogen  which  has  been  alluded  to  above,  is  formed 
when  chlorine  is  brought  in  contact  with  a  weak  solution  of  chlo- 
ride or  nitrate  of  ammonium  at  the  temperature  of  15°  or  20°. 
As  the  chlorine  is  gradually  absorbed,  yellow  oily  drops  of  chlo- 
ride of  nitrogen  form  upon  the  surface  of  the  liquid,  and  soon 
fall  to  the  bottom  : 

NH4C1  +  6CI  =  4HC1  +  NC18. 
Chloride  of  nitrogen   is  a  volatile   yellow  oil,  bf  peculiar,  pene- 


CHLORIDE    OF    NITROGEN.  Ill 

trating  odor ;  it  is  insoluble  in  water,  and  does  not  congeal  when 
exposed  to  cold.  Its  specific  gravity  is  1.653.  It  decomposes 
very  easily.  Upon  being  heated  to  nearly  100°,  or  touched  with 
any  fat  or  oil,  with  turpentine,  or  with  various  other  substances, 
it  explodes  with  extreme  violence ;  indeed,  it  often  explodes 
spontaneously,  without  any  apparent  cause.  A  single  drop  of  it, 
exploded  upon  a  glass  or  porcelain  dish,  shatters  the  vessel  to 
atoms.  The  preparation  and  handling  of  this  body  require  the 
greatest  caution  ;  it  should  never  be  prepared  by  the  novice  in 
chemistry. 

118.  We  have  heretofore  adduced  experimental  proof  of 
every  proposition  and  statement  so  far  as  was  possible  at  such  a 
stage  of  the  student's  progress.  The  chemical  properties  of  the 
four  elements,  oxygen,  nitrogen,  hydrogen,  and  chlorine,  have 
been  exhibited  by  experiment ;  the  composition  of  many  of  their 
most  important  compounds  has  been  demonstrated  by  analysis,  or 
by  synthesis,  or  by  both  these  methods,  and  the  chemical  proper- 
ties of  these  compounds  have  been  illustrated  by  actual  experi- 
ment. Several  objects  have  been  thus  attained; — first,  experi- 
mental methods  of  research  have  been  illustrated  by  tangible 
examples ;  secondly,  the  foundation,  scope,  and  application  of 
important  laws  of  chemical  combination  have  been  explained  ; 
thirdly,  four  leading  elements  have  been  minutely  studied, — 
hydrogen,  the  standard  atom  and  the  unity  of  specific  gravity 
for  gases,  —  oxygen,  nitrogen,  and  chlorine,  three  widely  dif- 
fused elements,  each  of  which  is  the  first  member  and  proto- 
type of  an  important  group  of  elements,  many  of  whose  proper- 
ties we  shall  hereafter  find  we  have  already  become  acquainted 
with  in  studying  the  prototypes  ;  fourthly,  three  compounds  of 
these  elements  have  been  carefully  studied,  —  chlorhydric  acid, 
water,  and  ammonia,  —  compounds  which  are  not  only  interesting 
in  themselves,  but  of  great  significance  as  types,  or  models,  of 
three  large  groups  of  compounds  whose  properties  we  have 
really  been  studying  while  we  studied  their  types. 

From  this  point  forward  the  student  will  be  asked  to  accept  on 
trust  many  facts,  drawn  from  the  accumulated  stores  of  the 
science  and  resting  on  satisfactory  evidence,  the  full  exposition 
of  which  would  be  both  tedious  arid  inappropriate.  The  subject- 


112 


OXIDES    OF    CHLORINE. 


matter  of  chemistry  is  so  vast  and  various  that  it  will  be  neces- 
sary to  select  from  the  great  mass  of  material  only  the  most 
valuable  portions,  and  to  dwell  on  those  elements  and  compounds 
only,  which  are  of  practical  importance  in  the  useful  arts,  or 
which  are  of  interest  in  connection  with  instructive  theories  or 
recognized  laws  of  the  science. 

119.  Compounds  of  Chlorine  and  Oxygen.  —  Free  chlorine 
does  not  combine  directly  with  free  oxygen.  But  by  resorting 
to  indirect  methods  several  compounds  of  the  two  elements  can 
be  obtained.  As  many  as  five  different  oxides  of  chlorine, 
enumerated  below,  have  been  described,  though  as  yet  some  of 
them  are  known  only  in  combination  with  water  or  other  sub- 
stances, and  not  in  the  free  condition. 


NAMES. 


COMPOSITION. 


FORMULAE. 


By  volume. 

H 

<P 

By  weight. 

Chlorine.   Oxygen. 

Chlorine.               Oxygen. 

Hypochlorous  acid  .  2  vols.-|-l  vol. 
Chlorous  acid       .  .  2  vols.-|-3  vols. 

2 
3 

35.5X2=71                      16 
35.5x2=71     16X3=  48 

C120 
C1203 

Hypochloric  acid  .  .  1  vol.  +2  vols. 
Chloric  acid         .  .  2  vols.-|-5  vols. 

2 

35.6                   16X2=  32 
35.5X2=71    16X6=  80 

C102 

C1203 

Perchloric  acid    .  .  2  vols.-|-7  vols. 

? 

35.6x2=71     16X7=112 

C1207 

120.  Hypochlorous  Acid  (C12O).  —  If  a  small  quantity  of  slaked 
lime  (hydrate  of  calcium)  be  thrown  into  a  bottle  of  chlorine 
gas,  and  the  mixture  be  then  left  to  itself  during  several  hours, 
the  chlorine  will  be  completely  absorbed,  and  there  will  be 
formed  two  compounds,  one  of  which  will  be  found  to  be  hypo- 
chlorite  of  calcium,  the  other  chloride  of  calcium.  The  reaction 
may  be  thus  formulated : 

2CaO  +  4C1  =  CaO,Cl2O  -f  CaCl2 

This  mixture  is  a  substance  much  used  in  the  arts  under  the 
technical  names  "  chloride  of  lime,"  or  "  bleaching  powder ;  "  it 
will  be  again  referred  to  hereafter. 

Hydrated  hypochlorous  acid  can  be  prepared  by  adding  dilute  sul- 
phuric acid,  little  by  little,  to  a  mixture  of  bleaching  powder  and 
water  in  a  retort  which  is  agitated  as  the  acid  is  poured  into  it.  The 
dilute  solution  of  hypochlorous  acid,  thus  obtained,  can  subsequently  be 
distilled  over  into  a  receiver.  No  excess  of  sulphuric  acid  should  be 


PROPERTIES    OF    HYPOCLOROUS    ACID.  113 

added,  lest  a  portion  of  the  chloride  of  calcium  be  decomposed  and 
chlorhydric  acid  evolved ;  for  this  last  acid,  reacting  upon  the  hypo- 
chlorous  acid,  would  destroy  it,  and  chlorine  only  would  be  evolved. 
The  two  reactions  may  be  thus  expressed  : — 

CaO,Cl.,O  +  H2O,SO3  =  CaO,SO3  -f  H2O,CLO 
H2O,C12O  -f       2HC1    =r      2H2O     -f        4C1.' 
Another  method  of  obtaining  a  solution  of  hypochlorous  acid  is  to  pass 
a  current  of  chlorine  into  water  in  which  red  oxide  of  mercury  is 
kept  suspended  by  frequently  agitating  the  liquid. 

2HgO  +  11*0  -f  4C1  =  HgCl2,HgO  +  H20,C120. 
There  are  formed  an  insoluble  oxychloride  of  mercury  readily  separa- 
ble by  filtration  and  a  solution  of  hypochlorous  acid. 

121.  The  aqueous  solution  of  hypochlorous  acid  has  a  yellowish 
color,  an  acrid  taste,  and  a  peculiar  sweet  odor.  When  concen- 
trated it  decomposes  rapidly,  even  if  kept  upon  ice.  Dilute 
solutions  are  more  stable,  but  they  decompose  slowly  upon  being 
boiled. 

Hypochlorous  acid  is  a  powerful  oxidizing  and  bleaching  agent. 
Its  solution  produces  at  once  effects  which  are  only  slowly  ob- 
tained when  chlorine  water  is  employed. 

Anhydrous  hypochlorous  acid  may  be  prepared,  by  removing  the 
water  from  the  aqueous  solution.  To  do  this,  a  bottle  full  of  the  con- 
centrated solution  may  be  inverted  over  mercury,  and  bits  of  dry 
nitrate  of  calcium  passed  up  into  the  bottle  ;  the  nitrate  of  calcium 
will  absorb  the  water  and  leave  the  gas  free.  Or  the  gas  can  be  ob- 
tained directly  by  passing  a  slow  current  of  dry  chlorine  through  a 
tube  filled  with  well-dried  red  oxide  of  mercury,  and  kept  cool  by 
means  of  ice  :  — 

HgO  +   4C1  •=  HgCl2  -f   C1.0. 

The  gas  may  be  collected  by  displacement,  or  over  mercury,  though  it 
gradually  acts  upon  this  metal. 

The  gas  is  of  pale  yellow  color  and  offensive  odor,  somewhat 
resembling  that  of  chlorine.  It  decomposes  very  easily  into  2 
volumes  of  chlorine  and  1  volume  of  oxygen  ;  even  the  warmth 
of  the  hand  is  sufficient  to  decompose  it,  and  it  is  difficult  to 
preserve  it  unchanged  even  for  a  few  hours.  At  low  tempera- 
tures, such  as  are  produced  by  a  mixture  of  ice  and  salt,  the  gas 
condenses  to  a  dark  orange-colored  liquid,  heavier  than  water 
and  very  explosive. 


114  CHLOROUS— HYPOCHLORIC  — CHLORIC    ACIDS. 

122.  Chlorous  Acid,  C12O3,  may  be   obtained  by  deoxidizing 
chloric  acid  by  means  of  nitrous  acid.     When  in  the  Anhydrous 
condition   it  is  a  gas  of  a  yellowish-green  color,  liquefiable  by 
extreme  cold.     It  is  a  dangerous  compound  to  prepare,  since  at 
temperatures    above    57°    it    decomposes,  with    explosion,   into 
chlorine  and  oxygen.     It  is  readily  soluble   in  water,  and  the 
solution  possesses  strong  bleaching  and  oxidizing  properties.     It 
is  a  weaker  acid   than  chloric  acid,  §  124,  but  resembles  it  in 
many  respects.     With   metallic  oxides   it   unites  to  form  com- 
pounds called  chlorites. 

123.  HypocJdoric  Acid,  C1O2.     This  very  explosive  compound 
may  be  prepared  by  gently  heating  a  mixture  of  chlorate  of 
potassium  and  concentrated  sulphuric  acid. 

Finely  pulverized  chlorate  of  potassium  is  added,  little  by  little,  to 
sulphuric  acid,  made  cool  by  a  mixture  of  ice  and  salt  until  a  pasty 
mass  is  produced.  This  mixture  is  placed  in  a  small  retort,  which  is 
then  very  cautiously  heated  with  warm  water.  The  hypochloric  acid 
gas  which  is  evolved  must  be  collected  by  displacement,  since  water 
absorbs  and  mercury  decomposes  it. 

The  gas  is  of  a  bright  yellow  color  and  aromatic  odor.  Upon 
being  exposed  to  daylight  or  to  a  temperature  somewhat  below 
the  boiling  point  of  water,  it  decomposes  into  oxygen  and  chlo- 
rine, the  decomposition  being  usually  attended  with  explosion. 
The  preparation  of  the  gas  is  dangerous,  and  should  never  be 
attempted  unless  upon  a  very  small  scale.  At  the  temperature 
of  a  mixture  of  ice  and  salt,  the  gas  condenses  to  a  ^yellow,  highly 
explosive  liquid. 

124.  Chloric  Acid,  C12O5 .  In  the  present  state  of  science  this 
is  the  most  important  of  the  compounds  of  oxygen  and  chlorine. 
It  is  not  known  in  the  free  state,  and  in  the  hydrated  condition 
has  never  been  obtained  with  less  than  I  molecule  of  water, 

HACIA. 

When  a  current  of  chlorine  is  made  to  flow  into  a  cold  dilute  aqueous 
solution  of  caustic  potash,  a  mixture  of  hypochlorite  and  of  chloride  of 
potassium  is  produced : 

2K2O  -f   4C1  =  K2O,CLO   -f   2KC1; 

the  reaction  being   analogous   to   that   between   lime   and   chlorine, 
described  in  §  120.     But  if  the  conditions  as  to  the  concentration  and 


PERCHLORIC    ACID.  115 

temperature  of  the  solution  of  potash  be  changed  ;  if,  instead  of  using  a 
dilute  solution,  chlorine  be  passed  into  a  moderately  strong  hot  solution 
of  caustic  potash,  or  of  carbonate  of  potassium,  hypochlorous  acid  will 
no  longer  be  formed,  but  instead  of  it  chloric  acid.  The  reaction 
be  expressed  as  follows  :  — 

6K2O  -f   12C1  =  K2O,C12O5  -j-   10KC1. 

Chloride  of  potassium  is  formed  as  before,  but  the  remainder  of  the 
chlorine  is  now  more  highly  oxidized.  Chlorate  of  potassium  is  less 
soluble  than  chloride  of  potassium;  it  separates  in  flat  tabular  crystals, 
after  the  liquid  has  been  concentrated  by  evaporation  and  cooled.  It 
is  the  substance  which  was  employed  for  making  oxygen  in  Exp.  7. 

Chloric  acid  could  be  prepared  directly  from  chlorate  of  potassium  by 
boiling  a  solution  of  this  substance  with  fluosilicic  acid.  An  almost  in- 
soluble fluosilicate  of  potassium  would  be  formed,  and  chloric  acid  set 
free.  But  an  easier  method  is  to  first  convert  the  chlorate  of  potassium 
into  chlorate  of  barium,  and  to  liberate  the  chloric  acid  from  this  salt  by 
means  of  sulphuric  acid,  with  which  barium  forms  a  remarkably  in- 
soluble compound :  — 

BaO,Cl3O5  +   H2O,SO3  =  BaO,SO3  -f   H2O,C12O5. 

The  solution  of  chloric  ?cid  is  separated  from  the  insoluble  sulphate 
of  barium  by  filtration,  and  concentrated  by  evaporation  over  sulphuric 
acid  in  the  exhausted  receiver  of  an  air-pump.  By  cautious  evapora- 
tion the  acid  may  be  brought  to  a  syrupy  consistence,  but  is  then 
rather  easily  decomposed,  especially  if  it  be  heated  or  exposed  to  light. 
At  the  temperature  of  boiling  it  is  rapidly  converted  into  perchloric 
acid,  water,  chlorine,  and  oxygen.  It  is  a  strong  acid,  and  a  powerful 
oxidizing  and  bleaching  agent. 

Perchloric  Acid,  C12O7,  is  formed,  as  above  stated,  when 
an  aqueous  solution  of  chloric  acid  is  boiled  ;  being  volatile  it 
may  be  distilled  off  and  collected.  A  compound  of  this  acid  and 
potassium,  perchlorate  of  potassium,  can  be  obtained  by  heating 
chlorate  of  potassium  to  a  certain  temperature. 

Exp.  69. —  Melt  40  or  50  grms.  of  chlorate  of  potassium  in  a  porce- 
lain dish,  and  heat  the  liquid  to  such  a  degree  that  oxygen  shall  be 
slowly  given  off.  If  the  heat  be  controlled  so  that  it  shall  not  increase, 
the  fused  salt  will  soon  become  pasty  ;  and  on  allowing  the  mass  to 
cool,  the  residue  will  be  found  to  consist  principally  of  perchlorate  and 
chloride  of  potassium  :  — 

2(K2O,C12O5)   =  K2O,CloOr  +   2KC1  +  O4. 
At  the  moderate  heat  employed,  only  a  portion  of  the  chlorate  of 


116  BROMINE. 

potassium  decomposes,  and  most  of  the  oxygen  from  this  decomposed 
portion  unites  with  the  remaining  chlorate  of  potassium  to  form  the 
perchlorate.  Perchlorate  of  potassium  being  much  less  soluble  in  water 
than  chloride  of  potassium,  may  be  readily  separated  from  the  chloride 
by  solution  and  re-crystallization. 

From  perchlorate  of  potassium  the  acid  may  be  obtained  by  means 
of  fluosilicic  acid,  the  nearly  insoluble  fluosilicate  of  potassium  being 
filtered  oft'  from  the  solution  of  perchloric  acid  ;  or  the  perchlorate  of 
potassium  may  be  treated  with  sulphuric  acid,  and  the  volatile  per- 
chloric acid  distilled  off  from  the  sulphate  of  potassium. 

Perchloric  acid  is  a  more  stable  compound  than  either  of  the 
other  oxides  of  chlorine.  The  dilute  aqueous  solution  may  be 
concentrated  by  evaporation  over  fire,  even  at  high  temperatures. 
The  hydra  ted  acid  H2O,C12O7  is  a  colorless,  oily  liquid,  which 
boils  at  203°,  and  has  a  specific  gravity  of  1.782.  It  is  a  power- 
ful oxidizing  agent. 

The  student  will  do  well  to  compare  this  series  of  oxides  of 
chlorine  with  that  of  the  oxides  of  nitrogen,  and  to  note  the 
points  in  which  the  two  series  resemble  and  those  in  which 
they  differ  from  each  other. 


CHAPTER    IX. 

BROMINE. 

125.  Bromine  is  an  element  closely  allied  to  chlorine.  It  is 
found  in-  small  quantities  in  sea-water  ;  and  in  the  water  of  many 
saline  springs.  1  litre  of  sea-water  contains  from  0.1005  to 
0.0143  grm.  of  it.  As  it  exists  in  nature  it  is  combined  with 
metals,  bromide  of  magnesium  being  the  compound  most  com- 
monly met  with. 

Bromide  of  magnesium  is  a  constituent  of  the  uncrystallizable 
residue,  called  bittern,  which  remains  after  the  chloride  of  sodium 
lias  been  crystallized  out  from  the  natural  brines  ;  at  several 
saline  springs  this  bittern  contains  so  large  a  proportion  of  the 
bromide  that  bromine  can  be  profitably  extracted  from  it. 

In  order  to  obtain  bromine  from  the  bittern,  the  latter  is  mixed  with 
black  oxide  of  manganese  and  chlorhydric  acid,  and  heated  in  a  retort. 


PROPERTIES    OF    BROMINE.  117 

Chlorine  is  of  course  evolved  from  these  materials  in  the  midst  of  the 
liquid;  it  reacts  upon  the  bromide  of  magnesium  and  sets  free  bromine, 
which  distilp  over  into  the  receiver  as  a  dark-red,  very  heavy  liquid. 

MgBr2   -f    201   =   MgClj,   +    2Br. 

All  the  metallic  bromides  are  readily  decomposed  by  chlorine  ;  bro- 
mine being,  as  a  rule,  a  less  energetic  chemical  agent  than  chlorine. 

126.  At  the  ordinary  temperature  bromine  is  a  liquid  of  dark 
brown-red  color,  about  three  times  as  heavy  as  water,  and  highly 
poisonous.     Its  odor  is  irritating  and  disagreeable,  whence  the 
name  bromine,  derived  from  a   Greek   word  and   signifying  a 
stench.     It  boils  at  about  60°,  but  is  very  volatile  even  at  the 
ordinary  temperature  of  the  air. 

Exp.  70.  —  By  means  of  a  small  pipette,  throw  into  a  flask  or  bottle 
of  the  capacity  of  1  or  2  litres  3  or  4  drops  of  bromine.  Cover  the 
bottle  loosely,  and  leave  it  standing.  In  a  short  time  it  will  be  filled 
with  a  red  vapor,  which  is  bromine  gas.  This  vapor  is  very  heavy, 
more  Ihan  5  times  as  heavy  as  air  and  80  times  heavier  than  hydrogen. 

At  about  7°  bromine  crystallizes  in  brittle  plates.  It  dissolves 
sparingly  in  water,  but  is  soluble  in  alcohol,  and  in  all  proportions 
in  ether. 

127.  In  its  chemical  behavior,  as  well  as  in  many  of  its  physi- 
1  properties,  bromine  closely  resembles  chlorine.     Its  affinity 
r  hydrogen,  though  weaker  than  that  of  chlorine,  is  still  power- 

ul.  Like  chlorine,  it  is  an  energetic  bleaching  and  disinfecting 
agent,  and  it  decomposes  the  vapor  of  water  when  passed  with  it 
through  a  tube  heated  to  bright  redness,  bromhydric  acid  and 
oxygen  being  the  products  of  the  reaction.  A  lighted  taper 
burns  for  an  instant  in  bromine  vapor  and  is  then  extinguished. 
Phosphorus,  antimony,  potassium,  and  the  like,  take  fire  on  be- 
ing thrown  into  bromine,  in  the  same  way  as  in  chlorine,  a 
bromide  of  the  other  element  being  produced. 

Exp.  71. —  To  a  wide-mouthed  litre  bottle  fit  a  thin  cork.  Perfo- 
rate this,  and  through  the  hole  pass  a  tube  of  thin  glass,  No.  2,  closed 
at  one  end.  The  tube  should  reach  nearly  to  the  bottom  of  the  bottle, 
and  ^should  project  two  or  three  inches  above  the  cork.  Within  the 
tube  place  a  few  drops  of  bromine  ;  throw  in  upon  this  a  very  small 
quantity  of  finely  powdered  antimony,  and  instantly  cover  the  mouth 
of  the  tube  with  an  inverted  crucible  or  wide-mouthed  phial,  in  order 
that  nothing  may  be  thrown  out  of  the  tube  by  the  violent  action 


118  BROMHYDRIC    ACID. 

which  attends  the  combination.     If  the  tube  be  broken,  its  fragments 
will  be  retained  within  the  bottle. 

Bromine  is  used  to  a  certain  extent  in  medicine,  ahd  largely 
in  photography.     In  the  chemical    laboratory  it  is  often    em- 
ployed, not  only  for  its  own  sake,  but  as  a  substitute  for  chlorine  ; 
for  though  less  energetic  it  is  more  manageable  than  the  latter, 
^especially  in  those  cases  where  a  liquid  is  desirable. 

128.  Bromhydric  Acid,  (HBr).  —  In  spite  of  the  strong  affin- 
ity of  bromine  for  hydrogen   these  elements  cannot  readily  be 
made  to  unite  directly.     A  mixture  of  equal  volumes  of  hydro- 
gen and  bromine  vapor  cannot  be  made    to  combine  with  ex- 
plosion by  exposure  to  the  sun's  rays,  or  by  the  introduction  of  a 
burning  lamp,  though  a  certain  amount  of  combination  occurs  in 
the  immediate  neighborhood   of  the  flame.     But  by  immersing 
in  the  mixture  a  platinum  wire,  kept  red-hot  by  a  galvanic   cur- 
rent, the  two  elements  may  be  made  to  unite  slowly,  and  a  simi- 
lar result  is  obtained  by  passing  the  mixed  gases  through  a  red- 
hot  tube.     Bromhydric  acid  gas   can  however,  be  readily  pre- 
pared by  decomposing  bromide  of  potassium  with  sulphuric  acid, 
or,  better,  with  a  concentrated  solution  of  phosphoric  acid.     The 
reaction  is  analogous  to  that  in  which  chlorhydric  acid  is  ob- 
tained from  chloride  of  sodium  : 

2KBr  +  H2O,SO3  —  K2O,S03  +  2HBr. 
If  sulphuric  acid  were  employed,  there  would  occur  a  secondary 
reaction ;  a  small  part  of  the  bromhydric  acid  would  suffer  de- 
composition, and   the  product  would   be   slightly  .contaminated 
with  free  bromine  and  with  sulphurous  acid. 

2EBr  +  H2O,SO3  —  2H2O  +  2Br+S02. 

Since  phosphoric  acid  does  not  thus  decompose  bromhydric 
acid,  the  latter  can  be  obtained  in  a  state  of  purity  by  distilling  a 
mixture  of  bromide  of  potassium  and  phosphoric  acid. 

129.  Bromhydric  acid  is  a  colorless,  irritating  gas,  which  on 
coming  in  contact  with  the  moisture  of  the  air,  fumes  even  more 
strongly  than  chlorhydric  acid.     By  powerful  pressure  it  can  b<j 
reduced  to  the  liquid  condition,  and  upon  being  exposed  to  intense 
cold  it  may  be  obtained  in  the  form  of  a  crystalline  solid. 

It  is  readily  soluble  in  water,  forming  a  strongly  acid  solution 


BROMIC    ACID.  119 

which  resembles  chlorhydric  acid  in  many  respects,  and,  like  it, 
fumesln  the  air.  A  ready  method  of  preparing  the  solution  is 
to  decompose  a  strong  solution  of  bromide  of  barium  with  sul- 
phuric acid  diluted  with  its  own  weight  of  water.  The  solution 
of  the  free  acid  may  then  be  separated  from  the  insoluble  sul- 
phate of  barium  by  filtration  or  by  distillation. 

When  the  solution  of  this  acid  is  mixed  with  nitric  acid,  there 
is  obtained  another  aqua  regia  capable  of  dissolving  gold  and 
platinum  like  the  mixture  of  chlorhydric  and  nitric  acids,  though 
less  readily. 

130.  The  gas  undergoes  no  change  when  passed  through  a 
red-hot  tube  ;  but  it  is  readily  decomposed  by  metals  like  potas- 
sium at  the  ordinary  temperature,  and  by  tin  gently  heated.     A 
bromide  of  the  metal  is  formed  in  either  case,  and  there  remains 
a  volume  of  hydrogen  equal  to  half  that  of  the  original  gas.    Ob- 
servation has  shown  that  the  specific  gravity  of  the  gas  is  very 
nearly  40.5,  or  half  the  sum  of  the  specific  gravities  of  bromine 
vapor  and  hydrogen.     From  this  fact  and  the  above  decompo- 
sition of  the  gas  by  metals,  it  follows  that  bromhydric  acid  is 
composed  of  equal   volumes  of    bromine  and  hydrogen  united 
without  condensation. 

131.  Bromic  Acid,   H2O,Br2O5.     Only  one  oxide  of  bromine 
has  been  studied,  and  even  this  has  never  been  obtained  free 
from  water.     It  is  bromic  acid,  a  substance   corresponding  to 
chloric  acid  in  composition  and  properties.     Its  compounds,  also, 
known  as  bromates,  generally  resemble  very  closely  the  corre- 
sponding chlorates. 

Bromate  of  potassium  can  be  obtained  by  the  action  of  bromine 
upon  potash-lye,  in  the  same  way  that  chlorate  of  potassium  is  obtained 
by  the  action  of  chlorine  :  — 

6K2O  -f  12Br  =  K2O,Br2O5  -f  lOKBr. 

The  bromate,  which  is  less  soluble  than  the  bromide,  can  subse- 
quently be  separated  by  crystallization.  In  order  to  obtain  the 
hydrated  acid,  bromate  of  barium  may  be  decomposed  with  dilute 
sulphuric  acid :  — 

BaO,Br2O5  -f  H2O,SO3  =  BaO,SO3  -f  H,,O,Br2O5. 

The  insoluble  sulphate  of  barium  is  separated  by  filtration.  The  acid 
solution  can  be  concentrated  to  a  certain  extent  by  evaporation  at  a 


120  HYPOBROMOUS    ACID. 

gentle  heat,  but  cannot  readily  be  brought  to  a  syrupy  consistency 
without  decomposition.  It  decomposes,  also,  on  being  heated  to  100, 
and  in  general  gives  up  oxygen  on  being  brought  in  contact  with  sub- 
stances which  readily  combine  with  that  element. 

132.  ffypobromous  Acid,  H2O,Br2O  ?     There    can    be    little 
doubt  of  the  existence  of  an  oxide  of  bromine  corresponding  to 
hypochlorous  acid,  though  neither  the  acid  itself  nor  any  of  its 
salts  have  yet  been  obtained  in  definite  form.     When  cold  dilute 
alkaline  solutions  are  mixed  with  bromine  they  acquire  a  power 
of  bleaching,  and  in  general  behave  like  the  alkaline  hypochlo- 
rites,  which  are  formed  under  similar  conditions.     So  too  when 
bromine-water  is  treated  with  red  oxide  of  mercury,  a  sparingly 
soluble  oxybromide  of  mercury  is  formed,  together  with  a  bleach- 
ing liquor  supposed  to  contain  hypobromous  acid. 

The  analogies  which  subsist  between  chlorine  and  bromine  are, 
however,  everywhere  so  clearly  defined  that  there  is  good  reason 
to  believe  that  other  oxides  of  bromine,  corresponding  to  those 
of  chlorine,  will  be  sooner  or  later  discovered. 

133.  Chloride  of  Bromine.     Liquid  bromine  absorbs  a  large 
quantity   of  chlorine-gas   when   the  two  elements  are  brought 
together,  and  there  is  formed  a  very  volatile  liquid  called  chloride 
of  bromine.     It  exhales  a  pungent,  irritating  odor,  and  is  solu- 
ble in    water ;    the   solution   possesses    considerable    bleaching 
power. 

134.  Bromide  of  Nitrogen  is  an  explosive  compound  analogous 
to  chloride  of  nitrogen,  from  which  it  may  be  prepared  by  means 
of  bromide  of  potassium. 


CHAPTER    X. 

IODINE. 

135.  In  its  chemical  properties  iodine  bears  a  striking  resem- 
blance to  bromine,  and  consequently  to  chlorine  also.  It  exists 
in  sea-water  and  in  the  water  of  many  saline  and  mineral  springs. 
The  proportion  of  iodine  in  sea- water  is  exceedingly  small,  being 


EXTRACTION    OF    IODINE.  121 

even  smaller  than  that  of  bromine.  But  iodine  is  obtained  more 
readily  than  bromine  ;  for  iodine  is  absorbed  from  sea-water  by 
various  marine  plants,  which,  during  their  growth,  collect  and 
concentrate  the  minute  quantities  of  iodine  which  the  sea-water 
contains,  to  such  an  extent  that  it  can  be  extracted  from  them 
with  profit. 

Upon  the  coasts  of  Scotland,  Ireland,  and  France,  where  labor  is 
cheap,  the  sea-weeds  which  contain  iodine  are  collected,  dried,  and 
burned,  at  a  low  heat,  in  shallow  pits.  The  half-fused  ashes  thus 
obtained,  called  kelp  or  varec  contains,  among  other  things,  iodine 
in  the  form  of  iodide  of  sodium  and  iodide  of  potassium.  It  is  lix- 
iviated with  boiling  water,  and  the  solution  is  then  evaporated  and 
set  aside  to  crystallize.  The  iodides,  being  much  more  soluble  than 
the  other  constituents  of  the  ash,  remain  dissolved  in  the  mother- 
liquor  after  most  of  the  other  salts  have  crystallized  out.  If  this 
mother-liquor,  or  iodine-lye,  be  now  mixed  with  a  small  quantity  of 
sulphuric  acid  and  left  to  itself  for  a  day  or  two,  it  may  be  freed 
from  a  further  portion  of  impurity  ;  it  is  then  transferred  to  a 
leaden  retort,  mixed  with  a  suitable  quantity  of  powdered  black 
oxide  of  manganese,  and  gently  heated.  Iodine  is  set  free,  just  as 
chlorine  would  be  from  chloride  of  sodium  under  similar  circumstances, 
and  may  be  collected  in  appropriate  receivers :  — 

2NaI+  2(HAS08)  +  MnO,  =  NaASO,-f  MnO,SO,+  2H,O  + 12. 

136.  At  the  ordinary  temperature  iodine  is  a  soft,  heavy, 
crystalline  solid  of  bluish-black  color  and  metallic  lustre.  Its 
specific  gravity  is  4.948.  It  melts  at  a  temperature  (107°)  a 
little  above  that  at  which  water  boils,  and  boils  at  a  somewhat 
higher  temperature  (178° -180°);  but  in  spite  of  this  high 
boiling  point  it  evaporates  rather  freely  at  the  ordinary  tem- 
perature of  the  air,  and  the  more  rapidly  when  it  is  in  a  moist 
condition.  It  may  be  entirely  volatilized  by  heating  it  upon 
writing-paper.  Its  odor  is  peculiar,  somewhat  resembling  that 
of  chlorine,  but  weaker,  and  easily  distinguished  from  it.  Its 
atomic  weight  is  127. 

The  vapor  of  iodine  is  of  a  magnificent  purple  color,  whence 
the  name  iodine,  derived  from  a  Greek  word  signifying  violet- 
colored  ;  it  is  very  heavy,  indeed  the  heaviest  of  all  known 
gases  ;  it  is  nearly  9  times  as  -heavy  as  air.  The  specific  gravity 
of  the  vapor  is  127. 


122  PROPERTIES    OF  IODINE, 

Exp.  72. —  Hold  a  dry  test-tube  in  the  gas-lamp  by  means  of  the 
wooden  nippers,  and  warm  it  along  its  entire  length,  in  so  far  as  this 
is  practicable.  Drop  into  the  hot  tube  a  small  fragment  of  iodine,  and 
observe  the  vapor  as  it  rises  in  the  tube.  Jf  only  a  small  portion  of 
the  tube  were  heated,  the  vapor  would  be  deposited  as  solid  iodine 
upon  the  cold  part  of  its  walls. 

137.  Solid  iodine  is  never  met  with  in  the  amorphous,  shape- 
less state  in  which  glass,  resin,  coal,  and  many  other  substances 
occur.     No  matter  how  obtained,  its  particles  always  exhibit  a 
definite  crystalline  structure.     If  the  iodine  be  melted  and  then 
allowed  to  cool,  or  if  it  be  converted  into  vapor  and  this  vapor 
be   subsequently  condensed,   crystals   will  be   formed  in   either 
case.     Perfect  crystals   can  be    still  more   readily  obtained  by' 
dissolving  iodine  in  an  aqueous  solution  of  iodohydric  acid,  and 
exposing  this  solution  to  the  air  in  a  narrow-necked  or  loosely 
stoppered  bottle;  the  iodohydric  acid  will  be  slowly  decomposed 
by  the  action  of  the  atmospheric  oxygen,  and,  as  it  decomposes, 
well-defined  crystals  of  iodine  will  be  deposited. 

In  whichever  way  prepared,  the  crystals  of  iodine  are  octo- 
hedrons  with  a  rhombic  base,  belonging  to  the  crystalline  system 
called  trimetric.  As  commonly  seen,  the  crystals  are  thin,  flat- 
tened tables,  distorted  by  excessive  elongation  in  one  direction. 

138.  Iodine  is  scarcely  at  all  soluble  in  water,  though  enough 
dissolves  to  impart  a  brown  color  to  the  water ;    but  it  dissolves 
readily  in  alcohol  and  ether.     These  solutions  are  much  used  in 
medicine,  particularly  the  alcoholic  solution  which  is  called  tinc- 
ture of  iodine.     When  swallowed  in  the  solid   state,,  iodine   acts 
as  an  energetic  corrosive  poison ;  but  several  of  its  compounds, 
and  the  element  itself  when   taken   in   small   doses,   are  highly 
prized  as  medicaments.     It  is  also  largely  employed  in  photo- 
graphy, and  is  a  useful  reagent  in  the  chemical  laboratory. 

139.  As  has  been  already  stated,  iodine,  in  its  chemical  be- 
havior, resembles  chlorine  and   bromine,   only  its  affinities   are 
more  feeble.     It  enters  into  combination  with  less   energy  than 
either  of  these  elements,  and  is  displaced  by  them  from  most 
of  its    combinations.     Like   them,   it   unites   directly  with    the 
metals  and  with  several  other  elements.     It  gradually  corrodes 
organic  tissues,  and  destroys  coloring  matters,  though  but  slowly. 


TESTING    FOR    IODINE.  123 

No  oxygen  is  given  off  from  the  aqueous  solution  when  this  is 
exposed  to  sunlight,  but  the  color  of  the  solution  slowly  disap- 
pears, and  a  mixture  of  iodohydric  and  iodic  acids  is  formed  in  it. 
A  singular  property  of  iodine  is  its  power  of  forming  a  blue 
compound  with  starch. 

Exp.  73.  —  Prepare  a  quantity  of  thin  starch  paste  by  boiling  30  c.c. 
of  water  in  a  porcelain  dish,  and  stirring  into  it  0.5  grm.  of  starch 
which  has  previously  been  reduced  to  the  consistence  of  cream  by 
rubbing  it  in  a  mortar  with  a  few  drops  of  water. 

Pour  3  or  4  drops  of  the  paste  into  10  c.  c.  of  water  in  a  test-tube 
and  shake  the  mixture  so  that  the  paste  may  be  equably  diffused 
through  the  water,  then  add  a  drop  of  an  aqueous  solution  of  iodine, 
and  observe  the  beautiful  blue  color  which  the  solution  assumes.  If 
the  solution  be  heated  the  blue  coloration  will  disappear,  but  it  reap- 
pears when  the  liquid  is  allowed  to7cool. 

Dip  a  strip  of  white  paper  in  the  starch-paste  and  suspend  it,  while 
still  moist,  in  a  large  bottle,  into  the  bottom  of  which  two  or  three 
crystals  of  iodine  have  been  thrown.  As  the  vapor  of  iodine  slowly 
diffuses  through  the  air  of  the  bottle  it  will  at  last  come  in  contact 
with  the  starch,  and  after  some  minutes  the  paper  will  be  colored  blue. 

This  reaction  furnishes  a  very  delicate  test  for  iodine.  By 
its  means  it  has  been  proved  that  iodine,  though  nowhere  very 
abundant,  is  very  widely  distributed  in  nature ;  traces  of  it  have 
been  detected  in  land  plants,  and  in  many  well,  river,  and  spring 
waters ;  also  in  rain  water,  and  even  in  the  air ;  indeed  it  would 
be  difficult  to  say  where  iodine  is  not. 

In  order  that  it  may  be  detected  by  this  test,  the  iodine  must 
be  free,  or  uncombined.  But,  as  has  been  stated,  chlorine  readily 
expels  iodine  from  most  of  its  combinations.  In  case,  then,  we 
have  reason  to  suspect  the  presence  of  a  compound  of  iodine  — 
iodide  of  potassium,  for  example  —  in  any  substance,  a  small 
quantity  of  chlorine-water,  or  of  some  other  agent  capable  of 
expelling  iodine,  must  be  added  to  this  substance.  Once  dis- 
placed from  its  combination,  the  iodine  may  be  at  once  detected 
by  means  of  starch. 

Exp,  74. —  Place  in  a  test-tube  10  c.  c.  of  water,  a  drop  of  a  con- 
centrated aqueous  solution  of  iodide  of  potassium,  and  3  or  4  drops  of 
the  starch-paste  of  Exp.  73.  If  the  iodide  of  potassium  be  pure,  no 
coloration  will  occur.  Add  now  2  or  3  drops  of  chlorine- water, 


124  DELICACY    OF    THE    IODINE-TEST- 

and  shake  the  tube.  The  characteristic  blue  coloration  at  once 
appears. 

In  order  to  illustrate  the  extreme  delicacy  of  this  reaction,  dissolve 
0.14  grm.  of  iodide  of  potassium  in  1  litre  of  water,  and  to  this  solu- 
tion, which  contains  1  part  of  iodine  in  10,000  parts  of  water,  add  some 
of  the  starch-paste  and  several  drops  of  red  fuming  nitric  acid,  a  reagent 
on  some  accounts  better  fitted  than  chlorine  to  disengage  iodine  in  this 
experiment  (see  §  150).  After  a  time  the  solution  will  exhibit  the  blue 
color,  though  in  solutions  so  dilute  as  this  it  sometimes  happens  that 
the  coloration  appears  only  after  the  lapse  of  several  hours. 

It  follows,  of  course,  from  the  foregoing  experiment,  that  the 
reaction  of  iodine  upon  starch  can  be  used  as  a  test  for  those 
substances  which,  like  chlorine  or  nitric  acid,  are  capable  of  set- 
ting free  iodine,  as  well  as  for  iodine  itself.  In  the  chemical 
laboratory  it  is  customary  to  keep  on  hand  for  this  purpose  a 
store  of  paper  upon  which  has  been  spread  a  mixture  of  starch- 
paste  and  iodide  of  potassium,  prepared  as  follows  :  — 

Exp.  75.  —  Dissolve  1  grm.  of  pure  iodide  of  potassium  (free  from 
iodate)  in  200  c.  c.  of  water;  stir  into  the  solution  10  grins,  of  finely 
powdered  starch,  and  heat  the  mixture  moderately  in  a  porcelain  dish, 
taking  care  not  to  burn  the  starch,  and  stirring  until  the  mass  gelatin- 
izes. Remove  the  lamp,  allow  the  paste  to  become  cold,  and  by  means 
of  a  wooden  spatula  spread  it  thinly  upon  one  side  of  white  glazed 
paper.  The  paper  is  then  dried,  cut  into  strips  about  8  c.  m.  long  by  2 
wide,  and  preserved  in  stoppered  bottles  kept  carefully  closed. 

Exp.  76.  —  Place  in  a  test-tube  a  small  quantity  of  binoxide  of  man- 
ganese, pour  upon  it  4  or  5  c.  c.  of  chlorhydric  acid,  heat  the  mixture,  and 
hold  at  the  top  of  the  tube  a  moistened  strip  of  the  test-paper  which  was 
prepared  in  the  preceding  experiment.  The  chlorine  evolved  by  the 
reaction  of  the  chlorhydric  acid  upon  the  binoxide  of  manganese  sets 
free  iodine  from  the  iodide  of  potassium  upon  the  test  paper,  and  the 
starch  is  thereby  colored  blue.  The  presence  of  chlorine  in  chlor- 
hydric acid  is  thus  made  apparent.  By  this  test  we  might  discriminate, 
for  example,  between  dilute  nitric  and  chlorhydric  acids. 

140.  lodobydric  Acid,  (HI),. —  Hydrogen  and  iodine  do  not 
readily  unite  together  directly.  There  is  here  nothing  to  recall 
the  explosive  violence  with  which  chlorine  and  hydrogen  com- 
bine. Sunlight  has  no  power  to  bring  about  the  union  of  the 
two  elements  at  the  ordinary  temperature  ;  but  when  a  mixture 
of  hydrogen  gas  and  iodine  vapor  is  passed  through  a  red-hot 


IODOHYDRIC    ACID.  125 

tube,  iodohydric  acid  is  formed.  It  has  been  observed,  also,  that 
spongy  platinum  will  cause  the  union  of  the  two  elements  even 
at  ordinary  temperatures.  Even  when  indirect  methods  are  re- 
sorted to,  it  is  less  easy  to  prepare  iodohydric  acid  than  chlor- 
hydric  or  bromhydric  acids. 

If  iodide  of  sodium  be  distilled  with  sulphuric  acid,  there  will  be  ob- 
tained but  little  iodohydric  acid  ;  for  most  of  that  which  is  produced 
at  first  will  be  subsequently  destroyed  by  the  action  of  sulphuric  acid 
in  the  same  way  as  happens  to  a  less  extent  with  bromhydric  acid, 
§128. 

As  fast  as  iodohydric  acid  is  formed  in  accordance  with  the  reaction, 

2NaI  -f  H2SO4  =  Na,SO4  +  2HI , 

most  of  it  is  decomposed  by  another  portion  of  sulphuric  acid,  in  a 
manner  which  may  be  thus  represented :  — 

2HI  +  H2SO4  =  2H2O  +  SO,  -f  21. 

Solutions  of  iodohydric  acid  can,  however,  be  readily  obtained  by  the 
action  of  iodine  upon  a  compound  of  sulphur  and  hydrogen,  called 
sulphydric  acid.  In  practice,  a  current  of  sulphydric  acid  gas  is  made 
to  pass  through  water  in  which  finely  divided  iodine  is  kept  suspended 
by  agitation.  The  sulphydric  acid,  the  formula  of  which  is  H2S,  reacts 
.  upon  21,  and  there  is  formed  2HI,  and  free  sulphur,  which  is  deposited. 
A  solution  of  iodohydric  acid  may  also  be  obtained  by  distilling  a 
mixture  of  iodine,  phosphorus  and  much  water,  in  which  case  the 
phosphorus  unites  with  the  oxygen  of  a  portion  of  the  water,  while 
the  iodine  takes  the  hydrogen.  Or  it  may  be  prepared  by  decom- 
posing an  aqueous  solution  of  iodide  of  barium  with  an  equivalent 
quantity  of  dilute  sulphuric  acid,  and  filtering  off  the  solution  from  the 
insoluble  sulphate  of  barium. 

141.  The  dilute  acid  obtained  by  either  of  these  methods  can 
be  concentrated,  by  evaporation,  to  a  liquor  of  1.7  specific  gravity, 
boiling  at  127°,  and  composed  of  one  molecule  of  iodohydric 
acid  united  with  11  molecules  of  water.  The  aqueous  solution 
has  a  sour,  suffocating  odor,  and  pungent,  acid  taste.  When  con- 
centrated it  fumes  strongly  in  the  air.  It  cannot  be  long  pre- 
served when  exposed  to  contact  with  the  air,  for  the  oxygen  of 
the  air  unites  with  its  hydrogen,  and  iodine  is  set  free  ;  at  first 
this  iodine  dissolves  in  that  portion  of  the  iodohydric  acid  which 
has  not  yet  been  decomposed,  but  after  the  acid  has  become  satura- 
ted, crystals  of  iodine  are  deposited,  as  has  been  stated  in  §  137. 


126  lODOHYDRIC    ACID. 

The  decomposition  of  iodohydric  acid  is  so  rapid  that  the  pure,  • 
colorless  solution  of  it  becomes  red  from  separation  of  iodine 
after  a  few  'hours'  exposure  to  the  air,  no  matter  whether  it  be 
dilute  or  concentrated.  The  easy  decomposition  of  this  acid 
shows  clearly  with  how  much  less  force  hydrogen  holds  iodine 
in  combination  than  it  holds  either  chlorine  or  bromine. 

142.    The  usual  method  of    preparing  anhydrous  iodohydric 
acid  is  as  follows  :  — 

In  the  bottom  of  a  test-tube  place  a  mixture  of  9  parts  of  iodine  and 
1  part  of  phosphorus.  Cover  the  mixture  with  coarsely-powdered  ' 
glass,  and  bring  about  chemical  union  between  the  iodine  and  the  phos- 
phorus by  gently  heating  them.  Place  now  a  few  drops  of  water  in 
the  tube,  and  connect  with  it  a  gas  delivery-tube  by  means  of  a  caout- 
chouc stopper.  Iodohydric  acid  will  be  immediately  given  off,  and  may 
be  collected  by  displacement. 

Another  method  is  to  pack  a  test-tube  with  alternate  layers  of  phos- 
phorus, iodine,  and  moistened  glass-powder,  and  then  to  gently  heat  the 
tube.  The  operation  depends  upon  the  formation  of  an  iodide  of  phos- 
4  phorus  and  the  subsequent  decomposition  of  this  body  by  contact  with 
water  into  iodohydric  acid  and  a  compound  of  phosphorus,  oxygen, 
and  water,  called  hydrated  phosphorous  acid.:  — 

2PI3  -f  6H20  =  6HI  -f  3H20,P203. 

143.  Iodohydric  acid  is  a  colorless,  acid  gas,  of  suffocating 
odor ;  it  fumes  strongly  in  the  air,  and  is  very  soluble  in  water. 
It  can  be  liquefied  rather  easily  by  pressure,  and  solidified  at 
— 51°  to  a  colorless  mass  like  ice.  The  gas  is  more  than  four 
times  as  heavy  as  air,  its  specific  gravity  having  been  found  by 
observation  to  be  64.11.  From  this,  taken  in  connection  with  the 
striking  arialogy  which  the  compound  bears  to  bromhydric  and 
chlorhydric  acids,  it  follows  that  the  gas  is  composed  of  equal 
volumes  of  iodine  vapor  and  hydrogen  united  without  condensa- 
tion, for  the  theoretical  density  of  a  gas  thus  composed  would  be 
(127  -|-  l)-r-  2=  64,  a  number  with  which  the  observed  specific 
gravity  closely  agrees.  The  chemical  effect  of  the  small  propor- 
tion of  hydrogen  contained  in  iodohydric  acid  is  most  remark, 
able.  The  acid  bears  no  resemblance  to  either  of  its  constituents. 
144.  Iodohydric  acid  is  a  compound  which  decomposes  easily. 
When  a  mixture  of  the  gas  and  oxygen  is  passed  through  a  red- 


TODIC    ACID.  127 

hot  tube,  water  and  free  iodine  are  the  products.  Chlorine  and 
bromine  abstract  hydrogen  from  it,  and  leave  iodine  free ; 
and  the  same  effect  is  produced  by  many  oxygen  compounds 
which  readily  part  with  oxygen.  With  many  of  the  metals  it 
forms  iodides,  while  hydrogen  is  set  free ;  and  it  reacts  upon 
most  of  the  metallic  oxides,  forming  water  and  a  metallic  iodide. 
Though  the  hydrogen  of  iodohydric  acid  is  readily  removed 
by  means  of  oxygen  in  numerous  instances,  it  appears,  upon  the 
other  hand,  that  iodine  can  abstract  hydrogen  from  most  of  its 
combinations  with  the  other  elements.  Only  oxygen,  chlorine, 
bromine,  arid  an  element,  still  to  be  studied,  called  fluorine,  ex- 
hibit a  stronger  tendency  than  it  to  unite  with  hydrogen.  Iodine 
separates  hydrogen  from  its  compounds  with  nitrogen,  sulphur, 
and  phophorus,  and  from  many  organic  compounds,  such  as  alco- 
hol and  ether,  iodohydric  acid  being  formed  in  each  case. 

145.  Compounds  of  Iodine  and  Oxygen.  —  Of  the  compounds 
of  iodine    and    oxygen,    only    two   have  as  yet  been  carefully 
studied.     These  correspond  respectively  to  chloric  and  perchloric 
acids.     Compounds  analogous  to  hypochlorous  and  hypochloric 
acids  appear  to  exist,  but  have  not  been  described  with  much 
accuracy. 

146.  Iodic  acid  (I205),  may  be  obtained  directly  by  oxidizing 
powdered  iodine  with  monohydrated  nitric  acid  at  a  moderate 
heat.    After  all  the  iodine  has  disappeared,  and  the  excess  of  nitric 
acid  employed  has  been  evaporated,  iodic  acid  will  be  left  as  a 
white  residue. 

Iodic  acid  is  readily  soluble  in  water  and  crystallizes  from  an 
acidulated  solution  in  colorless,  six-sided  tables,  of  the  formula 
HI03  or  H20,I.205 .  It  has  a  peculiar  odor,  and  acid,  disagree- 
able taste.  At  the  temperature  of  170°,  water  is  given  off  and 
the  anhydrous  acid  remains.  This  melts  upon  being  heated 
more  strongly,  and  suffers  decomposition. 

Iodic  acid  readily  gives  up  oxygen  to  many  other  substances, 
or,  in  other  words,  it  is  easily  decomposed  by  reducing  agents  ; 
for  example,  when  mixed  with  iodohydric  acid  it  reacts  upon  it, 
with  formation  of  water  and  deposition  of  iodine :  — 

10HI  +  LA  =  5H2O  +  121. 
All  of  the  metals  are  oxidized  by  it,  excepting  gold  and  platinum. 


128  IODIDE    OF    NITROGEN. 

With  metallic  oxides  it  forms  compounds  called  iodates,  which 
are  analogous  to  the  corresponding  chlorates  and  bromates  in 
composition  and  properties. 

147.  Periodic  Acid  (I207)  may  be  prepared  by  passing  chlo- 
rine gas  through  a   solution    of   iodate  of   sodium   mixed  with 
caustic  soda.     Chloride  of  sodium  and  basic  periodate  of  sodium 
will  be  formed,  and  the  latter,  being  sparingly  soluble  in  water, 
will  be  deposited  in  crystals  :  — 

Na2O,I2O5+3(Na2O,H2O)+4Clnr:2Na2O,I2O7+4NaCl+3H2O. 
If  now  the  sodium  salt  be  collected  and  dissolved  in  water,  and 
the  solution  be  mixed  with  nitrate  of  lead,  a  periodate  of  lead 
will  be  obtained  ;  this  may  be  decomposed  by  means  of  dilute 
sulphuric  acid  into  periodic  acid  and  insoluble  sulphate  of  lead. 
The  latter  may  then  be  separated  by  filtration,  and  the  clear  solu- 
tion of  the  acid  finally  concentrated  by  evaporation. 

From  the  concentrated  aqueous  solution  periodic  acid  separates 
in  colorless  hydrated  crystals,  which,  upon  being  carefully  heated, 
give  off  water  and  yield  as  a  residue  the  anhydrous  acid  I2O7 . 
At  a  still  higher  temperature,  the  anhydrous  acid  decomposes 
and  gives  off  oxygen.  It  is  decomposed  also  by  reducing  agents 
in  the  same  way  as  iodic  acid. 

The  other  compounds  of  iodine  and  oxygen  have  but  little  inter- 
est for  us,  except  in  so  far  as  they  serve  to  increase  the  number 
of  analogies  which  subsist  between  iodine,  bromine,  and  chlorine. 

148.  Iodide  of  Nitrogen  (?).  —  There  appear  to  be  a  number 
of  compounds,  which  have  hitherto  been  usually  classed  under 

is  title.  They  are  produced  by  the  action  of  ammonia  upon 
iodine,  and  are  mostly  of  a  highly  explosive  character,  though 
their  properties  and  composition  vary  to  a  certain  extent  accord- 
ing to  the  mode  of  their  preparation. 

Exp.  77.  —  Place  0.25  grm.  of  finely  powdered  iodine  in  a  porcelain 
capsule,  aiid  pour  upon  it  so  much  concentrated  ammonia-water  that 
the  iodine  shall  be  somewhat  more  than  covered  ;  allow  the  mixture  to 
stand  during  15  or  20  minutes,  when  an  insoluble  dark  brown  powder 
will  be  found  at  the  bottom  of  the  liquid.  This  powder  is  the  so-called 
iodide  of  nitrogen.  It  should  be  collected  upon  two  or  three  very 
small  filters  and  well  washed  with  cold  water.  Remove  the  filters, 
together  with  their  contents,  from  the  funnels,  pin  them  upon  bits  of 
board,  and  leave  them  to  dry  spontaneously. 


CHLORIDES    OF    IODINE.  129 

As  soon  as  the  powder  has  become  thoroughly  dry  it  will  explode 
upon  being  rubbed,  even  with  a  feather,  or  jarred,  as  by  the  shutting 
of  a  door,  or  by  a  blow  upon  the  wall  or  table.  Though  incomparably 
less  dangerous  than  chloride  of  nitrogen,  and  therefore  better  suited 
than  the  chloride  to  illustrate  the  explosive  character  of  this  obscure 
class  of  nitrogen  compounds,  iodide  of  nitrogen  must  nevertheless  be 
handled  with  great  care,  and  should  never  be  prepared  by  the  student 
except  in  very  small  quantities. 

149.  Chlorides  of  Iodine.  —  Iodine  combines  directly   with 
chlorine  in  several  proportions,  a  protochloride,  IC1 ,  and  a  ter- 
chloride,  IC13 ,  being  the  best  known  of  these  compounds. 

The  protochloride  is  obtained  by  passing  dry  chlorine  over  dry 
iodine,  the  current  of  chlorine  being  checked  at  the  moment  when  all 
the  iodine  has  become  liquid.  Or  it  may  be  made  by  distilling  iodine  with 
chlorate  of  potassium,  and  collecting  the'product  in  a  cooled  receiver. 

3KC1O3  •+  I2  ==  KC1O4  -f  KIO3  -f  KC1  -f  O2  -f  IC1 . 
Protochloride  of  iodine  is  a  reddish-brown,  oily  liquid,  volatile,  irritat- 
ing  and  of  penetrating  odor.     It  decolorizes  litmus   and  indigo,  but 
does  not  give  a  blue  color  with  starch. 

The  tcrchloride  may  be  produced  by  treating  iodine  with  an  excess 
of  chlorine  gas.  Or  by  acting  upon  anhydrous  iodic  acid  with  dry 
chlorhydric  acid  gas  :  — 

I2O5  -f-  10HC1  =  2IC13  +  5H2O  +  4C1. 

It  is  a  yellow  crystalline  solid,  melting  at  20° —  25°.  It  acts  upon 
other  substances  in  the  same  manner  as  the  protochloride ;  like  the 
protochloride,  it  decolorizes  indigo  and  does  not  turn  starch  blue. 

150.  A  knowledge  of  the  properties  of  the  chlorides  of  iodine 
is  of  some  practical  importance,  since  they  are  liable  to  be  formed 
incidentally  in  several  chemical  processes  which  their  presence 
perturbs.     Thus,  in  the  manufacture  of  iodine,  as  described  under 
§  135,  the  iodine  lye  almost  always  contains  a  certain  proportion 
of  chloride  of  sodium.     It  is  evident  that  if  the  chlorine  in  this 
compound  were  to  be  evolved  at  the  same  time  as  the  iodine, 
by  the  action  of  the  black  oxide  of   manganese  and  sulphuric 
acid,  there  would  be  formed  a  quantity  of  the  very  volatile  pro- 
tochloride of  iodine   which  would  escape   condensation.     What- 
ever of  iodine  was  thus  combined  with  chlorine  would  be  lost  to 
the  manufacturer.     But,  as  has  been  repeatedly  stated,  iodine  is 
an  element  which  can  be  much  more  readily  expelled  from  its 


130 


THE    CHLORINE    GROUP. 


combinations  than  chlorine ;  and  in  the  case  in  point  it  is  found 
that  the  iodine  in  the  mixture  of  iodide  of  sodium  and  chlo- 
ride of  sodium,  which  the  iodine-lye  contains,  will  all  come 
off  before  the  chlorine,  if  the  distillation  be  slowly  conducted. 
If  through  irregular  heating  any  portion  of  the  contents  of 
the  retort  should  become  hotter  than  the  rest,  and  so  lose  all 
its  iodine,  chlorine  would  be  disengaged  from  that  portion,  and 
would  unite  with  the  vaporized  iodine  which  fills  the  retort.  To 
ensure  the  necessary  slow  and  equable  heat,  the  retort  is  set 
upon  a  stove  suitable  for  the  maintenance  of  a  slow  fire,  and  is 
provided  with  an  agitator,  by  means  of  which  its  contents  may 
be  continually  stirred. 

Again,  in  testing  for  iodine,  as  in  Exp.  74,  chlorine  is  a  far 
less  convenient  agent  for  setting  free  the  iodine  from  its  com- 
binations than  fuming  nitric  acid,  for  if  the  slightest  excess  of 
chlorine  be  employed,  the  iodine  will  all  be  converted  into  chlo- 
ride of  iodine,  and  the  starch  will  not  be  colored  blue. 

151.  Bromides  of  Iodine.  —  There  are  two    compounds   of 
bromine  and  iodine,  and  their  properties  are  analogous  to  those 
of  the  chlorides  of  iodine. 

152.  Chlorine,  bromine,  and  iodine  constitute  one  of  the  most 
remarkable  and  best  defined  natural  groups  of  elements.    Whether 
we  regard  the  uncombined  elements  or  their  compounds,  it  is  im- 
possible not  to  be  struck  with  the  close  analogies  which  subsist 
between  them.     With  hydrogen,  all  of  these  elements  unite  in 
the  proportion  of  one  volume  to  one  volume,  without  condensa- 
tion, to  form  acid  compounds  extremely  soluble  in  water  and  pos 
sessing  throughout  analogous  properties. 


II 


THE    CHLORINE    GROUP.  131 

With  oxygen,  each  of  them  forms  a  powerful  acid  containing  five 
atoms  of  oxygen,  besides  divers  other  compounds  of  obvious* 
likeness.  The  compounds  furnished  by  their  union  with  any  one 
metal  are  always  isomorphous  (like-formed)  ;  the  chloride,  bro- 
mide and  iodide  of  potassium,  for  example,  all  crystallize  in 
cubes.  With  nitrogen  they  all  form  explosive  compounds.  Many 
similar  analogies  will  be  made  manifest  as  we  proceed  to  study 
the  other  elements,  and  their  compounds  with  this  chlorine  group. 

There  is  a  distinct  family  resemblance  between  these  three 
elements  as  regards  their  physical  as  well  as  their  chemical 
characteristics ;  but  with  all  their  properties,  a  distinct  pro- 
gression is  observable  from  chlorine  through  bromine  to  iodine. 
At  the  ordinary  temperature  chlorine  is  a  gas,  bromine  a 
liquid,  and  iodine  a  solid,  though  at  temperatures  not  widely 
apart  they  are  all  known  in  the  gaseous  and  liquid  states.  The 
specific  gravity  of  bromine  vapor  is  greater  than  that  of  chlo- 
rine, and  that  of  iodine  greater  than  that  of  bromine.  Chlorine . 
gas  is  yellow,  the  vapor  of  bromine  is  reddish-brown,  that  of 
iodine  violet.  So  with  all  their  other  properties,  —  chlorine  will 
be  at  one  end  of  the  scale,  iodine  at  the  other,  while  bromine  in- 
variably occupies  the  intermediate  position. 

The  properties  of  many  of  the  compounds  of  chlorine,  bro- 
mine, and  iodine  exhibit  a  similar  progression  as  we  pass  from 
the  chlorine  compounds  to  those  of  iodine.  For  example,  the 
specific  gravity  of 

Chlorhydric  acid  gas  is 18.2 

Bromhydric    "      "    " 40.5 

lodohydric      "      "    " 64.0 

Chlorhydric  acid  can  be  liquefied  at  about  — 80°,  and  has  not  yet 
been  solidified.  Bromhydric  acid  liquefies  at  about  — 60°,  and 
solidifies  at  about  — 92°.  lodohydric  acid  liquefies  at  about 
—40°,  and  solidifies  at  about  — 50°. 

Chlorhydric  acid  is  a  more  energetic  acid  than  bromhydric^ 
and  brornhydric  acid  is  more  powerful  than  iodohydric.  The 
aqueous  solution  of  Chlorhydric  acid  can  be  kept  without  change 
in  contact  with  air ;  that  of  bromhydric  acid  becomes  colored 
after  a  while,  from  separation  of  bromine ;  but  the  solution  of 
iodohydric  acid  decomposes  rapidly  and  much  iodine  is  deposited. 


132  FLUORINE. 

As  regards  the  relative  chemical  power  of  these  elements,  it 
has  already  been  sho\yn  that  the  intensity  of  this  force  becomes 
less  as  we  descend  from  chlorine  to  iodine.  It  is  easy,  for  exam- 
ple, to  displace  iodine  from  its  combinations  by  means  of  bromine, 

Nal  +  Br  =  NaBr  -+-  I, 

and  equally  easy  to  displace  bromine  from  its  compounds  by 
means  of  chlorine, 

NaBr  +  Cl  =  NaCl  +  Br . 

153.  It  is  an  important  principle,  borne  out  by  most  of  the 
other  groups  of  elements,  and   emphatically  true  of  the  natural 
family  now  under  consideration,  that  with  kindred  elements  the 
chemical  power  of  each  is  great,  in  comparison  with  that  of  the 
related  elements,  in  proportion  as  its  atomic  weight  is  low. 

Among  the  members  of  a  natural  chemical  group,  chemical 
energy  seems  to  be  inversely  proportional  to  atomic  weight. 
Thus,  the  atomic  weight  of  chlorine  is  35.5,  that  of  bromine  80, 
and  that  of  iodine  127;  while  the  chemical  energy  of  these  ele- 
ments follows  the  opposite  order. 

154.  It  is  noteworthy  that  elements  of  like  character  almost 
always  occur  associated  with  one  another  in  nature.     Bromine 
and  iodine  are  always  found   in   company  with  chlorine.     That 
this  should  be  so  is  in  no  wise  surprising.     Those  elements  which 
are  similar  in  character  and  properties  must  necessarily  be  simi- 
larly acted  upon  by  the  natural  forces  to  which  they  are  exposed, 
and  must  therefore  inevitably  tend  to  be  gathered  or  deposited 
in  like  places  under  like  conditions. 


CHAPTER    XI. 

t  FLUORINE. 

155.  There  is  another  substance,  called  fluorine,  which  is 
closely  analogous  to  chlorine.  This  element  cannot  be  readily 
obtained  in  the  free  state,  and  scarcely  anything  is  known  of  it 
in  that  condition.  Special  interest  attaches  to  it  upon  this  very 


FLUORHTDRIC    ACID.  133 

account,  and  many  fruitless  efforts  to  isolate  it  have  been  made. 
Of  all  the  elements,  it  appears  to  have  the  strongest  tendency 
to  enter  into  chemical  combination ;  at  all  events  it  is  the  most 
difficult  to  obtain,  and  to  keep,  in  the  free  and  uncombined  condition. 

It  is  not  only  difficult  to  expel  fluorine  from  the  minerals  in 
which  it  is  found  in  nature  ;  but  on  being  set  free  from  one 
compound  it  immediately  attacks  whatever  substance  is  nearest 
at  hand,  and  so  enters  into  a  new  combination.  Hence  it  is 
well-nigh  impossible  to  collect  it.  It  destroys  at  once  glass,  por- 
celain and  metal,  the  materials  from  which  chemical  apparatus 
is  usually  constructed.  Vessels  made  of  the  mineral  fluor-spar 
fa  compound  of  fluorine  and  calcium),  are  the  only  ones  which 
have  as  yet  been  found  capable  of  withstanding  its  action.  By 
operating  in  such  vessels,  a  small  quantity  of  impure  fluorine 
gas  appears  to  have  been  really  obtained,  but  the  process  is  dif- 
ficult, expensive,  and  not  uniformly  successful.  Little  or  no 
doubt,  however,  is  entertained  as  to  the  general  nature  of  fluor- 
ine, since  its  compounds  are  closely  analogous  in  many  respects 
to  the  corresponding  compounds  of  chlorine,  bromine,  and  iodine. 

The  symbol  of  fluorine  is  Fl.  Its  atomic  weight  is  19.  It 
occurs  tolerably  abundantly  in  nature  as  fluoride  of  calcium 
(CaFl2),  in  the  mineral  known  as  fluor-spar.  Small  quantities 
of  fluorine  are  found  also  in  several  other  minerals,  in  vegetable 
and  animal  substances,  particularly  in  bones,  and  traces  of  it 
occur  in  sea-water,  and  in  various  rocks  and  soils.  It  appears 
to  be  almost  as  widely  disseminated  as  iodine,  though,  from  the 
lack  of  delicate  tests  for  fluorine,  it  is  far  less  readily  detected. 
Of  late  years  a  considerable  mine  of  a  fluorine  mineral  called 
cryolite  (fluoride  of  sodium  and  aluminum)  has  been  worked  in 
Greenland. 

156.  Fluorhydric  Acid  (HF1).  —  With  hydrogen,  fluorine 
forms  a  powerful  acid  corresponding  to  chlorhydric  acid  and  the 
other  hydrides  of  the  chlorine  group.  It  is  a  more  energetic 
acid  than  either  of  these,  but  is  specially  characterized  by  its 
corrosive  action  upon  glass.  It  may  be  readily  prepared  by  dis- 
tilling powdered  fluor-spar  with  strong  sulphuric  -acid ;  the 
reaction  being  analogous  to  that  which  occurs  when  common 
salt  is  treated  with  sulphuric  acid :  — 


134  FLUORHYDR1C    ACID. 

CaFl2  +  H2SO4  =  CaSO4  +  2HF1 . 

Since  the  acid  rapidly  corrodes  glass,  the  process  must  be  con- 
ducted in  metallic  vessels.  Ordinarily,  retorts  of  lead  or  pla- 
tinum are  employed,  and  the  distillate  is  collected  in  receivers 
made  of  the  same  metals,  and  carefully  cooled  by  means  of  ice. 

157.  The  product  of  the  distillation  is  a  very  volatile,  color- 
less liquid,  a  little  heavier  than  water.     It  is  strongly  acid,  emits 
copious,  white  and  highly  suffocating  fumes  in  the  air,  boils  at 
15°  and  remains  unfrozen  at  — 20°.     On  account  of  its  corrosive 
power,  this  substance  is  highly  dangerous ;  if  any  of  it  happens 
to  come  in  contact  with  the  skin,  wounds  are  produced  which  are 
very  difficult  to  heal ;  a  single  drop  of  it  is  sufficient  to  occasion 
a  deep  and  painful  sore.     In  preparing  the  acid  special  provision 
must  be  made  for  carrying  away  from  the  operator  any  fumes 
which  may  escape  condensation. 

The  acid  may  be  kept  in  bottles  made  of  lead  or  silver,  or  of 
gutta-percha,  substances  upon  which  it  has  no  action.  It  unites 
with  water  with  great  avidity,  so  much  heat  being  evolved  that 
a  hissing  noise  is  produced,  as  if  a  bar  of  red-hot  iron  had  been 
immersed  in  the  water.  In  its  concentrated  form  the  acid  has  a 
specific  gravity  of  1.061,  but  on  the  addition  of  a  certain  amount 
of  water  the  density  increases  to  1.15 ;  a  definite  hydrate 
(HFl-j-2H2O)  being  formed,  which  boils  at  120°,  and  may  be 
distilled  unchanged.  The  further  addition  of  water  to  this 
hydrate  is  attended  with  a  regular  decrease  in  density. 

According  to  some  chemists,  the  liquid  acid  obtained  as  above 
described  is  not  anhydrous.  It  is  asserted  that  if  it  be  distilled 
with  an  excess  of  anhydrous  phosphoric  acid,  —  a  substance 
which  has  a  very  strong  affinity  for  water,  —  the  anhydride  will 
be  set  free  in  the  form  of  a  colorless,  extremely  irritating  gas. 

158.  Upon  metals  and  metallic  oxides,  fluorhydric  acid  acts 
like  chlorhydric  acid,  only  more  powerfully ;  but  its  most  striking 
peculiarity  is  its  action  upon  silica  and  the  compounds  of  silica, 
such  as  glass  or  porcelain.     If  a  drop  of  the  concentrated  acid 
be  allowed  to  fall  upon  a  piece  of  glass,  it  becomes  hot,  boils,  and 
partially  distils  off  as  a  fluoride  of  silicon,  while  the  glass  is  corrod- 
ed and  becomes  covered  with  a  white  powder  consisting  of  com- 
pounds of  fluorine  and  various  constituents  of  the  glass.     If  this 


ETCHING   BY   FLUORHYDRIC    ACID.  135 

powder  be  washed  away  a  deep  depression  will  be  found  upon  the 
glass  at  the  point  where  the  acid  has  acted. 

This  corrosive  power,  which  is  possessed  by  fluorhydric  acid 
gas  as  well  as  its  aqueous  solution,  is  made  use  of  for  etching 
glass.  The  graduations  on  the  glass  stems  of  thermometers  and 
eudiometers  may  thus  be  made  with  great  precision  and  facility ; 
the  acid  is  largely  employed  also  in  ornamenting  glass  with 
etched  patterns. 

Exp.  78. —  Warm  a  slip  of  glass  and  rub  it  with  beeswax  so  that  it 
shall  be  everywhere  covered  with  a  thim,  uniform  layer  of  the  wax. 
With  a  needle,  or  other  pointed  instrument,  write  a  name,  or  trace 
any  outline  through  the  wax,  so  as  to  expose  a  portion  of  the  glass. 
Lay  the  etching,  face  downward,  upon  a  bowl  or  trough  of  sheet-lead, 
in  which  has  been  placed  a  tea-spoonful  of  powdered  fluor-spar  and 
enough  strong  sulphuric  acid  to  convert  it  into  a  thin  paste ;  if  the 
glass  be  smaller  than  the  opening  of  the  dish,  it  may  be  supported 

tupon  wires  laid  across  the  latter. 
Cover  the  glass  and  the  top  of  the  dish  with  a  sheet  of  paper,  and 
then  gently  heat  the  leaden  vessel  for  a  few  moments,  taking  care  not 
to  melt  the  wax ;  then  set  the  dish  aside  in  a  warm  place  and  leave  it 
at  rest  during  an  hour  or  two.  Finally  melt  the  wax  and  wipe  it  off 
the  glass  with  a  towel  or  bit  of  paper ;  the  glass  will  be  found  to  be 
etched  and  corroded  at  the  places  where  it  was  laid  bare  by  the  re- 
moval of  the  wax. 

This  experiment  can  be  performed  more  rapidly  by  covering  the 
outside  of  a  watch-glass  with  wax,  tracing  characters  upon  this  layer, 
and  then  placing  the  glass  upon  a  small  platinum  crucible  containing  a 
mixture  of  fluor-spar  and  sulphuric  acid,  which  is  heated  over  the  gas- 
lamp.  The  watch-glass  is  mean*while  kept  full  of  water  in  order  to 
prevent  the  wax  from  melting.  In  this  way  the  etching  can  be  effected 
in  the  course  of  a  few  minutes. 

Instead  of  the  gas,  a  dilute  aqueous  solution  of  the  acid  may  be  em- 
ployed in  this  experiment.  The  concentrated  acid,  of  §  157,  diluted 
with  six  parts  of  water,  answers  a  good  purpose.  In  this  case  the 
etched  surface  will  appear  smooth  like  the  rest  of  the  glass,  while  in  case 
the  gas  is  employed  the  etched  portion  of  the  glass  will  be  dull  and  rough. 

159.  No  compounds  of  fluorine  with  chlorine,  bromine,  iodine, 
nitrogen,  or  oxygen  have  yet  been  discovered ;  though  a  sul- 
phur compound  has  been  obtained,  as  a  fuming  liquid,  by  distill- 
ing fluoride  of  lead  with  sulphur.  Fluorine  is  the  only  element 


136  OZONE     AND     ANTOZONE. 

of  which  no  oxygen  compound  is  known  ;  this  fact  will,  however, 
appear  less  remarkable  if  it  be  remembered  that,  in  order  to  ob- 
tain oxygen  compounds  of  chlorine,  bromine,  and  iodine,  it  is 
necessary  first  to  isolate  these  elements,  and  to  have  them  in  the 
free  and  imcombined  condition.  Analogy  would  therefore  teach 
that  a  practicable  method  of  preparing  free  fluorine  must  be 
discovered  before  we  can  hope  to  prepare  oxides  of  fluorine. 
1 60.  The  fact  that  fluorine  forms  a  powerful  acid  with  hydrogen, 
connects  this  element  with  the  three  elements,  chlorine,  bromine, 
and  iodine,  which  have  last  been  studied.  Many  of  its  com- 
pounds with  the  metals  are  analogous  in  composition  to  the 
compounds  of  chlorine,  bromine,  and  iodine,  and  not  a  few  of 
these  compounds  are  isomorphous  with  one  another.  It  is  cus- 
'  ternary  therefore  to  study  fluorine  in  connection  with  the  chlorine 
group,  but  the  student  should  remember  that  in  several  respects 
it  differs  widely  from  chlorine,  and  that  its  connection  there- 
with is,  in  any  event,  less  intimate  than  that  of  either  bromine 
or  iodine. 


CHAPTER    XII. 

OZONE     AND      ANTOZONE. 

161.  Besides  ordinary  oxygen,  such  as  is  found  in  the  air,  and 
has  been  prepared  in  Exps.  5  and  7,  two  other  kinds  or  forms  of 
this  element  are  known  to  chemists.     These   new  modifications 
of  oxygen   have  received  special  names,  and  are  called  ozone 
and  antozone  respectively. 

162.  Several  other  elements,  notably  sulphur,  phosphorus  and 
carbon,  occur,  as  oxygen  does,  in  very  unlike  states,  or  with  very 
different   attributes,    while   the    fundamental   chemical  identity 
of  the  substance   is   preserved.      The  word  allotropism  is  em- 
ployed to  express  this  capability  of  some  of  the  elements  ;  it  is 
derived  from   Greek  words  signifying  of  a  different  habit,  or 
character.     This  word  serves  merely  to  bring  into  one  category 
a  considerable  number  of  conspicuous  facts,  of  whose  essential 
nature  we  have  no  knowledge  ;  there  is,  of  course,  no  virtue  in 


OZONE.  137 

the  word  itself  to  explain   or  account  for   tlie    phenomena   to 
which  it  refers. 

163.  Ozone  is  an  exceedingly  energetic  chemical  agent,  which 
resembles  chlorine  in  some  respects ;  it  can  therefore  be  advan- 
tageously studied  in  connection  with  the  chlorine  group.     More- 
over, since  ozone  and  antozone  were  for  a  long  time  confounded 
with  one  another,  and  since  they  are  really  intimately  related, 
they  should,  of  course,  be  s'tudied  together.      Oxygen  and  ozone 
belong  together,  but  we  are  better  able  to  appreciate  what  is 
known  of  the  properties  of  these  somewhat  obscure  bodies,  now 
that  we  have  become  acquainted  with  a  number  of  the  elements, 
and  have  made  ourselves  familiar  with  a  considerable  variety  of 
chemical  processes  and  reactions,   than  we   were   at  the  very 
outset,  when  common  oxygen  was  necessarily  studied. 

164.  It  had  long  been  noticed  that  when  an  electrical  machine 
was  put  in  operation  a  peculiar,  pungent  odor  was    developed. 
But  it  is  only  at  a  comparatively  recent  period  that  it  has  been 
observed  that  the  same  odor  is  manifested  during  the  electrolysis 
of  water  (§  35),  and  that  this  odor  resembles  that  evolved  by 
moistened  phosphorus  when  exposed  to  the  air.     It  has  gradually 
been  made  out,  that  the  odor  in  each  of  these  cases  is  due  to  the 
presence  of  a  peculiar  modification  of  oxygen,  called  ozone  from 
a  Greek  word  signifying  to  smell.     This  modification  of  oxygen 
was  at  one  time   erroneously  supposed  by  some  to  be  a  high 
oxide  of  hydrogen,  of  composition  H202 ,  or  H2O3 ,  but  this  view 
has  lately  been  completely  disproved. 

Of  the  methods  of  obtaining  ozone  above  suggested,  that  by 
phosphorus  will  usually  be  found  most  convenient. 

Exp.  79.  — In  a  clean  bottle,  of  1  or  2  litres  capacity,  place  a  piece 
of  phosphorus  2  or  3  c.  m.  long,  the  surface  of  which  has  been  scraped 
clean  (under  water)  with  a  knife ;  pour  water  into  the  bottle  until  the 
phosphorus  is,  half  covered ;  close  the  bottle  with  a  loose  stopper,  and 
set  it  aside  in  a  place  where  the  temperature  is  20°  or  30°. 

In  the  course  of  ten  or  fifteen  minutes  a  column  of  fog  will  be  seen 
to  rise  from  that  portion  of  the  phosphorus  which  projects  above  the 
water ;  the  original  garlic  odor  of  the  phosphorus  will  soon  be  lost,  and 
the  peculiar  odor  of  ozone  will  gradually  pervade  the  bottle.  After 
five  or  six  hours,  the  bottle  will  be  found  to  contain  an  abundance  of 
ozone  for  use  in  the  subsequent  experiments. 


138  OZONE    BY    ELECTRICITY. 

The  chemical  changes  which  occur  during  this  experiment  are 
complicated ;  it  will  be  enough  to  say  of  them  that  the  phospho- 
rus unites  with  oxygen  from  the  air  in  the  bottle  to  form  an 
oxide  of  phosphorus,  which  will  be  studied  hereafter  under  the 
name  of  phosphorous  acid ;  that  during  this  process  of  oxidation 
a  portion  of  the  oxygen  in  the  bottle  is  changed  into  ozone  and 
antozone,  and  that  some  of  the  ozone  remains,  even  after  many 
hours,  diffused  in  the  air  of  the  bottle. 

165.  It  must  be  distinctly  understood  that  no  very  large  quan- 
tity of  ozone  is  obtained  in  the  foregoing  experiment.     At  the 
best,  only  a  very  minute  proportion  of  it  will  be  found  in  the  air 
of  the  bottle.     But  ozone  is  a  substance  possessing  great  chemi- 
cal power,  and  but  little  of  it  is  needed  in  order  to  exhibit  its 
characteristic  properties. 

If  it  be  desired  to  prepare  ozone  by  passing  electric  discharges 
through  air  or  oxygen,  either  of  these  gases  may  be  sealed  up  in 
narrow  glass  tubes,  through  the  centres  of  which  are  passed  platinum 
wires,  welded  tightly  into  the  glass,  as  shown  in  Fig.  33,  and  a  series 
of  sparks  from  an  electrical  machine  is  thrown  through  the  gas  in  the 
tube,  during  ten  or  twelve  hours.  If  the  experiment  be  continued 
longer  than  this,  nothing  is  gained,  for  the  sparks  after  this  time  appear 
to  destroy  the  ozone  previously  produced. 

To  avoid  the  difficulty  last  named,  a  slow  current  of  oxygen  may  be 
forced  through  a  tube  open  at  both  ends,  and  electrical  discharges  may 
be  passed  through  the  gas  in  its  transit ;  a  constant  stream  of  ozonized 
air  will  be  thus  obtained. 

Fi  a .  33.  Instead  of  the  sparks,  the  gas  within  the  tube  may  be  subjected 
to  silent  discharges  of  electricity  obtained  by  connecting  one  of 
the  platinum  wires  with  the  ground,  the  other  with  the  prime 
conductor  of  an  electrical  machine,  and  slowly  turning  the 
crank  of  the  latter.  By  using  a  tube  having  wires  near  the  top, 
as  in  Fig.  33,  and  closing  the  lower  end  of  the  tube  by  immer- 
sing it  in  a  bath  filled  with  an  aqueous  solution  of  iodide  of  potas- 
sium, so  that  the  ozone  may  be  absorbed  as  fast  asi  it  is  formed,  it 
has  been  found  possible,  by  some  experimenters,  to  transform 
and  remove  all  the  original  oxygen  contained  in  the  tube. 

166.  Ozone  is  produced  not  only  during  the  slow  oxidation  of 
phosphorus,  and  by  the  action  of  electricity  upon  air  or  oxygen ; 
a  certain  quantity  of  it  appears   to  be  produced  also  during 
processes  of  oxidation.     It  is  readily  formed,  for  example,  during 


OZONE    BY    OXIDATION.  139 

the  slow  combustion  of  ether  and  of  various  other  volatile 
liquids ;  it  can  be  at  once  produced  by  plunging  a  heated  glass 
rod  or  iron  wire  into  a  mixture  of  air  and  ether-vapor. 

Into  a  wide-mouthed  bottle,  a  small  quantity  of  ether  is  poured ;  the 
bottle  is  shaken  for  a  moment,  that  the  air  within  it  may  become 
charged  with  the  vapor  of  ether ;  the  liquid  ether,  if  auy  remain,  is 
then  poured  away,  and  a  large  glass  rod,  or  thick  iron  wire,  heated  to 
about  250°,  is  thrust  into  the  bottle.  The  rod  must  not  be  too  hot,  lest 
the  ozone  formed  be  reconverted  into  ordinary  oxygen  ;  if  it  be  insuf- 
ficiently heated  no  ozone  is  produced. 

During  the  slow  oxidation  of  oil  of  turpentine,  oil  of  cinnamon,  oil 
of  lemons,  and  others  of  the  so-called  essential  oils,  at  the  ordinary 
temperature  of  the  air  a  considerable  quantity  of  ozone  is  produced. 
This  may  be  seen  in  oil  of  turpentine  which  has  been  kept  for  a  long 
time  in  half-filled  bottles,  exposed  to  sunlight,  and  frequently  opened 
and  shaken.  The  formation  of  ozone  under  these  circumstances  ex- 
plains the  familiar  fact  that  the  corks,  employed  to  close  bottles  contain- 
ing oil  of  turpentine  and  the  analogous  oils,  are  soon  bleached  and 
corroded.  At  the  same  time,  antozone  is  also  produced  in  large  quan- 
tity, as  will  be  explained  hereafter. 

If  quicksilver,  to  which  a  little  water  and  a  few  drops  of  a  solution 
of  indigo  have  been  added,  be  shaken  up  violently  in  a  large  bottle  full 
of  air,  the  indigo  will  soon  be  bleached  as  if  by  the  action  of  ozone. 

167.  One  of  the  best  methods  of  preparing  ozone  is  by  treat- 
ing  a   compound    known  as  permanganate  of    potassium  with 
sulphuric  acid.     It  should  be  observed,  however,  that  in  this 
process,  as  in  all  the  others,  the  ozone  obtained  is  mixed  with 
common  oxygen ;  no  available  method  of  isolating  ozone  in  a  con- 
dition of  purity  has  yet  bee'n  made  known. 

A  small  quantity  of  concentrated  sulphuric  acid  is  placed  in  the 
bottom  of  a  bottle,  and  a  quantity  of  pure,  dry,  permanganate  of-  potas- 
sium, in  fine  powder,  is  added;  the  proportion  of  acid  to  permanganate 
should  be  three  parts  to  two,  by  weight.  A  strong  smell  of  ozone  will 
be  at  once  perceived,  and  the  pasty  mass  will  continue  to  give  off 
ozone  for  a  long  time. 

In  this  case  it  is  conjectured  that  a  portion  of  the  oxygen  of  the 
permanganate 'of  potassium,  the  empirical  formula  of  which  is  K2Mn2O8, 
actually  exists  in  the  compound  as  ozone,  and  is  given  off  as  such 
when  the  compound  is  decomposed. 

168.  As  has  been  already  mentioned,  the  chemical  behavior 
of  ozone  is  analogous  to  that  of  chlorine  ;  it  bleaches  and  de- 


140  PROPERTIES    OF    OZONE. 

s troys  vegetable  coloring  matters,  and  is  a  powerful  disinfectant. 
Like  chlorine  it  instantly  decomposes  the  iodides  of  the  metals ; 
upon  this  property  is  based  a  ready  method  of  testing  for  its 
presence. 

Exp.  80. —  Into  the  bottle  of  ozonized  air  (Exp.  79),  thrust  a  moist- 
ened slip  of  the  test-paper,  saturated  with  starch  and  iodide  of  potas- 
sium, which  was  prepared  in  Exp.  75  ;  the  paper  will  instantly  acquire 
a  deep  blue  tint. 

As  in  the  case  where  the  test-paper  was  employed  for  detecting 
chlorine  (Exp.  76),  so  here,  the  reaction  depends  upon  the  displace- 
ment of  the  chemically  feeble  iodine  by  the  more  powerful  ozone :  — 

2KI  +  O  =  K2O  +  21 . 

The  ozone  here  acts  as  oxygen,  in  one  sense ;  at  all  events,  the  oxide 
of  potassium  formed  is  not  to  be  distinguished  from  oxide  of  potassium 
prepared  with  common  oxygen,  but  this  in  no  wise  contradicts  the  fact 
that  ozone  is  an  extraordinarily  active  and  energetic  variety  of  oxy- 
gen, inasmuch  as  common  oxygen  will  not  effect  this  decomposition. 

169.  Ozone  is  an  irritating,  poisonous  gas  ;  air  which  is  highly 
charged  with  it  is  irrespirable,  and  produces  effects  on  the  human 
subject  similar  to  those  produced  by  chlorine.  Its  odor,  which 
has  been  compared  to  that  of  weak  chlorine,  is  so  powerful  that 
it  can  be  recognized  in  air  containing  only  one-millionth  part  of 
the  gas.  Its  oxidizing  power  is  intense.  When  moisture  is 
present  it  oxidizes  all  the  metals  excepting  gold,  platinum,  and 
the  platinum  metals  ;  even  silver  is  oxidized  by  it  at  the  ordinary 
temperature,  and  becomes  covered  with  a  brown  coating  of  per- 
oxide of  silver.  It  destroys  many  hydrogen  compounds,  such  as 
those  of  sulpliur,  phosphorus,  and  iodine,  the  hydrogen  being 
oxidized  as  well  as  the  element  with  which  the  hydrogen 
is  associated;  iodohydric  acid,  for  instance,  is  converted 
into  water  and  iodic  acid.  In  the  same  way,  free  iodine  is 
oxidized  by  ozone,  and  if  test-paper  which  has  become  blue 
by  exposure  to  ozone,  as  in  Exp.  80,  be  left  long  in  ozon- 
ized air,  it  will  become  white  from  oxidation  of  the  iodine. 
Ozone  will  even  oxidize  nitrogen,  at  the  ordinary  tempera- 
ture, when  in  contact  with  water,  and  such  alkaline  oxides 
as  caustic  soda,  caustic  potash,  or  caustic  lime ;  thus  if  lime- 
water  (a  solution  of  caustic  lime  in  water),  be  left  exposed  to 
ozonized  air,  a  certain  quantity  of  nitrate  of  lime  will  be  formed. 


TESTING  FOR   OZONE.  141 

• 

Ammonia  is  oxidized  by  it  also,  and  it  converts  nitrous  and  sul- 
phurous into  nitric  and  sulphuric  acids. 

Many  salts  of  the  metals  are  oxidized  by  it,  for  example,  the 
sulphates  of  iron  and  of  manganese.  A  valuable  test  for  the 
presence  of  ozone  is  furnished  by  its  behavior  towards  sulphate 
of  manganese. 

Exp.  81.  —  Dissolve  a  gramme  or  two  of  sulphate  of  manganese  in 
water ;  soak  in  this  solution  strips  of  thin,  white,  blotting-paper ;  dry 
the  paper,  and  preserve  it  in  a  bottle.  If  a  slip  of  this  paper  be 
moistened,  and  then  hung  in  ozonized  air  (Exp.  79),  it  will  quickly 
become  brown  from  the  formation  upon  it  of  black  oxide  of  manganese. 

In  like  manner,  most  organic  substances  are  quickly  oxidized 
by  ozone  ;  when  substances  such  as  saw-dust,  garden-mould, 
powdered  charcoal,  milk,  or  flesh,  are  thrown  into  a  bottle  of 
ozonized  air  the  odor  of  ozone  instantly  disappears ;  corks 
and  caoutchouc  tubes  are  attacked  by  it,  and  must  not  be 
used  in  experimenting  with  the  gas.  It  destroys  the  color  of  in- 
digo, and  bleaches  litmus  without  first  reddening  it.  Some 
organic  bodies,  on  the  other  hand,  become  colored  when  exposed 
to  its  action  ;  thus,  the  cut  surface  of  an  apple  becomes  brown, 
and  fresh  surfaces  of  certain  mushrooms  become  blue.  Gum 
guaiacum  also  becomes  blue.  Papers  soaked  in  a  dilute  alcoholic 
solution  of  gum  guaiacum  are,  indeed,  often  employed  as  a  test 
for  ozone. 

Exp.  82. —  Dissolve  one  part  of  gum  guaiacum  in  thirty  parts  of 
ninety  per  cent,  alcohol ;  add  a  few  drops  of  this  solution  to  2  c.  c.  of 
ordinary  eighty  per  cent,  alcohol ;  dip  in  this  dilute  solution  strips  of 
thin,  white,  blotting  paper,  and  dry  them  in  the  dark.  By  exposure  to  . 
ozonized  air  this  test-paper  acquires  a  bright  blue  color. 

170.  By  virtue  of  its  strong  oxidizing  power,  ozone  is  of 
great  importance  as  a  disinfecting  agent.  It  destroys  instantly  a 
multitude  of  offensive  gases,  such  as  arise  from  decaying  ani- 
mal and  vegetable  matter,  and  has  been  frequently  recommended 
of  late  as  a  substance  well  fitted  for  the  purification  of  sick- 
rooms and  hospital-wards.  Where  ozone  is  employed  for  pur- 
poses of  disinfection,  it  must  be  borne  in  mind  that  the  action  of 
the  gas  depends  solely  upon  oxidation.  A  given  quantity  of 
ozone  can  destroy  only  a  certain  definite  amount  of  the  offensive 


142  OZONE    A    DISINFECTANT. 

organic  matter  ;  wherever  these  emanations  are  incessantly  gen- 
erated, ozone  must  be  as  constantly  produced  in  order  to  destroy 
them.  This  disinfecting  power  of  ozone  is  interesting  in  con- 
nection with  the  observed  facts,  that  ozone  is  abundant  in  the 
air  of  pine  forests,  where  turpentine  abounds,  and  that  pine  for- 
ests are,  as  a  general  rule,  remarkably  free  from  malaria.  The 
well-known  disinfecting  power  of  tar  is  supposed  in  like  manner 
to  be  partly  due  to  the  formation  of  ozone  during  the  oxidation 
of  some  of  its  ingredients. 

Coal-tar,  mixed  with  plaster-of-Paris,  coal-ashes,  or  dry  earth,  in 
quantity  sufficient  to  destroy  its  stickiness,  has  been  found  to  be  a  very 
efficient  disinfectant.  The  dry  powder  obtained  as  above,  is  simply 
scattered  freely  about  the  offensive  locality.  The  coal-tar,  of  course, 
evolves  a  slight  odor,  peculiar  to  itself,  which  tends  to  mask  or  conceal 
Other  odors,  and  also  acts  as  an  antiseptic,  or  arrester  of  putrefaction, 
but  its  chief  merit  does  not  appear  to  defend  upon  either  of  these 
properties ;  it  seems  really  to  destroy  the  gases  which  are  evolved 
from  putrescent  matter,  and  probably  does  so  by  generating  ozone. 

171.  It  is  supposed  that  a  minute  proportion  of  ozone  exists 
in  normal  atmospheric  air :  at  all  events,  there  is  usually  present 
in  air  a  substance  which  exhibits  the  various  reactions  of  ozone, 
and  behaves  as  ozone  would  if  it  were  there.  This  atmospheric 
ozone,  which  is  supposed  to  be  formed  in  the  processes  of  oxida- 
tion which  are  always  going  on  in  nature,  varies  in  quantity  with 
the  locality,  the  season  of  the  year,  the  hour  of  the  day,  and 
many  other  circumstances. 

Ozone  is  seldom  found  in  the  air  of  thickly  inhabited  localities  ; 
it  often  happens  that  it  cannot  be  detected  in  the  air  of  cities  at 
the  very  time  when  it  is  abundant  in  the  neighboring  country. 
It  is  often  found  to  be  abundant  on  the  windward  side  of  a  city, 
and  altogether  absent  from  the  air  upon  the  leeward  side,  the  in- 
ference being  that  it  is  destroyed  by  the  exhalations  which  arise 
from  a  dense  population.  Ozone  appears  to  be  more  abundant 
in  the  air  in  winter  than  in  summer,  in  cloudy  j  than  in  clear 
weather,  and  by  night  than  by  day  ;  it  has  been  observed  to  be 
specially  abundant  at  times  when  dew  was  falling  heavily.  As 
might  be  expected,  comparatively  large  quantities  of  it  are 
found  during  thunder-storms,  and  its  odor  has  been  recognized 
in  the  neighborhood  of  objects,  struck  by  lightning.  Ozone  is 


OZONE    IN    THE     ATMOSPHERE.  143 

abundant  during  snow-storms,  and  it  is  probable  that  upon  its 
presence  depends  the  well-known  bleaching  power  of  newly 
fallen  snow. 

In  searching  for  ozone  in  the  air,  test-paper  containing  iodide  of 
potassium  and  starch,  such  as  was  prepared  in  Exp.  75,  is  usually  em- 
ployed. Dry  slips  of  the  prepared  paper  are  exposed,  during  from  six 
to  twenty-four  hours,  to  a  free  current  of  air,  in  a  place  well  sheltered 
from  light  and  rain.  By  exposure,  the  dry  paper  becomes  brown,  and 
when  wetted  acquires  shades  of  color  varying  from  pinkish-white  and 
iron-gray  to  blue.  The  shade  of  color  obtained  in  this  way  is  then 
compared  with  a  standard  chromatic  scale,  which  includes  all  the 
shades  possible  under  the  circumstances,  and  the  proportion  of  ozone 
present  in  the  air  is  thus  roughly  estimated. 

Although  observations  of  this  kind  are  far  from  possessing  that 
degree  of  accuracy  and  certainty  which  is  desirable,  they  have  never- 
theless been  considered  trustworthy  by  numerous  observers  and  have 
given  rise  to  much  speculation  concerning  the  functions  of  atmospheric 
ozone,  more  particularly  with  regard  to  its  probable  influence  upon 
health  and  disease.  If  there  be  ozone  in  the  atmosphere,  it  will,  on 
the  one  hand,  oxidize  and  destroy  many  volatile  organic  substances 
which  are  supposed  to  be  prejudicial  to  health ;  hence  many  physicians 
are  of  opinion  that  the  atmospheric  ozone  plays  an  important  part  in 
controlling  or  preventing  epidemic  diseases  through  its  power  of  re- 
moving infectious  matter  from  the  air ;  and  it  has  been  noticed  that 
with  the  advent  of  an  ozone-bearing  wind  such  diseases  have  abated 
or  ceased.  But,  on  the  other  hand,  ozone  is  a  highly  irritating  gas,  and 
in  the  opinion  of  some  physicians  occasions  many  diseases  of  the  respira- 
tory organs.  Numerous  statements  are  upon  record  to  the  effect  that 
epidemics  of  catarrh,  colds,  sore  throat,  and  influenza,  have  been  co- 
incident with  the  beginning  of  a  spell  of  ozoniferous  wind. 

173.  Ozone  is  usually  considered  to  be  completely  insoluble 
in  water,  but  it  has  been  recently  ascertained  that  water  can  take 
up  a  small  quantity  of  it,  and  so  acquire  some  of  the  properties 
of  ozone.  When  ozonized  air  is  passed  through  a  solution  of 
caustic  soda  or  caustic  potash,  a  certain  amount  of  ozone  is  ab- 
sorbed at  first,  perhaps  by  combination  with  some  oxidizable  im- 
purity of  the  solution,  but  after  a  little  time  the  ozone  will  pass 
through  without  apparent  alteration.  Acids  do  not  absorb  ozone. 
It  is  readily  absorbed,  however,  by  aqueous  solutions  of  iodide 
of  potassium  and  of  pyrogallic  acid,  with  the  constituents  of 


144  ANTOZONE. 

which  it  enters  into  combinations  not  to  be   distinguished  from 
those  made  with  oxygen. 

174.  At  moderately  high  temperatures  ozone  loses  its  pecu- 
liarities and  changes  into  ordinary  oxygen  ;  if  ozonized  air,  such 
as  was  obtained  in  Exp.  79,  is  made  to  pass  through  a  narrow 
glass  tube  heated  to  250°,  its  peculiar  odor,  and  its  power  of  de- 
composing iodide   of  potassium   will   entirely   disappear.      The 
same    change    occurs    gradually  if  the  tube  is  heated  only   to 
100°  ;  or  instantly  if  steam  be  thrown  into  the  ozonized  air,  so 
that  the  whole  of  it  can  be  heated  at  once  to  100°  ;  hence  it  may 
be  stated,  in  general  terms,  that  ozone  is  converted  into  ordinary 
oxygen  at  temperatures  greater  than  100°. 

175.  Ozone  is   supposed  to  exist  as  such  in  several  of  the 
oxides.     Black  oxide  of  manganese,  for  example,  is  thought  to 
contain  it  as  a  constituent ;  and  a  method  of  obtaining  it  from 
permanganic  acid  has  been  already  given,  §  167.     The  oxygen 
compounds    which   are    supposed    to    contain    ozone  are  called 
ozonides.     The  formula  of  the  following  compounds,  recognized 
as  ozonides,  are  here  given  for  the  sake  of  reference :  — 

PbO2      OO3      MnO2      Co2O3     N2O5 
Ag2O2     MnO3     Mn267     Ni2O      Bi2O5 

176.  Antozone  (the  opponent  or  opposite  of  ozone)   appears 
to  be  produced  simultaneously  with  ozone  whenever  the   latter 
is   formed,    whether    by   electrical   action   or  during    processes 
of  oxidation.     It  may  even   be  that,  as  some  chemists  believe, 
ordinary   oxygen   is  in  a  certain  sense  a  compound  substance, 
and   that  when   in    contact  with   phosphorus,  and  in  the  other 
circumstances  under  which  ozone  is  produced,  the  neutral  oxy- 
gen is  split  or  decomposed  into  two  opposite  and  dissimilar  modi- 
fications,—  we  had  almost  said   elements; — one  of   which  is 
ozone,  the  other  antozone.     It  is  thought  that  while  the  greater 
part  of  the  ozone  thus  engendered  enters  into  combination  with 
the  phosphorus,  or  other  substance,  undergoing  oxidation,  a  cer- 
tain portion  of  it,  together  with  some  of  the  antozone,  becomes 
mixed  with  the  surrounding  air,  and  so  escapes  combining  with 
the  body  which  is  being  oxidized. 

Only   a  comparatively  short  time  has  elapsed  since  antozone 
has  been  recognized  as  a  distinct  substance ;  hence  its  properties. 


PREPARATION  OF  ANTOZONE.  145 

have  been  even  less  thoroughly  studied  than  those  of  ozone. 
Many  of  its  characteristics  and  properties  are  still  involved  in 
great  obscurity,  very  various  and  even  conflicting  statements 
having  been  published  concerning  them. 

177.  Of  the  methods  devised  for  preparing  antozone,  the  fol- 
lowing deserve  notice  :  — 

By  passing  dry  electrized  air  (§  165)  through  a  concentrated  aque- 
ous solution  of  iodide  of  potassium,  or  of  pyrogallic  acid,  all  the  ozone 
contained  in  the  air  will  be  at  once  absorbed,  and  the  antozone  left 
behind,  free  from  any  admixture  of  ozone. 

During  the  slow  oxidation  of  oil  of  turpentine  and  other  volatile 
or  essential  oils  (§  166),  a  considerable  quantity  of  antozone  is  pro- 
duced, as  well  as  of  ozone.  While  most  of  the  ozone  at  once  combines 
with  the  constituents  of  the  oil,  to  form  resins  and  other  products  of 
oxidation,  the  antozone,  which  does  not  oxidize  the  oil,  is  dissolved  by 
it.  In  what  state  the  antozone  exists  within  the  oil  is  still  uncertain, 
but  it  is,  in  any  event,  very  loosely  held,  and  is  readily  given  up  to 
other  substances. 

In  the  same  way  that  ozone  can  be  prepared,  by  chemical  decompo- 
sition, from  permanganate  of  potassium,  a  compound  supposed  to  con- 
tain ozone  (§  167),  antozone  maybe  obtained  by  decomposing  certain 
compounds,  which  are  believed  to  contain  this  variety  of  oxygen, 
such,  for  example,  as  peroxide  of  barium,  BaO2 .  A  little  con- 
centrated sulphuric  acid  is  poured  into  a  small  bottle,  and  into  this 
acid  is  thrown  a  number  of  small  fragments  of  peroxide  of  barium 
(free  from  any  admixture  of  nitrate  of  barium) ;  so  soon  as  an 
evolution  of  gas  ensues,  the  air  of  the  bottle  will  be  found  charged 
with  antozone.  This  reaction  is  sometimes  capricious ;  usually  it  occurs 
at  the  ordinary  temperature  of  the  air,  but  it  is  often  necessary  to 
place  the  bottle  in  a  water-bath  heated  to  50°  or  60°,  in  order  to  start" 
the  evolution  of  gas,  and  on  the  other  hand,  the  violence  of  the  reac- 
tion must  sometimes  be  allayed  by  immersing  the  bottle  in  cold  water. 

In  the  preparation  of  ozone  by  means  of  phosphorus  in  moist  air 
(Exp.  79),  or  by  the  electrolysis  of  water  (§  35),  the  antozoiie  which 
is  formed  at  the  same  time  with  the  ozone,  unites  with  the  water  pres- 
ent, and  must  there  be  sought.  (See  §  181.) 

Antozone  has  been  found  in  nature  in  a  dark-blue  variety  of  fluor- 
spar from  Wolsendorf,  in  Bavaria.  Upon  being  rubbed,  this  mineral 
emits  a  peculiar  odor  which  was  formerly  thought  to  be  that  of  chlo- 
rine or  cf  hypochlorous  acid.  More  recent  investigations  have  shown 
that  the  odor  is  that  of  antozone  ;  and  that  by  grinding  the  mineral 
with  water  the  antozone  can  be  transferred  to  the  water. 
10 


146  PROPERTIES  OF  ANTOZONE. 

178.  Antozone  is  a  gas,  the  odor  of  which  somewhat  resem- 
bles  that   of    ozone ;    there   is,   however,  a   decided  difference 
between  the  two  odors,  that  of  antozone  being  disgusting,  while 
that   of    ozone   is   merely   pungent   and   irritating.      Antozone 
changes  at  once  to  ordinary  oxygen  on  being  heated.     Even  at 
the  ordinary  temperature   it   reverts   to   common  oxygen   very 
readily,  —  much  more  readily  than   ozone.     Most  of  the  anto- 
zone  usually  disappears  from  dry  electrized   air  in  the   course 
of  an  hour,  or  an  hour  and  a  half;  and  if  the  air  be  moist,  the 
change  is  still  more  rapid.      Ozone,  on  the  contrary,  is  compara- 
tively permanent,  under  the  same  conditions,  and  although  when  a 
mixture  of  ozone  and  antozone  is  left  in  contact  with  water  in  a 
glass-stoppered  bottle,  some  ozone  is  destroyed  during  the  rever- 
sion of  the  antozone,  the  larger  portion  of  it  will  remain  almost,  if 
not  quite,  unaltered  for  months.     Antozone,   whether  moist  or 
dry,  also  reverts  to  the  condition  of  ordinary  oxygen   on  being 
brought  in  contact  with  black  oxide   of  manganese,   peroxide  of 
lead,  or  finely  divided  platinum. 

179.  A  very   remarkable    characteristic   of   antozone    is    its 
power  of  forming  fogs  and  clouds  with  water.     It  may  even  be 
found,  after  the  matter  has  been  more  thoroughly  studied,  that 
all  the  fogs  and  clouds  which  occur  in  nature  are  dependent  for 
their  existence  upon  the  presence  of  antozone. 

If  air,  charged  with  antozone,  be  made  to  bubble  through  water,  it 
will  emerge  from  the  water  in  the  form  of  a  thick  white  mist,  similar  to 
that  formed  by  the  cooling  of  steam.  The  same  thing  occurs  when 
electrized  air,  or  electrized  oxygen,  issues  into  a  moist  atmosphere, 
though  the  effect  is  less  marked  when  ozone  is  present,  than  when-  it 
has  been  removed  by  means  of  iodide  of  potassium.  The  mist  pro- 
duced by  slowly  passing  antozonized  air  through  water  is  heavy;  it 
remains  hanging  over  the  surface  of  the  liquid,  and  may  be  readily 
poured  from  one  vessel  to  another.  By  conducting  it  through  a  tube 
to  the  bottom  of  a  dry,  ta.ll,  bottle,  it  displaces  the  air,  all  the  while 
preserving  a  sharply  defined  boundary ;  by  gentle  agitation  it  is  easily 
broken  up  into  cloud-like  masses. 

When  a  large,  dry,  bottle  is  nearly  filled  with  this  antozone  mist, 
then  closed  and  left  to  itself,  the  mist  gradually  becomes  thinner  and 
less  opaque,  and  in  the  course  of  half  or  three-quarters  of  an  hour, 
vanishes  altogether.  As  the  cloud  thus  disappears,  water  is  deposited 


THE     ANTOZONE     CLOUD.  147 

upon  the  sides  of  the  bottle,  at  first  as  a  mere  dew,  but  afterwards  ac- 
cumulating in  droplets,  which  finally  flow  together  to  the  bottom  of  the 
vessel.  When  the  air  in  the  bottle  has  become  clear,  no  antozone  can 
be  detected  in  it. 

It  thus  appears  that  antozone  has  the  property  of  taking  up  water 
in  such  a  manner  that  the  water  assumes  the  peculiar  physical  condi- 
tions of  a  cloud  or  mist.  While  the  antozone  lasts  the  cloud  is  perma- 
nent ;  but  the  antozone  is  soon  transformed  into  ordinary  oxygen,  and  as 
fast  as  this  change  occurs,  the  water  of  the  cloud  is  deposited  in  droplets. 

By  passing  the  antozone  mist  through  tubes  filled  with  desiccating 
substances,  such  as  chloride  of  calcium  (Appendix,  §  15),  the  water 
may  be  removed,  and  transparent  antozonized  air  obtained,  capable  of 
again  producing  a  mist  on  being  brought  in  contact  with  water.  Many 
strong  saline  solutions  likewise  deprive  antozone  of  water ;  hence  the 
non-appearance  of  the  cloud  when  electrized  air  is  passed  through  a 
strong  solution  of  iodide  of  potassium.  The  cloud  does  appear,  how- 
ever, when  the  solution  is  sufficiently  dilute. 

It  has  been  proved  by  experiment  that  electrized  air  can  sup- 
port or  carry  nearly  twice  as  much  moisture  as  ordinary  air  or 
oxygen  at  the  same  temperature,  and  that  this  air  is  much  more 
difficult  to  dry  than  the  gases  with  which  chemists  usually  have 
to  deal.  This  explains  how  it  happened  that,  before  the  dis- 
covery of  th'e  cloud-forming  property  of  antozone,  so  many 
observers  had  been  led  to  consider  ozone  an  oxide  of  hydrogen. 
One  experimenter  would  pass  recently  electrized  air  through  an 
ordinary  drying-tube,  such  as  long  experience  had  shown  to  be 
capable  of  drying  common  air  perfectly,  and  would  then  heat 
the  gas ;  by  this  treatment,  both  the  ozone  and  the  antozone 
would  be  changed  to  ordinary  oxygen,  and  the  water  which  had 
been  carried  through  the  drying  tube  by  the  antozone  would  be 
made  visible.  The  remarkable  capacity  of  antozone  for  moisture 
being  rnknown,  the  water  thus  obtained  was  naturally  enough 
supposed  to  have  been  derived  from  some  compound  of  hydro- 
gen and  oxygen  other  than  water,  and  capable  of  passing  unab- 
sorbed  through  the  drying-tube.  Other  chemists,  performing,  as 
they  supposed,  the  same  experiment,  but  in  reality  operating 
upon  air  less  recently  electrized,  and  so  containing  no  antozone, 
were,  of  course,  unable  to  obtain  any  water  at  the  point  where  it 
had  been  observed  by  their  predecessors  ;  hence  arose  a  series  of 
controversies  which  have  only  recently  been  composed. 


148  ANTOZONE    IN    COMBUSTION. 

180.  As  has  been  already  mentioned,  antozone,  like  ozone,  is 
formed  in  all   processes  of  oxidation   and   combustion.     During 
combustion  most  of  the  ozone  produced  enters  into  combination 
with  the  substance  burned,  while   the  antozone  is  left  free,  or 
enters  into  combination  with  water  to  form  peroxide  of  hydrogen. 
When  the  combustion  is  slow  or  smouldering,  antozone  appears 
in  large  quantities,  and  in  presence  of  moisture  forms  the  char- 
acteristic  mist   or    cloud.     Tobacco-smoke,   the   gray   smoke  of 
chimneys  and  of  gun-powder,  and  all  such  smokes,  are  antozone 
clouds,  —  facts  which  support  the  idea  that  all  clouds,  fogs,  and 
mists  are  caused  by  the  presence  of  antozone  in  the  atmosphere. 

The  oxidation  of  phosphorus  affords  a  ready  method  of  exhibiting 
the  antozone  cloud.  During  the  oxidation  of  phosphorus  in  moist  air, 
white  fumes  are  formed,  which  were  long  a  great  puzzle  to  chemists. 
Whether  the  phosphorus  be  allowed  to  oxidize  slowly,  as  in  Exp.  79, 
or  burned  rapidly,  as  in  Exp.  13,  there  is  always  produced  a  white  mist 
of  very  considerable  permanence,  which  remains  long  after  the  oxides  of 
phosphorus,  which  are  also  formed,  have  been  taken  up  and  removed 
by  the  water.  This  mist  is  the  antozone  cloud  ;  it  is  nothing  but  water 
held  suspended  by  antozone. 

In  the  rapid  combustion  of  phosphorus,  little  or  no  ozone  is  left  free  ; 
all  of  it  seems  to  unite  directly  with  the  phosphorus ;  bat  much  more 
antozone  is  produced  when  the  combustion  is  rapid  than  when  it  is 
slow.  The  formation  of  antozone  in  this  connection  explains  the  fact 
already  alluded  to  (Exp.  13),  that  phosphorus  burning  with  flame,  in 
a  confined  volume  of  air,  does  not  wholly  exhaust  the  latter  of  oxygen. 
The  phosphorus  cannot  combine  with  antozone,  but  only  with  ozone ; 
hence,  when  no  oxygen  other  than  that  in  the  form  of  antozone  re- 
mains, the  combustion  must  cease. 

During  the  burning  of  a  jet  of  hydrogen  under  a  bell-glass  through 
which  a  stream  of  air  is  drawn,  antozone  is  formed,  as  is  proved  by 
passing  the  issuing  stream  through  water;  the  antozone  cloud  is  pro- 
duced without  difficulty,  and  peroxide  of  hydrogen  appears  as  a 
product.  The  formation  of  the  antozone  mist,  and  of  peroxide  of  hy- 
drogen, may  be  observed  with  any  other  flame,  if  care  be  taken  that 
the  air  which  streams  over  the  flame  be  not  too  strongly  heated.  A 
high  temperature  destroys  the  antozone  as  fast  as  it  is  formed. 

181.  Besides  its  power  of  forming  clouds  or  mists  with  water, 
which  is  interesting  rather  as  a  physical  than  as  a  chemical  fact, 
antozone,  particularly  when  newly  formed,  also  unites  with  water 


ANTOZONE    OXIDIZES    WATER.  149 

chemically,  the  substance  called  peroxide  of  hydrogen  (see 
§  61),  whose  composition  is  expressed  by  the  formula  H2O2, 
being  the  result  of  the  combination. 

A  simple  method  of  exhibiting  the  formation  of  peroxide  of  hydro- 
gen by  the  action  of  antozone  upon  water,  is  to  place  a  short,  narrow 
tube,  containing  concentrated  sulphuric  acid,  within  a  bottle  2  or  3 
c.  m.  in  width,  furnished  with  a  ground-glass  stopper,  and  filled  with 
water  nearly  to  the  top  of  the  tube.  Small  portions  of  peroxide  of 
barium  are  now  added,  at  intervals,  to  the  sulphuric  acid  in  the  tube, 
elevation  of  temperature  being  voided  as  far  as  possible;  the  stop- 
per should  be  replaced  in  the  botue  after  each  addition  of  the  peroxide. 
Most  of  the  oxygen  evolved  in  this  process  appears,  however,  to  be  in 
the  ordinary  inactive  state,  and  the  solution  of  peroxide  of  hydrogen 
obtained  is  consequently  extremely  dilute.  A  belter  method  of  pro- 
cedure is  to  pass  a  current  of  carbonic  acid  gas  into  a  mixture  of 
water  and  peroxide  of  barium, 

BaO2  +  H2O  +  CO2  =  BaO,CO2  +  H2O2. 

In  this  way  a  highly  concentrated  solution  of  the  peroxide  can  be 
obtained.  • 

Another  easy  method  of  preparing  peroxide  of  hydrogen  is  by  the 
oxidation  of  amalgams  of  lead  or  zinc.  In  this  case  also,  as  in  the 
preceding,  the  peroxide  of  hydrogen  is  probably  formed  by  the  union 
of  antozone  with  water. 

One  hundred  grammes  of  lead  amalgam,  containing  so  much  mer- 
cury that  it  shall  be  fluid  at  the  ordinary  temperature,  is  shaken  in  a 
bottle  of  the  capacity  of  a  litre,  together  with  200  c.  c.  of  water, 
acidulated  with  2  grms.  of  sulphuric  acid;  the  water  soon  becomes 
milky  from  separation  of  sulphate  of  lead,  and,  in  the  course  of  ten  or 
twelve  minutes,  contains  enough  peroxide  of  hydrogen  to  exhibit  the 
characteristic  reactions  of  this  substance. 

So,  too,  if  pulverulent  zinc-amalgam  be  loosely  thrown  into  a  glass 
funnel,  with  narrow  throat,  and  a  thin  stream  of  water  be  allowed  to 
flow  through  it  in  such  manner  that  the  metal  may  be  at  the  same 
time  acted  upon  by  both  air  and  water,  the  water  will  become  charged 
with  peroxide  of  hydrogen.  By  repeatedly  pouring  back  the  dilute 
solution  of  the  peroxide  upon  the  amalgam,  it  can  be  very  considerably 
strengthened.  In  order  to  prepare  the  zinc  amalgam,  equal  weights  of 
zinc-filings  and  of  mercury  are  placed  in  a  beaker-glass,  covered  with 
water  acidulated  with  sulphuric  or  chlorhydric  acid,  and  thoroughly 
mixed  by  stirring  with  a  glass  rod  ;  the  acid  is  then  poured  away,  and  the 
last  portions  of  it  removed  from  the  amalgam  by  washing  with  water. 


150        DIFFERENCES    BETWEEN    OZONE    AND    ANTOZONE. 

This  power  of  antozone  to  oxidize  water  distinguishes  it  com- 
pletely from  ozone,  which  has  little  or  no  action  upon  water. 

182.  Peroxide  of  hydrogen,  like  peroxide  of  barium,  is  sup- 
posed to  contain  one  atom  of  oxygen  in  the  form  of  antozone  ; 
the   peroxides    of   potassium,   sodium,    and    strontium    al>o   are 
placed  in  the  same  category.     They  are  all  called  antozonides. 

183.  Antozone  can  be  distinguished  from  ozone  by  the  follow- 
ing tests  :  — 

Strips  of  paper,  charged  with  a  solution  of  sulphate  of  manganese, 
(Exp.  81),  do  not  become  brown  when  exposed  to  the  action  of  antozone ; 
on  the  contrary,  manganese  papers,  which  have  been  browned  by  ozone, 
are  bleached  by  antozone.  Guaiacum  paper  (Exp.  82)  does  not  become 
blue  in  antozonized  air.  The  yellow  compound  called  ferroeyanide  of 
potassium,  which  is  converted  into  red  ferricyanide  of  potassium  by  the 
action  of  azone,  is  not  changed  by  antozone.  In  the  absence  of  acids, 
antozone  has  no  action  upon  iodide  of  potassium. 

The  chemical  behavior  of  antozone  may  be  conveniently  studied  by 
resorting  to  its  compound  with  water,  the  antozonide  peroxide  of  hy- 
drogen. If  peroxide  of  hydrogen  be  brought  in  contact  with  an  ozon- 
ide  like  peroxide  of  lead,  for  example,  both  of  the  peroxides  will  be 
reduced,  and  there  will  result  water,  protoxide  of  lead,  and  free 
ordinary  oxygen.  Whenever  an  antozonide  is  mixed  with  an  ozonide,  a 
similar  reaction  occurs ;  the  two  active  varieties  of  oxygen  disappear, 
and  common  oxygen  is  evolved ;  hence  it  has  been  assumed  that 
ordinary  inactive  oxygen  is  a  sort  of  compound,  resulting  from  the 
union  or  neutralization  of  ozone  with  antozone.  Several  important 
tests  for  antozone  are  dependent  upon  this  fact  of  the  decomposition  of 
antozonides  by  ozonides. 

If  a  liquid  suspected  to  contain  peroxide  of  hydrogen  be  shaken  in  a 
test-tube,  with  a  small  quantity  of  ether,  the  ether  will  dissolve  the 
peroxide,  and  will  finally  collect  upon  the  surface  of  the  liquid  ;  on  adding 
to  it  a  small  drop  of  a  solution  of  the  ozonide  chromic  acid,  or  what  comes 
to  the  same  thing,  a  drop  of  a  solution  of  bichromate  of  potassium  acid- 
ulated with  sulphuric  acid,  the  ethereal  solution  will  become  blue. 

If  a  liquid  containing  peroxide  of  hydrogen  be  added  to  a  dilute  red 
solution  of  permanganate  of  potassium,  this  solution  will  be  Decolorized, 
while  common  oxygen  will'be  evolved,  and  in  the  same  way  the  brown 
peroxide  of  lead,  and  the  red-colored  salts  of  peroxide  of  iron  are 
bleached  by  it. 

Another  exceedingly  delicate  and  characteristic  test  for  antozone,  or 
rather  for  peroxide  of  hydrogen,  the  rationale  of  which  has  not  yet 


SULPHUR.  151 

been  well  made  out,  is  the  following:  — If  to  a  solution  containing  per- 
oxide of  hydrogen  there  are  added  a  few  drops  of  dilute  starch-paste 
charged  with  iodide  of  potassium,  and  subsequently  a  very  small 
quantity  of  a  solution  of  copperas  (protosulphate  of  iron),  iodine  will 
be  set  free,  and  the  starch  will  become  blue.  The  solution  to  be  tested 
must  be  as  nearly  neutral  as  possible.  The  addition  of  an  acid,  instead 
of  the  copperas  solution,  will  also  bring  about  the  same  reaction, 
though  less  readily. 

184.  We  have  thus  set  forth  whatever  is  best  known  concern- 
ing ozone  and  antozone,  in  spite  of  the  details  into  which  so  full 
an  exposition  has  necessarily  descended,  partly  because  the  sub- 
ject will  evidently  be  one  of  primary  importance,  both  theoretical 
and  practical,  in  the  near  future,  and  partly  from  a  desire  to 
show  the  student  how  vague  and  uncertain  the  prospect  is  when 
once  the  narrow  limits  of  established  knowledge  are  past  and 
the  inquirer  ventures  out  into  the  obscurity  which  perpetually 
separates  the  knowledge  of  to-day  from  that  which  shall  be 
knowledge  to-morrow,  but  also  because  of  the  impossibility, 
with  so  obscure  a  subject,  of  making  such  a  just  discrimination 
between  salient  and  unimportant  points  as  with  a  well-studied 
subject  is  both  easy  and  desirable. 


CHAPTER    XIII. 

SULPHUR. 

185.  Sulphur  occurs  somewhat  abundantly  in  nature,  both  in 
the  free  state  and  in  combination  with  other  elements.  Many 
ores  of  metals,  for  example,  are  sulphur  compounds.  It  is  a 
component  of  several  abundant  salts,  such  as  the  sulphates  of 
calcium,  barium,  and  sodium,  and  occurs  in  small  proportion  in 
many  animal  and  vegetable  substances.  Free  sulphur  is  found 
chiefly  in  volcanic  districts.  Generally  it  occurs  mixed  with 
earthy  matters,  but  it  often  forms  distinct  veins,  and  is  sometimes 
found  in  the  shape  of  well-defined  crystals  of  considerable  size. 
At  the  present  time  about  nine-tenths  of  the  sulphur  of  com- 
merce comes  from  Sicily. 


152  THE    SULPHUR    OF    COMMERCE. 

186.  Native  sulphur  is  usually  subjected  to  a  rough  purifica- 
tion at  the  place  of  its  occurrence.     This  purification  is  some- 
times effected  by  distilling  the  volcanic  earth  in   retorts   or  jars 
of  earthen-ware  ;  the  sulphur  being  volatile,  distils  over,  and  is 
collected  in  receivers,  from  which  it  is  drawn  off,  from  time  to 
time,  in  the  liquid  state  ;  or,  if  the  earth  be  very  rich  in   sul- 
phur, it  is  simply  heated  in  large  kettles  and  the  melted  sulphur 
dipped  off  from  above,  while  the  earthy  impurities  settle  to  the 
bottom  of  the  kettle.     The  product  thus  obtained  is  known  as 
crude  sulphur  ;  it  comes  to  us  in  irregular  lumps  of  a  dirty  light- 
yellow  color,  and  is  largely  employed  for  manufacturing  purposes. 

This  crude  sulphur  is  contaminated  with  more  or  less  earthy 
matter.  In  order  to  purify  it,  it  is  distilled  from  iron  retorts  into 
large  chambers  constructed  of  masonry,  in  which  it  is  deposited 
either  in  the  form  of  a  light  powder,  known  as  flowers  of 
sulphur,  or  in  the  liquid  state,  according  to  circumstances.  At 
the  beginning  of  the  operation,  while  the  chamber  is  cold,  the 
sulphur  vapor  condenses  as  an  exceedingly  fine,  soft,  powder 
(flowers  of  sulphur)  upon  the  walls  of  the  chamber.  But  heat 
is  given  off  as  the  sulphur  vapor  condenses,  and  after  a  while 
the  walls  of  the  chamber  become  so  hot  that  sulphur  will  melt 
upon  them.  After  this,  the  incoming  sulphur  vapor  of  course 
condenses  only  to  the  liquid  state,  and  a  layer  of  liquid  sulphur 
collects  upon  the  floor  of  the  chamber.  This  liquid  sulphur  is 
drawn  off  into  wooden  moulds,  and  thus  cast  into  the  sticks 
familiarly  known  as  roll-brimstone.  It  is  evident  that,  by  a  little 
management,  the  sulphur-refiner  can  obtain,  at  will,  either  flowers 
of  sulphur  or  roll-brimstone,  or  first  the  one  and  then  the  other. 

187.  At  the  ordinary  temperature  of   the  air,   sulphur  is  a 
brittle  solid,  of  a   peculiar  light  yellow   color.     It  has  neither 
taste  nor  smell,  excepting  that  when  rubbed  it  exhales  a  faint 
and  peculiar  odor.     Most  of  the   odors  which  in   everyday  life 
are  referred  to    sulphur  are  really  the  odors"  of"  various  com- 
pounds of  sulphur,  and  are  not  evolved  by  the  element   itself. 
It  is  a  bad  conductor  of  heat  and  electricity.     On  being  rubbed 
it  becomes  highly  (negatively)  electric,  and  is  still  employed  as  a 
source  of  electricity  in  some  cases.     The  symbol  of  sulphur  is 
S ;  its  atomic  weight  is  32,  being  precisely  twice  as  great  as  the 
atomic  weight  of  oxygen. 


CRYSTALLIZATION    OF    SULPHUR.  153 

188.  Sulphur  melts  easily  at  about  112°,  a  temperature  not 
very  far  above  that  at  which  water  boils.  A  fragment  of  it  may 
even  be  melted  by  heating  it  on  writing-paper  over  the  flame  of 
a  candle.  It  volatilizes  freely  at  temperatures  lower  than  its 
melting  point,  and  boils  at  440°.  Indeed,  as  is  the  case  with 
water,  it  is  a  substance  which  can  be  brought  into  either  of  the 
three  states  of  matter  without  any  difficulty ;  we  can  have  it  as  a 
solid,  a  liquid,  or  a  gas  as  we  please.  It  can  readily  be  obtained 
also  in  the  form  of  crystals. 

Exp.  83. — In  a  small  beaker  glass,  or  porcelain  capsule,  slowly 
heat  50  to  60  grms.  of  sulphur  until  it  has  entirely  melted.  Remove 
the  vessel  from  the  lamp,  and  allow  it  to  cool  slowly  until  about  a 
quarter  part  of  the  sulphur  has  solidified  ;  then  pour  off,  into  a  basin 
of  water,  that  portion  of  the  sulphur  which  is  still  liquid,  breaking 
through,  for  this  purpose,  the  crust  at  the  top  of  the  liquid,  if  any  such 
have  formed.  The  interior  of  the  vessel  will  be  found  to  be  lined  with 
transparent,  prismatic  crystals. 

Exp.  84.  —  In  a  test-tube,  melt  enough  sulphur  to  fill  one-quarter  of 
the  tube ;  place  the  tube  in  such  a  position  that  its  contents  may  cool 
slowly  and  quietly,  and  then  watch  the  formation  of  crystals  as  they 
shoot  out  from  the  comparatively  cold  walls  of  the  tube  towards  the 
centre  of  the  liquid. 

Exp.  83  represents  one  general  method  of  obtaining  crystals. 
Crystals  of  many  of  the  metals,  lead  and  bismuth  for  example, 
can  be  obtained  by  operating  in  this  way ;  it  is  only  neces- 
sary to  melt  the  metal  in  a  crucible  of  some  refractory  material, 
placed  in  a  furnace.  The  melted  metal  having  then  been  allowed 
to  cool  until  a  tolerably  firm  crust  has  formed  upon  its  surface, 
this  crust  is  pierced  with  an  iron  rod,  and  the  crucible  quickly 
inverted,  so  that  the  portion  of  the  metal  which  still  remains 
fluid  in  the  interior  shall  flow  out.  Upon  afterwards  breaking 
the  crucible,  crystals  will  be  found  lining  the  cavity  of  the 
metallic  cup  which  has  been  formed  within  it. 

189.  Exp.  84,  besides  illustrating  the  manner  in  which  crys- 
tals form,  teaches  us  something  of  the  physical  structure  of 
solid  bodies.  The  solid  mass  of  sulphur  which  is  left  in  the  test- 
tube,  when  it  has  become  cold,  is  evidently  nothing  more  than 
a  compact  bundle  of  interlaced  crystals.  If  the  mass  be  re- 
moved from  the  tube,  and  then  broken  across,  it  will  present  a 


154  CRYSTALLINE    STRUCTURE. 

glistening  appearance,  owing  to  the  reflection  of  light  from  the 
surfaces  of  the  minute  crystals  of  which  it  is  composed.  It  is 
said  to  have  a  crystalline  structure.  This  crystalline  structure  is 
apt  to  render  a  body  brittle ;  substances  which  possess  it  are 
liable  to  break  "  with  the  grain,"  or  to  split  in  certain  directions 
determined  by  the  shape  of  the  crystals,  and  called  lines  of 
cleavage  ;  a  stick  of  roll-brirnstone,  for  example,  may  be  readily 
broken  or  cut  across,  but  not  so  easily  in  the.  direction  of  its 
length.  The  same  remark  applies  to  many  samples  of  metal. 
In  all  cases  where  tenacity  is  required,  it  is  important  to  counter- 
act, or  to  prevent  as  much  as  possible,  the  tendency  towards 
crystallization.  Thus,  in  manufacturing  wrought  iron,  it  is 
the  -constant  endeavor  of  the  workman  to  render  the  metal 
stringy  or  fibrous,  and  not  crystalline,  and  he  seeks  to  accomplish 
this  by  appropriate  processes  of  kneading,  squeezing,  and  rolling. 
190.  Another  easy  way  to  crystallize  sulphur  is  by  the 
method  of  solution  and  evaporation,  such  as  was  employed  in 
the  preparation  of  nitrate  of  ammonium  (Exp.  33).  Sulphur 
is  not  soluble  in  water,  but  it  dissolves  readily  in  a  liquid  com- 
pound of  sulphur  and  carbon,  known  as  bisulphide  of  carbon, 
which  being  readily  volatile,  quickly  escapes,  on  exposure  to  the 
air,  and  so  deposits  the  sulphur. 

Exp.  85.  —  Place  in  a  test-tube  a  small  teaspoonful  of  flowers  of 
sulphur,  pour  upon  the  sulphur  10  or  12  c.  c.  of  bisulphide  of  carbon, 
close  the  tube  with  a  cork,  and  allow  the  mixture  to  stand  during  half 
an  hour,  shaking  it  occasionally.  Decant  the  clear  liquid  from  the 
sulphur  which  still  remains  undissolved,  and  pour  it  into  a  small  porce- 
lain capsule,  which  place  out  of  doors,  or  in  a  draught  of  air,  until  the 
highly  offensive  bisulphide  of  carbon  has  all  evaporated.  Crystals  of 
sulphur  will  then  be  found  at  the  bottom  of  the  dish. 

This  experiment  might  be  modified  by  preparing,  in  the  first  place,  a 
saturated  solution  of  sulphur  in  boiling  bisulphide  of  carbon,  and  then 
allowing  the  clear-  solution  to  cool  slowly.  Crystals  of  sulphur  would 
finally  be  found  beneath  the  cold  liquid.  The  method  by  evaporation, 
as  above  described,  is  to  be  preferred. 

It  will  be  noticed  that  the  crystals  of  Exp.  85  are  not  shaped 
like  those  obtained  by  the  method  of  fusion  in  Exp.  83.  The 
two  sets  of  crystals  belong  in  fact  to  entirely  different  systems  of 
crystallization. 


THE     SIX    SYSTEMS    OF    CRYSTALLIZATION.  155 

191.  The  researches  of  cry stallogra  pliers  have  proved  that 
the  crystals  of  natural  minerals  and  artificial  chemical  substances 
may  all  be  included  in  six  general  classes  of  form,  called  systems 
of  crystallization.  In  every  crystal,  certain  directions  may  be 
recognized,  with  reference  to  which  the  bounding  planes  of  the 
crystal  exhibit  a  more  or  less  symmetrical  arrangement.  These 
directions,  represented  by  straight  lines  drawn  through  the  centre 
of  the  crystal,  are  called  axes.  The  thousands  of  crystal-forms 
which  occur  in  nature,  or  are  produced  by  art,  have  been  divided 
into  six  systems,  or  groups,  by  observation  of  the  number,  rela- 
tive length  and  mutual  inclination  of  the  axes  around  which  they 
are  symmetrically  formed.  These  six  systems  are  defined  as 
follows :  — 

I.  Monometrie  (single-measure)  or  Regular  System.  —  The  axes  are 
three  in   number,  equal  in  length,  and  intersect  each  other  at  right 
angles.     The  cube,  regular  octahedron,  and  rhombic  dodecahedron, 
forms  of  perfect  symmetry,  belong  to  this  system. 

II.  Dimetric  (two-measure)  System.  —  The  axes  are  three  in  number, 
and  intersect  each  other  at  right  angles ;  but  one,  called  the  vertical,  is 
either  longer  or  shorter  than  the  two  lateral  'which  are  equal.     The 
right  square  prism  and  square  octahedron  are  of  this  system. 

III.  Trimetrlc    (three-measure)    System. —  The    axs  are    three   in 
number,  unequal  in  length  and  intersect  each  other  at  right  angles. 
The  system  includes  the  right  rectangular  prism,  the  right    rhombic 
prism,  and  the  rhombic  octahedron. 

IV.  Monoclinic  (single-inclination)  System.  —  The  axes  are  three  in 
number,  and  unequal  in  length,  and  one,  called  the  vertical,  is  at  right- 
angles  Avith  one  of  the  other  two  axes,  which  are  called  lateral,  but 
obliquely  inclined  to  the  other ;    the  two  lateral  axes  intersect  each 
other  at  right  angles.     The   right  rhomboidal   and   oblique    rhombic 
prisms  belong  to  this  system. 

V.  Triclinic   (three-inclination)   System.  —  The    axes  are   three   in 
number,  unequal  in  length,  and  all  their  intersections  are  oblique.     The 
oblique  rhomboidal  prism  is  of  this  system. 

VI.  Hexagonal    System.  —  The    axes   are   four  in   number ;  three, 
called  lateral,  lie  in  one  plane,  are  equal  in  length,  and  intersect  each 
other  at  angles  of  60°  ;  the  fourth  axis,  called  vertical,  is  either  longer 
or  shorter  than  the  other  three,  and  crosses  them  at  right  angles.     This 
system  includes  the  hexagonal  prism  and  the  rhombohedron. 

Under  these  systems  of  crystallization,  the  variety  of  possible  forms 
and  dimensions  is  unlimited.  Thus,  in  systems  in  which  the  axes  are 


156 


SULPHUR    IS    DIMORPHOUS. 


unequal,  the  inequality  may  be  great  or  small,  through  all  degrees  of 
discrepancy ;  in  oblique  systems  the  inclination  of  the  axes  may  vary 
indefinitely ;  rhombohedrons  may  occur  of  every  angle.  Thus  the 
actual  forms  of  crystallography  become  exceedingly  numerous,  although 
they  all  belong  to  a  few  simple  types. 

If  the  student  draws  in  perspective,  upon  paper,  the  axes  of  the 
several  systems  above  described,  or  better,  constructs  the  different  sets 
of  axes  out  of  bits  of  wood  or  wire,  he  will  appreciate  the  fact  that 
forms  belonging  to  different  systems  are  ordinarily  so  unlike  in  general 
appearance  as  to  be  readily  distinguishable,  even  by  those  who  have  no 
exact  knowledge  of  the  mathematical  science  of  crystallography. 

192.  As  a  general  rule,  a  substance  crystallizes  in  forms  be- 
longing to  only  one  system,  and  the  crystalline  form  of  a  sub- 
stance is  something  so  constant  and  characteristic  as  to  be  one  of 
the  chemist's  most  valued  means  of  recognition  and  definition. 
But  this  general  rule  is  not  without  exceptions.  Sulphur,  as 
has  just  been  proved,  may  be  made  to  crystallize  in  forms  be- 
longing to  two  distinct  systems  of  crystallization,  and  there  are 
other  substances,  not  a  few,  which  when  crystallized  under  differ- 
FIG.  34.  ent  conditions,  assume  forms  of  two  distinct  systems. 
Substances  which  are  thus  capable  of  assuming  erys- 
\  talline  forms  belonging  to  two  different  systems  are 
said  to  be  dimorphous  (two-formed).  Two  such  dif- 
ferent forms  of  the  same  substance  often  FIG.  35. 
have  quite  dissimilar  physical  properties ; 
they  are  apt  to  differ  from  each  other  in 
hardness,  specific  gravity,  color,  optical  prop- 
erties, and  in  their  relation  to  heat ;  the 
chemical  properties,  also,  of  two  such  differ- 
ent forms  are  seldom  entirely  the  same. 
The  crystals  of  sulphur  obtained  by  fusion  (Exp.  83),  are 
elongated  oblique  rhombic  prisms  (Fig.  34),  and  belong  to  the 
fourth,  or  monoclinic,  system.  The  crystals  of  sulphur,  which 
are  derived  from  its  solution  in  bisulphide  of  carbon  (Exp.  85), 
are  rhombic  octahedrons  (Fig.  35),  belonging  to  the  trimetric 
system.  The  specific  gravity  of  the  octahedral  crystals  is  greater 
than  that  of  the  prismatic  in  the  ratio  of  2.07  :  1.91.  The  spe- 
cific heat  of  the  octahedral  crystals  is  0.163,  and  that  of  the 
prismatic  somewhat  greater.  The  melting  point  of  the  prismatic 
crystals  is  about  120°. 


CHANGE    OF    PRISMATIC    INTO    OCTAHEDRAL    SULPHUR.      157 

The  prismatic  crystals  of  sulphur  (Exp.  83),  cannot  be  kept 
for  any  great  length  of  time.  They  soon  lose  their  transparency 
and  characteristic  amber  color,  becoming  opaque  and  light 
yellow,  like  ordinary  brimstone.  If  they  be  examined  under 
the  microscope  it  will  be  seen  that  the  prisms  are  now  composed 
of  a  multitude  of  little  octahedral  crystals.  The  change  of 
color  and  texture  is  due  to  a  rearrangement  of  the  particles  of 
the  original  crystals,  though  the  aggregation  of  octahedrons 
which  have  been  formed  within  the  prismatic  crystal  still  retains 
the  shape  of  the  prism.  If  the  prismatic  crystals  be  left  at  rest, 
this  change  of  form  usually  begins  in  the  course  of  a  few  hours, 
but  it  may  be  greatly  accelerated  by  scratching  the  crystals,  or 
shaking  them  together.  Under  ordinary  circumstances  the  pas- 
sage of  the  sulphur  from  the  one  molecular  state  to  the  other 
goes  on  very  slowly,  several  years  being  often  required  for  its 
completion ;  but  the  change  can  be  accomplished  immediately 
by  moistening  the  prismatic  crystals  with  bisulphide  of  carbon. 
A  considerable  amount  of  heat  is  developed  as  the  prismatic 
sulphur  changes  into  octahedral ;  this  can  readily  be  appreciated 
when  the  conversion  is  effected  by  means  of  bisulphide  of  carbon. 

In  the  same  way  that  prismatic  sulphur  slowly  changes  into 
the  octahedral  variety  at  the  ordinary  temperature,  octahedral 
sulphur  is  gradually  converted  into  prismatic  sulphur,  when  kept 
for  a  long  time  at  a  temperature  near  its  melting-point.  The 
change  in  specific  gravity  enables  us  to  follow  the  progress  of 
this  conversion. 

Sulphur,  which  has  been  melted  and  allowed  to  solidify  gradu- 
ally, is  always  in  the  prismatic  condition  immediately  after  the 
solidification.  Roll-brimstone,  for  example,  when  fresh  from  the 
moulds,  is  translucent,  and  of  a  dark  amber  or  brownish-yellow 
color,  like  the  prismatic  crystals  of  Exp.  83,  but  in  a  short  time, 
often  in  the  course  of  a  few  hours,  the  sticks  become  light-yellow 
'and  opaque,  as  we  find  them  in  commerce,  and  are  then  com- 
posed, at  least  externally,  of  a  mass  of  octahedral  crystals. 

193.  There  is  still  a  third  way  of  obtaining  crystals  of  sulphur, 
naruely,  by  sublimation.  At  slightly  elevated  temperatures,  sul- 
phur is  volatile,  and  if  the  circumstances  be  such  that  the  vapor 
shall  condense  very  slowly,  crystals  will  form.  The  natural 


158  METHODS    OF    OBTAINING    CRYSTALS. 

crystals  of  sulphur  found  in  volcanic  countries,  which  are  often 
very  large  and  of  great  beauty,  have  been  formed  in  this  way. 
These  native  crystals  are  octahedral,  like  those  obtained  by 
means  of  bisulphide  of  carbon  (see  Exp.  85). 

194.  As  appears  from  the  foregoing,  there  are  three  distinct 
methods  of  obtaining  crystals :  —  I.  By  fusion ;  that  is  to  say, 
by  the  slow  cooling  of  molten  matter.  II.  By  solution,  fol- 
lowed either  by  removal  of  the  solvent  by  evaporation  or  chem- 
ical means,  or  by  reduction  of  its  temperature.  III.  By 
sublimation. 

A  familiar  instance  of  the  first  method  is  seen  in  the  case  of 
ice,  as  when  a  part  of  the  water  in  any  hollow  vessel  freezes 
slowly  upon  the  sides  of  the  vessel ;  of  the  second,  in  the  manu- 
facture of  common  salt ;  and  of  the  third,  in  the  formation  of 
frost  upon  a  window-pane. 

There  is  still  a  fourth  general  method  of  obtaining  crystals, 
which  consists  in  very  slowly  decomposing  some  chemical  com- 
pound of  the  substance  to  be  crystallized,  either  by  the  addition 
of  some  other  chemical  agent,  or  by  means  of  the  galvanic  cur- 
rent. Crystals  of  sulphur  may  be  formed  in  this  way,  and  are 
in  fact  sometimes  found  in  the  pipes  used  to  convey  illuminating 
gas  through  the  streets  of  cities,  under  such  circumstances  that 
it  is  evident  that  they  have  resulted  from  the  decomposition 
of  some  one  of  the  sulphur  compounds  with  which  coal-gas  is 
always  contaminated. 

It  must  not  be  inferred,  from  the  above  enumeration  of  the 
ordinary  methods  of  obtaining  crystals,  that  either  fusion,  solu- 
tion, or  sublimation  is  a  necessary  condition  of  the  formation 
of  crystals.  Both  in  nature  and  in  art  examples  occur  of  the 
crystalline  arrangement  of  particles  within  solid  masses,  under 
circumstances  which  preclude  the  idea  that  either  fusion,  solu- 
tion, or  sublimation,  in  the  ordinary  sense  of  these  terms,  should 
have  occurred. 

1,95.  Sulphur  behaves  in  a  very  remarkable  manner  on  being 
heated.  When  melted  at  the  lowest  possible  temperature,  110° 
to  115°,  it  forms  a  limpid  liquid  of  a  light-yellow  color;  but  if 
this  liquid  be  heated  more  strongly,  it  begins  to  become  viscid 
and  dark-colored  at  about  150°,  and  at  170°  to  200°  it  is  almost 


SOFT    SULPHUR.  159 

black,  and  at  the  same  time  so  thick  and  tenacious  that  it  can- 
not be  poured  from  the  vessel  which  holds  it,  even  if  the 
vessel  be  inverted.  At  330°  to  340°  it  regains  its  fluidity  in 
part,  though  the  liquid  is  still  dark  colored,  and  finally,  at  about 
440°,  it  begins  to  boil,  and  is  converted  into  an  amber-colored 
vapor.  The  specific  gravity  of  sulphur  vapor,  referred  to 
hydrogen,  is  32. 

196.  If  melted  sulphur,   in  the  viscid  state,  or,   better,  that 
which  has  regained  its  mobility,  be  suddenly  cooled,  a  semi-solid 
modification  of  sulphur,  remarkably  different  from  the  ordinary 
form,  will  be  obtained. 

Exp.SG.  —  Place  in  a  test-tube,  of  about  30  c.  c.  capacity,  15  to  20 
grms.  of  coarsely-powdered  sulphur ;  melt  the  sulphur  slowly  over  the 
gas-lamp,  and  continue  to  heat  it  until  it  begins  to  boil,  noting,  mean- 
while, the  changes  which  the  sulphur  undergoes,  —  as  described  in  §  195. 
Finally  pour  the  hot  sulphur,  in  a  fine  stream,  into  a  large  dish  full 
of  cold  water.  There  will  be  obtained  a  soft,  elastic,  reddish-brown 
mass,  which  can  be  kneaded  and  moulded  like  wax,  and  drawn  out 
into  threads,  like  caoutchouc. 

This  soft  sulphur  cannot  be  preserved  for  any  great  length 
of  time.  When  left  to  itself,  at  the  ordinary  temperature  of  the 
air,  it  slowly  hardens  and  changes  into  ordinary  brittle  yellow 
sulphur.  This  change  is  accelerated  by  kneading,  and  is  in- 
stantaneous at  the  temperature  of  100°.  In  any  event,  a  certain 
amount  of  heat  is  evolved  as  the  soft  sulphur  changes  into 
ordinary  sulphur.  The  specific  gravity  of  soft  sulphur  is  some- 
what lower  than  that  of  the  prismatic  crystals. 

From  the  foregoing  facts  it  appears  that  sulphur,  like  oxygen, 
is  capable  of  assuming  different  allotropic  states.  (See  §  162.) 

197.  In  its  behavior  towards  solvents,  sulphur  presents  some 
curious    anomalies.      Some    specimens    of    sulphur    are   freely 
soluble  in  bisulphide  of  carbon,   while    of   other  samples  only 
a  comparatively  small  portion  dissolves.     We  distinguish,  there- 
fore, a  soluble  and  an  insoluble  modification  of  sulphur. 

Octahedral  sulphur,  the  bright  yellow,  translucent,  native  crys- 
tals for  example,  is  completely  soluble  in  bisulphide  of  carbon. 
But  of  the  soft,  elastic  sulphur,  such  as  was  prepared  in  Exp.  86, 
as  much  as  30  or  40  per  cent,  is  completely  insoluble  in  the 
bisulphide,  whether  this  liquid  be  hot  or  cold. 


160  MILK    OF    SULPHUR. 

No  method  has  as  yet  been  discovered  of  preparing  pure  in- 
soluble sulphur  directly,  but  it  can  always  be  readily  obtained 
by  dissolving  out  the  soluble  sulphur  from  a  mixture  of  the  two 
varieties,  such  as  the  soft  sulphur  above-mentioned.  Flowers  of 
sulphur  contain  a  considerable  portion  of  insoluble  sulphur ; 
roll-brimstone  much  less,  though  the  interior  of  the  sticks  con- 
tains decidedly  more  than  the  outside  portions.  It  may  be 
observed,  in  this  connection,  that  flowers  of  sulphur  are  prepared 
by  suddenly  cooling  the  vapor  of  sulphur,  while  the  soft  variety 
is  obtained  by  suddenly  cooling  melted  sulphur. 

Insoluble  sulphur  undergoes  no  change  at  the  ordinary  tem- 
perature, but  if  it  be  kept  for  a  long  time  at  100°,  or  if  it  be 
exposed  to  the  vapor  of  water  or  alcohol,  it  is  slowly  converted 
into  the  soluble  variety. 

198.  For  some  pharmaceutical  purposes,  sulphur  is  prepared 
as  a  powder  finer  even  than  flowers  of  sulphur.  This  prepara- 
tion is  known  as  milk  of  sulphur  or  precipitated  sulphur. 

Exp.  87.  —  Place  in  a  test-tube  as  much  flowers  of  sulphur  as  can  be 
taken  up  on  the  point  of  a  pen-knife  ;  half  fill  the  tube  with  a  solution 
of  caustic  soda,  and  boil  the  mixture  for  some  time.  Part  of  the  sul- 
phur will  dissolve  and  color  the  liquid  yellowish-brown. 

Pour  off  the  clear  liquid  from  the  undissolved  sulphur,  mix  it  with 
an  equal  volume  of  water  and  stir  in  dilute  chlorhydric  acid,  added  by 
small  portions,  until  a  drop  of  the  mixture  placed  upon  litmus  paper 
exhibits  an  acid  reaction.  As  the  acid  is  added,  the  liquid  assumes  a 
milky  appearance  from  the  separation  of  sulphur  in  the  form  of  an 
exceedingly  fine  powder.  This  powder  is  so  light  that,  for  a  long 
while,  it  will  not"  subside,  but  remains  suspended  in  the  liquor,  impart- 
ing to  it  a  milky  appearance. 

Collect  the  powder  on  a  filter,  wash  it  with  water,  and  dry  it  at  a 
gentle  heat.  It  will  now  appear  as  a  pale  yellowish- gray  impalpable 
powder.  If  it  be  heated  more  strongly,  so  that  it  melts,  the  color  will 
become  distinctly  yellow,  the  numberless  small  particles  of  the  powder 
being  now  compacted  into  a  single  mass.  . 

It  will  be  remarked  that  the  color  of  flowers  of  sulphur  is 
lighter  than  that  of  roll-brimstone,  while  the  color  of  the  pre- 
cipitated sulphur  is  far  lighter  than  that  of  the  flowers.  Such 
differences  as  these  are  common ;  they  depend  upon  a  difference 
of  mechanical  condition,  upon  differences  in  the  state  of  aggre- 
gation of  the  particles  of  the  substances  which  exhibit  them. 


METALS    BURN   IN    SULPHUR.  161 

The  method  of  pulverization  by  precipitation,  employed  in  this 
experiment,  is  a  general  method,  applicable  to  many  other  sub- 
stances besides  sulphur. 

199.  Sulphur  unites  energetically  with  most  of  the  other  ele- 
ments, such  union  being,  in  many  cases,  attended  with  evolution 
of  light.     Most  of  the  metals,  for  example,  combine  with  it 
directly,  just  as  they  do  with  oxygen. 

Exp.  88. —  Melt  in  an  ignition-tube  12  to  Fia.  36. 

15  c.  m.  long,  4  or  5  grms.  of  sulphur,  and 
heat  the  liquid  until  it  boils ;  then  throw  in 
small  portions  of  copper  filings,  or  fine  turn- 
ings, and  observe  the  violent  action  which 
ensues. 

Or  a  strip  of  very  thin  sheet  copper  or  a  coil 
of  fine  copper-wire  may  be  suspended  in  the 
hot  sulphur  vapor,  in  the  upper  part  of  the 
ignition-tube ;  it  will  glow  vividly  as  it  unites 
with  the  gaseous  sulphur,  much  in  the  same 
way  as  if  it  were  burning  in  oxygen  gas.  The 
product  of  the  reaction,  in  either  case,  is  called 
sulphide  of  copper. 

Exp.  89.  —  Mix  intimately  4  grms.  of  flowers  of  sulphur  and  7  grms. 
of  the  finest  iron  filings.  Place  the  mixture  in  an  ignition-tube  10  to 
12  c.  m.  long,  and  heat  the  lower  end  of  the  tube  over  the  gas-lamp. 
In  a  short  time  the  mass  will  begin  to  glow,  as  the  sulphur  and  iron 
enter  into  chemical  combination,  and  this  ignition  will,  of  itself,  pass 
through  the  entire  length  of  the  tube,  even  if  the  lamp  be  withdrawn. 
The  final  product  of  the  reaction  is  protosulphide  of  iron. 

200.  As  has  been  already  shown  (§§  2, 109),  phenomena  of 
combustion,  such  as   are  exhibited   in   these   experiments,  are 
directly  referable  to  chemical  union.     They  are  strictly  analo- 
gous to  the  ordinary  processes  of  combustion  in  which  oxygen 
is  involved,  though  the  technical  term,  combustion,  is  by  custom 
limited  to  the  act  of  combination  with  oxygen. 

Sulphur  combines  witli  chlorine,  bromine,  iodine,  and  phos- 
phorus at  the  ordinary  temperature,  and  with  carbon  at  a  red 
heat.  With  oxygen  it  unites  readily  at  a  comparatively  low 
temperature.  When  heated  in  the  air,  it  takes  fire  at  about 
250°,  and  burns  with  a  peculiar  blue  light.  This  easy  inflam- 
mability may  be  readily  illustrated  by  blowing  flowers  of  sulphur 
11 


162  USES    OF    SULPHUR. 

into  the  hot  air  issuing  from  the  chimney  of  an  Argand  gas-lamp  ; 
the  sulphur  takes  fire  at  a  considerable  height  above  the  flame. 
The  irritating,  suffocating  gas,  which  is  produced  by  the  union  of 
sulphur  and  oxygen,  will  be  shortly  described  under  the  name 
of  sulphurous  acid. 

Several  important  practical  applications  of  sulphur  depend 
upon  this  property  of  igniting  and  continuing  to  burn  at  a  mod- 
erate heat.  It  is,  in  fact,  largely  employed  as  a  kindling  material. 
By  means  of  it,  other  bodies  less  readily  combustible,  can  be 
heated  to  the  temperature  at  which  they  continue  to  burn. 
Hence  its  use  upon  matches  and  in  gunpowder  and  fire-works. 

201.  In  its  chemical  properties,  sulphur  is  closely  allied  to 
oxygen ;   like  oxygen,  it  forms  a  great  variety  of  compounds 
with  a  wide  range  of  different  elements,  and  the  series  of  com- 
pounds  thus   obtained  is   in    many  respects   parallel   with,   or 
comparable  to,  the  series  of  oxygen  compounds. 

It  is  an  important  raw  material  in  the  chemical  arts,  being  an 
ingredient  of  numerous  useful  compounds,  such  as  cinnabar, 
ultramarine,  vulcanized  caoutchouc,  bisulphide  of  carbon,  chlo- 
ride of  sulphur,  and  the  various  compounds  of  sulphur  and 
oxygen,  one  of  which,  sulphuric  acid,  is  the  most  important 
chemical  agent  at  present  employed  in  manufacturing  industry. 
Sulphur  is  largely  employed  in  medicine,  in  the  treatment  of 
cutaneous  diseases  of  both  men  and  domesticated  animals,  and 
has  been  of  late  years  extensively  used  in  the  vineyards  of 
Europe  for  destroying  a  parasitic  fungus  which  infests  the  vines. 

202.  Sulphydric  Acid  (H2S).     When  sulphur  is  sublimed  in 
hydrogen  gas,  or  when  hydrogen  is  passed  over  melted  sulphur, 
combination  takes  place  between  the  two  elements,  though  very 
slowly  and  imperfectly,  so  that  only  a  comparatively  small  quan- 
tity of  the  compound  is  obtained,  even  if  the  process  be  coji- 
/tinued  for  a  long  time.     Somewhat  larger  quantities  of  it  are 
formed  when  a  mixture  of  hydrogen  "gas  .and  sulphur-vapor  is 
passed   through  a  tube  filled  with  fragments  of  pumice-stone 
heated  to  about  500°. 

When  the  two  elements  meet  in  the  nascent  state,  they  com- 
bine readily;  thus,  when  organic  bodies  containing  sulphur 
putrefy,  and  when  they  are  subjected  to  destructive  distillation, 


PREPARATION    OF    SULPHYDRIC    ACID. 


163 


Fio.  37. 


sulphydric  acid  is  evolved,  just  as  ammonia  is  under  the  same 
circumstances.  In  either  event,  the  product  of  the  union  is  a 
colorless  gas  of  highly  offensive  odor,  like  that  of  rotten  eggs. 

An  easier  method  of  preparing  sulphydric  acid,  or  sulphu- 
retted hydrogen,  as  it  is  often  called,  is  by  acting  upon  a  compound 
of  sulphur  and  iron  with  dilute  chlorhydric  acid. 

Exp.  90. —  In  a  gas-bottle,  Fig.  37,  put  10  or  12  grms.  of  proto- 
sulphide  of  iron,  see  Exp.  89  ; 
replace  the  cork  in  the  bottle  and 
introduce  the  gas  delivery-tube 
into  another  small  bottle  contain- 
ing cold  water,  letting  it  dip  5  or 
6  c.  m.  beneath  the  surface  of  the 
water.  Through  the  thistle-tube, 
pour  into  the  gas-bottle  water 
enough  to  seal  the  lower  extrem- 
ity of  this  tube  ;  then  add,  through 
the  thistle-tube,  as  before,  2  or  3 
teaspoonfuls  of  muriatic  acid,  and 
observe  that  bubbles  of  gas  soon 
begin  to  pass  through  the  water 
in  the  absorption  bottle. 

Sulphydric  acid  is  soluble  in  water  to  a  considerable  extent,  and  is 
consequently  taken  up  by  the  water  in  the  absorption  bottle.  The 
solution  thus  obtained,  known  as  sulphuretted  hydrogen-water,  is  much 
employed  as  a  reagent  in  chemical  laboratories ;  it  will  serve  us  here 
as  a  convenient  source  of  sulphydric  acid. 

When  the  disengagement  of  gas  slackens,  a  new  portion  of  muriatic 
acid  may  be  added  through  the  thistle-tube,  and  this  process  continued 
until  the  water  in  the  absorption  bottle  smells  strongly  of  the  gas.  > 

This  experiment  should  be.  performed  out  of  doors,  or  in  a  draught  of 
air  so  arranged  that  those  portions  of  the  gas  which  escape  solution 
shall  be  carried  away  from  the  operator. 

203.  The  reaction  between  the  sulphide  of  iron  and  chlorhy- 
dric acid  in  the  foregoing  experiment  is  somewhat  analogous  to 
that  which  occurs  in  the  preparation  of  hydrogen,  §  50.  If 
metallic  iron  (or  zinc)  be  treated  with  chlorhydric  acid,  hydrogen 
is  evolved,  according  to  the  equation, 

Fe  +  2HC1  =  FeCl2  +  2H . 
But  if,  instead  of  simple  iron,  sulphide  of  iron,  whose  formula 


164 


PREPARATION    OF    SULPHYDRIC    ACID. 


is  FeS,  be  taken,  sulphur  will  be  eliminated,  as  well  as  hydrogen, 
by  the  action  of  the  acid,  and  these  elements,  as  they  come 
together  in  the  nascent  state,  will  unite  to  form  sulphydric  acid. 

FeS  +  2HC1  =  FeCl2  +  H2S  . 

Instead  of  absorbing  the  gas  evolved  in  the  foregoing  experi- 
ments in  water,  it  might  be  collected  as  such,  over  a  basin  hold- 
ing but  a  small  quantity  of  water,  or,  better,  filled  with  warm 
water  or  with  brine,  either  of  which  absorbs  less  of  the  gas 
than  cold  water.  Unless  absolutely  dry,  the  gas  cannot  be  col- 
lected over  mercury,  since,  when  moist,  it  acts  upon  this  metal. 

204.  At  the  ordinary  temperature  and  pressure,  sulphydric 
acid  is  a  gas  somewhat  heavier  than  air,  its  specific  gravity  being 
17,  referred  to  hydrogen  ;  but  under  a  pressure  of  about  15  atmos- 
pheres at  11°,  it  becomes  liquid.     The  specific  gravity  of  this 
liquid  referred  to  water  is  0.9.     At  —  85°  the  liquid  solidifies 
to  a  white  crystalline  mass. 

205.  Since  the  sulphide  of  iron,  employed  in  the  preparation 
of  sulphydric  acid,  is  usually  mixed  with  a  certain  quantity  of 
metallic  iron,  the  gas  is  liable  to  be  contaminated  with  free  hydro- 
gen.     For   all   ordinary   purposes,   the   gas   thus   mixed   with 
hydrogen  serves  as  well  as  if  it  were  pure,  but  in  some  cases  a 
gas  free  from  hydrogen  is  required.     In   order  to  prepare  it, 
sulphide  of  antimony  is  substituted  for  the  sulphide  of  iron. 

1  part  of  powdered  sulphide 
of  antimony  placed  in  a  thin 
bottomed  flask  is  treated  with 
3  or  4  parts  of  chlorhydric 
acid  of  1.1  sp.  gr.  and  the 
mixture  gently  heated.  The 
apparatus  may  be  arranged 
as  in  Fig.  38,  in  wlych  the 
bottle  into  which  the  gas  first 
enters  contains  a  very  small 
quantity  of  water;  this  water 
serves  to  remove  any  particles 
of  the  acid  or  of  solid  matter 
which  may  have  been  carried 
over  in  the  current  of  gas.  In 
case  a  dry  gas  be  needed,  a 


ANALYSIS    OF    SULPHYDRIC    ACID.  165 

chloride  of  calcium  tube  (see  Appendix,  §   15)   must  be  interposed 
between  the  wash-bottle  and  the  mercury-trough. 

206.  The  volumetric  composition  of  sulphydric  acid  gas  is 
one  volume  of  sulphur  vapor  and  two  volumes  of  hydrogen,  con- 
densed to  two  volumes.  Its  molecule,  therefore,  contains  one 
atom  of  sulphur  and  two  atoms  of  hydrogen,  and  is  strictly 
analogous  to  the  molecule  of  water. 

The  composition  of  sulphydric  acid  may  be  determined  experi- 
mentally by  heating  metallic  tin  in  a  confined  volume  of  the  gas. 
An  ignition  tube  20  or  30  c.  m.  long,  bent  at  an  obtuse  angle,  within 
5  to  6  c.  m.  of  the  .closed  extremity,  as  shown  in  Fig.  39,  should  be 
completely  filled  with  dry  sulphydric  acid  gas  over  the  mercury-trough, 
and  then  closed  with  the  thumb  and  inverted.  FIG.  39. 

Some  granulated  tin  should  be  dropped  into 
the  tube  and  made  to  lodge  in  the  bent  part, 
the  thumb  being  instantly  replaced  upon  the 
mouth  of  the  tube  the  moment  the  tin  has 
entered. 

The  tube  full  of  gas  is  now  replaced  in  the 
mercury  trough,  as  shown  in  the  figure,  and 
about  one-third  part  of  the  gas  is  allowed  to  escape  by  inclining  the 
tube  so  that  the  gas  may  bubble  out  through  the  mercury.  The  tube 
and  its  contents  are  left  at  rest  during  half  an  hour,  in  order  that  they 
may  acquire  the  temperature  of  the  surrounding  air,  a  caoutchouc  ring 
is  slipped  down  the  tube  to  mark  the  height  of  the  gas,  and  the  tin  is 
then  heated  with  the  flame  of  a  spirit  lamp.  The  hot  tin  combines 
with  the  sulphur,  and  hydrogen  is  set  free.  The  apparatus  is  left  at 
rest  during  another  half  hour,  and  the  height  of  the  gas  in  the  tube 
is  then  noted.  If  the  gas  employed  was  pure,  it  will  be  found  that  its 
volume  has  undergone  no  change.  The  hydrogen  which  has  been  set 
free  occupies  precisely  the  same  space  as  the  sulphydric  acid  did  before 
it  was  decomposed. 

It  is  evident  from  this  that  1  volume  of  sulphydric  acid  gas  contains 
1  volume  of  hydrogen,  or,  multiplying  these  numbers  by  2,  in  order  to 
arrive  at  the  composition  indicated  by  our  molecular  formula,  that  2 
volumes  of  sulphydric  acid  contain  2  volumes  of  hydrogen.  Now,  the 
specific  gravity,  or  unit-volume-weight  of  sulphydric  acid  has  been 
found,  by  experiment,  to  be  17.19,  that  of  hydrogen  being  1 ;  and  if 

From  the  weight  of  2  volumes  of  sulphydric  acid,       .         .         .     34.38 
We  subtract  the  weight  of  2  volumes  hydrogen,   .         ,         .  2.00 

There  will  remain  .        .        .     32.38 


166 


PROPERTIES    OF    SULPHYDRIC    ACID. 


which  is  very  nearly  equal  to  the  unit-volume-weight  of  sulphur-vapor, 
81.8,  as  experimentally  determined. 

The  composition    of   sulphydric   acid,  both   by  volume    and 
weight,  may,  therefore,  be  expressed  by  the  diagram. 


H2S 

34 

207.  The  gas  is  very  poisonous ;  when  respired  in  the  pure 
state  it  quickly  proves  fatal,  and  it  is  very  deleterious,   even 
though  largely  diluted  with  atmospheric  air.     Small  birds  soon 
die  in  air  which  contains  only  T3^^  of  its  volume  of  the  gas,  dogs 
in  air  which  contains  ¥^,  and  horses  in  air  which  contains  ^^  of 
its  volume.     Men  can  support  more  of  it,  but  in  experimenting 
with  it,  it  is  best  to  do  so  where  there  is  a  free  circulation  of  air. 
Nausea  and  headache  are  often  produced  when  an  atmosphere 
even  slightly  contaminated  with  sulphuretted  hydrogen  has  been 
breathed  for  any  length  of  time.     In  case  the  air  of  an  apart- 
ment become  contaminated  with  the  gas,  the  disgusting  smell  can 
readily  be    neutralized  by  sprinkling  the   room  with    chlorine 
water,  or  by  evolving  a  little  chlorine  gas  by  adding  some  dilute 
acid  to  a  small  quantity  of  bleaching  powder. 

The  gas  exists  as  a  natural  constituent  of  some  mineral  waters 
which  are  thence  called  sulphurous,  such  as  the  Virginia  Sul- 
phur Springs,  and  the  mineral  springs  at  Sharon,  N.  Y.  It  is 
also  found  in  the  air  and  water  of  foul  sewers,  and  wherever 
animal  matter  is  undergoing  putrefaction. 

208.  Sulphydric  acid  gas  is  readily  inflammable,  and,  like  hy- 
drogen, extinguishes  the  flame  of  a  burning  candle  immersed  in 
it.     It  burns  with  a  blue  flame,  producing  water  and  sulphurous 
acid  gas. 

H2s  +  30  =  H20  +  SO2. 

In  case  it  be  ignited  in  contact  with  a  quantity  of  air  insufficient 
to  burn  the  whole  of  it,  the  hydrogen  will  burn  first,  and  a  por- 
tion of  the  sulphur  will  escape  combustion. 

If  a  tall  glass  cylinder  be  filled  with  sulphydric  acid  gas,  and  the  gas 


SULPHURETTED  HYDROGEN  WATER.         167 

be  lighted  at  the  top,  the  flame  will  pass  down  the  cylinder  as  the  hy- 
drogen is  consumed,  and  a  quantity  of  very  finely  divided  solid  sulphur 
will  be  deposited  upon  the  walls  of  the  vessel. 

It  has  already  been  stated  that  sulphur  kindles  very  easily,  and  that 
it  has  a  strong  affinity  for  oxygen,  but  it  appears,  from  this  experiment, 
that  hydrogen  kindles  still  more  readily,  and  that  its  affinity  for  oxygen 
is  greater  than  that  of  sulphur. 

When  mixed  with  air,  in  certain  proportions,  it  is  explosive ; 
a  fact  which  should  be  borne  in  mind  by  the  experimenter. 

209.  Water  dissolves  about  three  times  its  own  volume  of  the 
gas  at  the  ordinary  temperature.  This  solution  (see  Exp.  90)  is 
transparent  and  'colorless  when  recently  prepared,  but,  when 
kept,  it  gradually  becomes  opalescent  and  turbid  from  deposition 
of  sulphur.  Oxygen  from  the  air  unites  with  the  hydrogen  of 
the  sulphydric  acid  to  form  water,  and  sulphur  is  set  free.  After 
the  lapse  of  several  weeks  or  months,  it  will  be  found  that  the 
solution  no  longer  contains  any  sulphydric  acid ;  it  has  lost  its 
nauseous  odor,  and  the  bottom  of  the  bottle  is  covered  with  sul- 
phur, the  result  of  the  decomposition  of  the  dissolved  gas. 

The  aqueous  solution  of  the  gas  reddens  litmus  slightly,  like 
the  very  weak  acids.  Towards  metals  and  metallic  oxides  it 
behaves  in  a  manner  somewhat  analogous  to  that  of  chlorhydric 
acid  and  its  congeners,  while,  with  regard  to  metallic  sulphides, 
it  stands  in  the  same  relation  as  water  to  the  oxides,  as  will  be 
explained  hereafter. 

Exp.  91. —  Place  a  drop  of  sulphuretted  hydrogen  water  (Exp.  90) 
upon  a  bright  piece  of  copper,  lead,  or  silver.  The  metal  will  quickly 
become  black.  The  sulphur  of  the  sulphydric  acid  unites  with  the 
metal,  to  form  sulphide  of  copper,  sulphide  of  lead,  or  sulphide  of  sil- 
ver, as  the  case  may  be,  while  the  hydrogen  escapes. 
Cu  +  H2S  =  CuS  -|-  2  H. 

Exp.  92.  —  Place  in  a  test  tube  as  much  litharge  (oxide  of  lead  = 
PbO)  as  can  be  held  upon  the  point  of  a  peii-knite,  pour  upon  it  a  tea- 
spoonful  of  sulphuretted  hydrogen-water,  and  observe  that  the  yellow 
litharge  immediately  becomes  black.  Sulphide  of  lead  is  formed,  as  in 
the  preceding  experiment,  together  with  a  quantity  of  water. 
PbO  -f  H2S  =  PbS  -f  H2O. 

Exp.  93.  — In  place  of  the  litharge  of  the  last  experiment,  take  a 
very  small  crystal  of  nitrate  of  lead  (Exp.  42)  ;  dissolve  it  in  as  much 


168  SULPHYDRIC    ACID    AS    A   REAGENT. 

water  as  will  half  fill  the  test  tube,  and  to  this  solution  add  a  few  drops 
of  the  sulphuretted  hydrogen-water.  Black  sulphide  of  lead  is  thrown 
down  as  a  precipitate,  and  nitric  acid  is  set  free. 

PbO,N205  -f  H2S  =  PbS  +  H20,N205. 

210.  Since  many  of  the  metallic  sulphides  are,  like  the  sul- 
phides of  lead,  copper,  and  silver,  insoluble  in  water  and  dilute 
acids,  sulphuretted  hydrogen  is  peculiarly  well  adapted  for  pre- 
cipitating the  metals  from  their  solutions.     After  having  been 
thrown  down  as  sulphides,  as  in  the  last  experiment,  they  can  be 
readily  separated  and  collected  by  filtration. 

Though  many  of  the  metallic  sulphides  are  black,  like  that  of 
lead,  this  is  not  true  of  all.  Several  of  them  exhibit  character- 
istic colors,  by  which  they  may  be  readily  recognized ;  thus,  the 
color  of  sulphide  of  antimony  is  orange,  that  of  sulphide  of 
arsenic  is  yellow,  and  that  of  sulphide  of  zinc,  white.  Upon  this 
fact,  the  application  of  sulphuretted  hydrogen,  as  a  test  or  re- 
agent, that  is,  as  a  means  of  detecting  and  identifying  many 
metals,  is  in  part  based. 

211.  In  the  same  way  that  sulphuretted  hydrogen  can  be  em- 
ployed for  detecting  the  presence  of  metals,  so,  conversely,  solu- 
tions of  the  metals,  or,  in  some  cases,  the  metals  themselves,  may 
be  used  as  tests  for  sulphuretted  hydrogen. 

Exp.  94.  —  Prepare  a  strong  aqueous  solution  of  nitrate  of  lead,  or 
better,  of  acetate  of  lead  (sugar  of  lead).  Wet  strips  of  white  paper 
3  to  4  c.  m.  wide  with  this  solution,  and  dry  them  in  air  which  is  free 
from  sulphuretted  hydrogen.  This  lead-paper,  as  it  is  called,  should 
be  kept  for  use  iri  tightly-stoppered  bottles. 

Moisten  a  bit  of  the  lead-paper  with  water  and  expose  it  to  some 
source  of  sulphuretted  hydrogen,  —  the  mouth  of  the  bottle  of  sulphy- 
dric  acid  prepared  in  Exp.  90,  for  example,  or  to  the  fetid  air  of  a 
sewer.  The  paper  will  immediately  be  blackened  from  formation  of 
sulphide  of  lead. 

The  blackening  of  silver-ware,  of  watches,  and  cards  which 
have  been  glazed  with  a  preparation  of  lead,  at  many  mineral 
springs,  and  other  localities  where  sulphuretted-hydrogen  is 
evolved,  is,  in  like  manner,  indicative  of  the  presence  of  this 
gas.  Tests  like  these,  which  are  continuous  and  cumulative,  are, 
of  course,  much  more  delicate  means  of  detection  than  the  mere 
odor  of  the  gas. 


DECOMPOSITION    OF    SULPHYDRIC    ACID.  169 

212.  Sulphydric  acid  is  a  compound  which  is  very  easily  de- 
composed.    When  simply  heated,  it  breaks  up  into  its  compo- 
nents, and  it  is  readily  destroyed  by  various  chemical  agents. 

•  Exp.  95.  —  To  a  gas-bottle,  such  as  was  employed  in  Exp.  90,  con- 
taining sulphide  of  iron,  attach  a  chloride  of  calcium  tube  (Ap- 
pendix, §  15)  and  a  piece  of  hard  glass  tubing,  No.  4,  about  20  c.  m. 
long.  To  the  end  of  this  glass  tube,  attach  another  tube  bent  at  right 
angles  and  dipping  into  a  bottle  of  water.  Pour  chlorhydric  acid  into 
the  gas-bottle,  so  that  sulphydric  acid  shall  be  freely  generated,  as  seen 
by  the  flow  of  bubbles  through  the  final  bottle  of  water.  After  the  lapse 
of  some  minutes,  when  the  apparatus  has  become  completely  filled  with 
the  gas  and  the  lasf  portions  of  air  have  been  expelled,  heat  the  middle 
of  the  tube  of  hard  glass  with  the  flame  of  the  gas-lamp,  and  observe 
the  ring  of  sulphur  which  will  collect  upon  the  walls  of  the  cold  portion 
of  the  tube  a  short  distance  in  front  of  the  flame. 

It  will  be  seen  in  subsequent  chapters  that  several  other  of  the 
gaseous  compounds  of  hydrogen  are  decomposed,  like  sulphydric  acid, 
upon  being  passed  through  hot  tubes. 

The  influence  of  oxygen,  in  decomposing  the  aqueous  solution 
of  sulphydric  acid,  has  been  already  alluded  to,  §  209 ;  it  has 
been  observed,  moreover,  that  air  contaminated  with  sulphydric 
acid  soon  becomes  odorless  of  itself,  oxygen  uniting  with  hydro- 
gen, as  before,  and  sulphur  being  set  free.  All  the  oxidizing 
agents,  that  is,  substances  which  readily  give  up  oxygen,  decora- 
pose  sulphydric  acid,  water  being  formed  and  sulphur  deposited. 

Exp.  96.  —  Into  a  test  tube  containing  4  or  5  c.  c.  of  sulphuretted 
hydrogen-water  (Exp.  90),  pour  half  as  much  concentrated  nitric  acid. 
Sulphur  will  be  deposited  and  nitrous  fumes  evolved. 

Very  dilute  nitric  acid  will  not  thus  decompose  sulphydric  acid. 

Chlorine,  bromine,  and  iodine  vapor  instantly  decompose  sul- 
phydric  acid,  uniting   with  its  hydrogen  to  form  chlorhydric, 
bromhydric,  or  iodohydric  acid,  while  sulphur  is  precipitated. 
H2S  +  2C1  =  2HC1  +  S  . 

Exp.  97.  — In  place  of  the  nitric  acid  of  the  preceding  experiment, 
pour  a  few  drops  of  chlorine  water  into  the  solution  of  sulphydric  acid, 
and  observe  that  the  odor  of  the  latter  is  destroyed  and  that  the  liquid 
becomes  turbid  from  deposition  of  sulphur. 

213.  Persulphide  of  Hydrogen,  (H2S2?).     This  is  an  exceed- 
ingly unstable  liquid,  the  composition  of  which  is  not  accurately 


170  PERSULPHIDE    OF   HYDROGEN. 

known,  though  it  is  supposed  to  be  analogous  to  that  of  the  per- 
oxide of  hydrogen.  It  can  be  prepared  by  adding  a  solution  of 
persulphide  of  calcium  to  diluted  chlorhydric  acid.  The  reaction 
may  be  conceived  to  take  place  in  accordance  with  the  following 
equation  :  — 

CaS5  +  2HC1  =  CaCl2  +  HaSa  +  3S . 

Exp.  98. —  Boil  75  to  100  grms.  of  flowers  of  sulphur  and  an  equal 
weight  of  slaked  lime  in  half  a  litre  of  water  for  about  an  hour,  then 
filter  off  the  liquor  from  the  undissolved  portions  of  sulphur  and  lime. 
The  solution  thus  obtained  is  a  mixture  of  several  sulphides  of  calcium, 
more  highly  sulphuretted  than  the  protosulphide,  but  will  serve  the 
present  purpose  as  well  as  if  it  were  the  pure  quinquisulphide. 

Pour  the  solution  of  sulphide  of  calcium  into  250  c.  c.  of  a  mixture 
of  2  parts  of  concentrated  chlorhydric  acid  and  1  part  of  water.  Per- 
sulphide of  hydrogen  will  separate  in  fine  oily  drops,  producing  a  milky 
turbidity  in  the  liquid.  These  drops  soon  coalesce  and  settle  out  be- 
neath the  water.  A  good  way  of  collecting  the  persulphide  is  to  per- 
form the  precipitation  in  a  large  glass  funnel,  provided  with  a  stopper. 
By  carefully  opening  this  stopper,  the  precipitated  oil  can  nearly  all 
be  drawn  off  without  disturbing  the  water  which  floats  above  it. 

Persulphide  of  hydrogen  emits  a  peculiar,  disagreeable  odor, 
and  is  very  irritating  to  the  eyes  and  mucous  membrane.  It 
tastes  sweet  and  bitter,  but  disorganizes  the  flesh  wherever  it 
touches  it.  Its  properties  closely  resemble  those  of  peroxide  of 
hydrogen ;  it  is  very  unstable,  and  is  decomposed  by  the  same 
substances  which  destroy  the  oxide.  It  even  decomposes  spon- 
taneously when,  left  at  rest  for  a  few  days ;  vegetable  colors  are 
quickly  bleached  by  it.  It  decolorizes  a  solution  of  indigo,  for 
example,  more  promptly  even  than  the  peroxide  of  hydrogen. 

214.  In  the  last  section  we  have  used,  for  the  first  time,  cer- 
tain technical  terms,  which,  perhaps,  need  brief  explanation. 
As  has  been  stated  in  §  76,  many  of  the  elements  are  capable  of 
uniting  with  other  elements  in  several  different  proportions  to 
form  chemical  compounds.  Sulphur,  for  example,  is  specially 
apt  to  form  more  than  one  compound  with  a  single  element. 
When  sulphur  unites  with  a  metal,  the  compound  formed  is 
called  a  sulphide,  just  as  a  compound  of  oxygen  and  a  metal  is 
called  an  oxide,  or  one  of  chlorine  and  a  metal,  a  chloride  ;  the 
termination  ide,  which  always  indicates  combination,  being  added 


COMPOUNDS    OF   SULPHUR   AND    OXYGEN.  171 

to  the  first  syllable  of  the  word  sulphur,  or  oxygen,  or  chlorine, 
and  the  new  word  ending  in  ide  being  then  connected  with  the 
name  of  the  metal,  as  in  the  case  of  sulphide  of  copper,  Exp.  91. 
But  when,  as  in  the  case  of  calcium,  there  are  several  distinct 
sulphides,  it  is  customary  to  distinguish  one  from  the  other  by 
means  of  various  Latin  arid  Greek  prefixes.  Thus,  the  com- 
pound which  contains  one  atom  of  sulphur  and  one  atom  of  cal- 
cium is  the  proto-sulphide,  or  simply  the  sulphide  of  calcium, 
the  prefix  proto  being  derived  from  the  Greek  word  for  first ;  the 
compound  which  contains  two  atoms  of  sulphur  to  one  of  calcium 
is  the  bisulphide  of  calcium,  from  the  Latin  for  twice,  and,  in 
like  manner,  we  have  a  tersulphide,  containing  three  atoms  of 
sulphur  to  one  of  calcium,  and  a  quinquisulphide  containing  five 
atoms  of  sulphur.  The  compound  containing  the  highest  pro- 
portion of  sulphur  is  often  called  the  jpersulphide.  A  good  cus- 
tom is  to  designate,  the  cpmpounds  which  contain  more  sulphur 
than  the  protosulphide  by  prefixes  of  Latin  origin,  and  to  dis- 
tinguish those  which  may  contain  less  sulphur  than  the  proto- 
sulphide by  means  of  Greek  prefixes;  thus,  if  there  were  a 
compound  of  two  atoms  of  calcium  and  one  of  sulphur  (Ca2S) 
it  would  properly  be  called  a  di-sulphide  of  calcium,  the  prefix 
being  from  the  Greek  dig*  The  same  prefixes  are  used  in  an 
analogous  manner  in  connection  with  the  words  oxide,  chloride, 
bromide,  iodide,  and  the  similar  words  ending  in  ide. 

215.  Compounds  of  Sulphur  and  Oxygen.  No  less  than  seven 
different  compounds  of  sulphur  and  oxygen  have  been  discovered, 
all  of  which  form  acids  by  union  with  water.  Thus  the  oxide  of 
sulphur  S03  forms,  by  union  with  the  elements  of  water,  common 
sulphuric  acid  H2S04  ;  the  name  sulphuric  acid  being  indiscrim- 
inately applied  to  both  bodies,  although  only  that  one  which 
contains  hydrogen  possesses  the  properties  commonly  described 
by  the  term  acid. 

Two  of  these  compounds,  viz.,  sulphurous  acid  (S02),  and 
sulphuric  acid  (S03),  have  long  been  known,  and  these  are  still, 
comparatively  speaking,  of  most  importance,  since  they  are  em- 
ployed upon  the  large  scale  in  the  arts.  Subsequently  there 
were  found  the  compounds  S2O5  (hyposulphuric  acid)  and  S2O2 
(hypostilphurous  acid)  ;  and  at  a  still  more  recent  period  the 
compounds  S305  ,  S405 ,  and  S505 . 


172  SULPHUROUS    ACID. 

Since  the  ordinary  rules  of  chemical  nomenclature,  as  described 
in  §§  70,  71,  were  inadequate  to  meet  this  new  case,  a  special  set 
of  names  was  given  to  these  new-found  acid  compounds  of  sul- 
phur and  oxygen  containing  five  atoms  of  oxygen.  They  were 
all  called  Thionic  acids,  from  the  Greek  word  for  sulphur,  and 
were  then  distinguished  from  one  another  by  the  prefixes  tri, 
tetra,  and  penta,  in  accordance  with  the  number  of  atoms  of  sul- 
phur in  each.  Strictly  speaking,  the  compound  S2O5  should  follow 
the  new  rule  and  be  called  dithionic  acid,  but  it  is  still  customary 
to  retain  the  old  name  hyposulphuric  acid. 

The  complete  list  of  the  names  of  the  compounds  of  sulphur 
and  oxygen  is  as  follows :  — 

Sulphurous  acid, SO2 

Sulphuric  acid,      .         .         .         .         .         .  SO3 

Hyposulphurous  acid,        .  '     i   .  -i        .        .        S2O2 
Hyposulphuric  acid  (or  Dithionic  acid),        .         .    S2O5 
Trithionic  acid,          .         .       ,.       \         « -...._,        S3O5 

Tetrathionic  acid,          *        *,        •     ,   r        •         •    S4O3 
Pentathionic  acid,      ...        .        .  -      .        S5O3 

Of  all  these  compounds,  only  sulphurous  acid  can  be  readily 
obtained  by  the  direct  union  of  sulphur  and  oxygen.  The  others 
must  be  prepared  by  circuitous  methods. 

21G.  Sulphurous  Acid  (SO2).  This  acid  is  produced  when 
sulphur  is  burned  in  the  air  or  in  pure  oxygen  gas. 

FIG.  40.  Exp.99. —  Light  a  piece  of  sulphur  in  a  deflagrating 
spoon  and  suspend  the  latter  in  a  half-litre  bottle  full  of 
air.  '  On  examining  the  contents  of  the  bottle,  after  the 
sulphur  has  ceased  to  burn,  there  will  be  found  an  irritat- 
ing, suffocating  gas  having  the  peculiar  odor  which  is  famil- 
iar as  that  of  a  burning  match.  The  bottle  is  now  full  of 
sulphurous  acid  gas,  mixed  with  the  nitrogen  originally 
present  in  the  air. 
217.  By  burning  sulphur  in  oxygen  gas,  instead  of  in  air,  as 
in  the  preceding  experiment,  a  much  purer  product  could,  of 
course,  be  obtained.  But  the  experiment  would  be  chiefly  inter- 
esting in  enabling  us  to  determine  synthetically  the  composition 
of  sulphurous  acid. 

If  sulphur  be  burned  in  a  confined  volume  of  dry  oxygen  gas,  it  will 
be  found,  after  the  combustion  has  terminated,  and  the  gas  has  been 


COMPOSITION    OF    SULPHUROUS    ACID. 


173 


allowed  to  regain  its  original  temperature,  that  the  volume  of  the  sul- 
phurous acid  produced  is  sensibly  the  same  as  that  of  the  original  oxy- 
gen, though  its  weight  is  twice  as  great.  Hence  1  volume  of  sulphurous 
acid  gas  contains  1  volume  of  oxygen.  Now,  if 

From  the  weight  of  1  unit-volume  of  sulphurous  acid,  .  .  32.256 
We  subtract  the  weight  of  1  unit-volume  of  oxygen,  .  15.969 


There  will  remain 


16.287 


or  not  far  from  one-half  the  number,  32,  which  represents  the  specific 
gravity  or  equal  volume  weight  of  sulphur  vapor.  Consequently  1 
volume  of  sulphurous  acid  gas  contains  half  a  volume  of  sulphur  vapor, 
besides  1  volume  of  oxygen.  Or,  multiplying  these  numbers  by  2,  in 
order  to  avoid  a  fractional  volume,  it  appears  that  the  volumetric  com- 
position of  sulphurous  acid  is  1  volume  of  sulphur  vapor  and  2  volumes 
of  oxygen  condensed  to  2  volumes  of  the  compound  gas.  Or,  ex- 
pressed in  the  form  of  a  diagram :  — 


I 

o 

16 

s 

39 

+1 

— 

SO2      64 

o 

I 

16 

218.  An  easier  method  of  preparing  pure  sulphurous  acid  is 
by  depriving  common  sulphuric  acid  of  part  of  its  oxygen.  This 
can  be  effected  by  a  variety  of  reducing  or  deoxidizing  agents. 
For  example,  when  concentrated  sulphuric  acid  is  heated  with 
metallic  copper  or  mercury,  these  metals  are  oxidized  at  the  cost 
of  a  part  of  the  oxygen  of  a  portion  of  the  sulphuric  acid,  and 
there  is  formed  a  sulphate  of  the  metal,  water,  and  sulphurous 
acid :  — 

Cu  +  2H2SO4  =  CuS04  +  2H2O  +  S02. 

Exp.  100.  — Into  a  thin-bottomed  glass  flask  of  half  a  litre  capacity, 
put  14  grms.  of  copper  clippings,  or  turnings,  and  50  grins,  of  concen- 
trated sulphuric  acid.  Attach  to  the  flask  a  delivery  tube  and  connect 
this  with  a  series  of  Woulfe's  bottles,  such  as  were  employed  in  the 
preparation  of  chlorhydric  acid  (Exp.  52) ;  heat  the  flask  over  the 
gas-lamp  until  the  acid  begins  to  react  upon  the  copper,  then  quickly 
withdraw  the  lamp  for  a  moment,  lest  the  contents  of  the  flask  boil 
over,  and  finally  regulate  the  flame  so  that  a  steady  current  of  sulphu- 
rous acid  shall  pass  through  the  water  in  the  Wouife-bottles.  After  the 


174         PREPARATION  OF  SULPHUROUS  ACID. 

first  tumultuous  evolution  of  gas  has  subsided,  the  flask  can  be  slowly 
heated  without  further  trouble.  The  current  of  gas  should  be  kept  up 
until  the  water  of  the  first  bottle  has  become  saturated,  or,  at  the  least, 
highly  charged  with  the  gas. 

Sulphurous  acid  is  freely  soluble  in  water,  which,  at  15°,  takes  up 
something  like  44  times  its  bulk  of  the  gas ;  hence  the  solution  obtained 
as  above  may  be  used  as  a  convenient  vehicle  for  sulphurous  acid.  On 
account  of  this  easy  solubility,  the  gas  cannot  be  collected  over  water ; 
but  it  can  be  collected  over  mercury,  or  by  displacement.  Since  the 
gas  is  more  than  twice  as  heavy  as  air,  the  method  by  displacement  is 
to,, be  recommended,  if  an  efficient  ventilating  flue  is  at  command  to 
carry  off  that  portion  of  the  suffocating  gas  which  must  escape. 

Mercury  is,  in  some  respects,  better  than  copper  for  use  in  this  ex- 
periment. It  affords  a  much  more  regular  evolution  of  gas,  and  the 
operation  requires  less  care,  but  copper  is  usually  employed  on  account 
of  its  comparatively  low  cost. 

Instead  of  copper  or  mercury,  as  in  the  foregoing  experiment,  other 

reducing  agents,  such  as  sulphur  or  charcoal,  may  be  employed.     If  1 

part  of  powdered  sulphur  be  boiled  with  12  parts  of  strong  sulphuric  acid, 

sulphurous  acid  is  set  free,  as  exhibited  by  the  following  equation :  — 

S  +  2H2SO4  =  3SO2  +  2H2O. 

The  evolution  of  gas,  in  this  case,  though  steady  and  uniform,  is  com- 
paratively slow,  as  contrasted  with  that  of  the  experiment  in  which 
copper  is  employed ;  hence  the  process  is  usually  less  convenient  than 
that  with  copper. 

If  bits  of  charcoal  or  dry  saw-dust  are  heated  with  sulphuric  acid,  a 
copious  evolution  of  sulphurous  acid  occurs,  though  the  gas  is  not,  in 
this  case,  pure,  being  mixed  with  half  a  volume  of  carbonic  acid. 

C  -f  2H2SO4  =  2SO2  -f  CO2  +  2H2O . 

For  many  purposes,  as  in  the  preparation  of  the  aqueous  solution  of 
sulphurous  acid,  this  method  with  charcoal  is  to  be  preferred,  on  the 
ground  of  economy  and  convenience  of  application.  In  the  laboratory 
it  is  perhaps  more  frequently  employed  than  either  of  the  others. 

Sulphurous  acid  may  also  be  readily  prepared  by  heating  in  an  ig- 
nition tube  a  mixture  of  4  parts  of  sulphur  and  5|  parts  of  black  oxide 
of  manganese,  both  in  fine  powder,  and  intimately  mixed.  A  mix- 
ture of  3  parts  of  black  oxide  of  copper  with  1  part  of  sulphur  an- 
swers the  same  purpose :  — 

2S  +  MnOa  =  SO2  -f  MnS . 
3S  +  2CuO=  SO2  -j-  2CuS. 

In  both  these  cases,  metallic  sulphides  are  left  as  a  residuum  in  the 


SULPHUROUS    ACID    STOPS    COMBUSTION.  175 

ignition-tube,  but  if  the  sulphur  and  black  oxide  of  manganese  be 
mixed  in  the  proportion  of  1  part  sulphur  to  5£  parts  of  the  oxide,  no 
sulphide,  but  only  protoxide  of  manganese,  will  be  formed. 

S  -f  2MnO2  =  SO2  +  2MnQ  . 

219.  As  has  been  already  stated,  sulphurous  acid  gas  is  trans- 
parent and  colorless.  It  is  irrespirable  and  suffocating,  and  when 
mixed  with  air,  even  in  small  proportion,  occasions  violent  cough- 
ing. It  is  not  inflammable,  but,  on  the  contrary,  it  stops  com- 
bustion. 

The  flame  of  a  taper  is  immediately  extinguished  on  being  immersed 
in  sulphurous  acid  gas,  just  as  it  is  by  nitrogen.  A  useful  application 
of  this  property  of  the  gas  is  in  extinguishing  burning  chimneys.  A 
handful  of  fragments  of  sulphur  being  thrown  upon  the  hot  coals  m 
the  grate,  and  the  openings  of  the  fire-place  being  closed  in  such  man- 
ner that  no  air  shall  enter  the  chimney,  excepting  that  which  passes 
through  the  fire,  the  chimney  will  quickly  become  filled  with  an  atmos- 
phere of  sulphurous  acid  mixed  with  nitrogen  from  the  air  employed 
in  burning  the  sulphur,  and  the  burning  soot  upon  the  walls  of  the 
chimney  will  be  immediately  extinguished. 

It  is,  of  course,  essential  that  the  chimney  should  then  be  closed  at 
the  top,  so  that  air  may  be  excluded  and  the  chimney  kept  full  of  the 
fire-extinguishing  atmosphere  until  its  walls  shall  have  cooled  to  below 
the  kindling  temperature  of  the  soot. 

The  oxygen  contained  in  the  sulphurous  acid  gas  is  so  firmly 
held  that  combustibles  are  powerless  to  take  it  away  under  ordi- 
nary circumstances,  though  at  high  temperatures  this  oxygen  can 
be  removed  by  means  of  hydrogen,  carbon,  and  easily  oxidizable 
metals  like  potassium. 

When  hydrogen  and  sulphurous  acid  gas  are  passed  together 
through  a  red-hot  tube,  water  is  formed  and  sulphur  deposited, 

4H  +  S02  =  2H20  +  S , 

and  when  sulphurous  acid  is  passed  through  a  tube  containing 
ignited  charcoal,  carbonic  acid  is  produced  and  sulphur  deposited, 
as  before. 

C  +  S02  =  C02  +  S  . 

In  case  nascent  hydrogen  come  in  contact  with  sulphurous  acid, 
it  will  decompose  it  at  the  ordinary  temperature,  though  in  a 
manner  somewhat  different  from  the  foregoing.  The  sulphur,  as 


176  LIQUID    SULPHUROUS    ACID. 

well  as  the  oxygen,  will,  in  this  case,  combine  with  hydrogen, 
and  there  will  be  formed  sulphydric  acid  as  well  as  water. 

6H  +  SO2  =  2H2O  +  H2S . 

This  reaction  may  be  made  visible  by  putting  a  few  drops  of  a  solu- 
tion of  sulphurous  acid  (Exp.  100)  into  a  gas-bottle  from  which  hydro- 
gen is  being  evolved  (Exp.  19),  and  testing  the  hydrogen  with  a  strip 
of  moistened  lead-paper  (Exp.  94)  both  before  and  after  the  addition 
of  the  sulphurous  acid. 

220.  Sulphurous  acid  can  readily  be  condensed  to  the  liquid 
state.     It  is,  in  fact,  one  of  the  most  easily  liquefiable  of  the 
gases.     By  mere  cooling  to  — 10°,  under  the  ordinary  pressure 
of  the  air,  it  is  converted  into  a  colorless,  transparent,  limpid 
liquid. 

In  preparing  small  quantities  of  the  liquid,  it  is  sufficient  to  lead  the 
gas,  prepared  from  copper  and  sulphuric  acid,  and  dried  by  passing  it 
through  sulphuric  acid  or  over  chloride  of  calcium,  into  a  U  tube  which 
is  immersed  in  a  freezing  mixture  of  ice  and  salt  (2  parts  of  pounded 
ice  and  1  part  salt). 

Liquid  sulphurous  acid  is  a  rather  heavy  liquid,  of  1.4911 
specific  gravity,  boiling  at  about  — 10°  and  solidifying  at  — 76°,  to 
a  colorless  crystalline  solid.  On  being  exposed  to  the  air  at 
ordinary  temperatures,  the  liquid  acid  evaporates  with  great 
rapidity,  and  consequently  occasions  very  intense  cold.  •  By 
means  of  it,  mercury  may  be  frozen,  and  chlorine  and  ammonia- 
gas  liquefied. 

If  a  quantity,  of  the  liquid  acid  be  poured  into  water,  the  tempera- 
ture of  which  is  a  few  degrees  above  0°,  a  portion  of  the  acid  will  evap- 
orate at  once,  another  portion  will  dissolve  in  the  water,  and  a  third 
portion  of  the  heavy  oily  liquid  will  sink  to  the  bottom  of  the  vessel. 
If  the  portion,  which  has  thus  subsided,  be  stirred  with  a  glass  rod,  it 
will  boil  at  once,  and  the  temperature  of  the  water  will  be  so  much 
reduced  that  a  portion,  or  even  the  whole,  of  the  water  will  be  frozen. 

221.  The  specific  gravity  of  the  gas,  as  determined  by  differ- 
ent observers,  is  32.256,  or  32.443,  or  32.558,  instead  of  32,  as 
would  be  indicated  by  theory.     This  variation  is  explained  by 
the  fact  that   sulphurous  acid,  like  all  the  easily  condensible 
gases,  ceases  to  conform  exactly  to  the  law  of  Mariotte  at  tem- 
peratures near  to  its  point  of  condensation.     Under  any  given 


SULPHUROUS    ACID    BLEACHES.  177 

pressure,  its  volume  decreases  in  somewhat  larger  proportion  than 
is  the  case  with  air  and  the  other  permanent  gases. 

An  important  property  of  sulphurous  acid  is  its  power  of 
bleaching  vegetable  colors.  It  is  extensively  employed  in 
bleaching  articles  of  straw,  wool,  silk,  &c.,  which  would  be  in- 
jured by  chlorine. 

Exp.  101. —  Into  a  bottle  in  which  sulphur  has  been  burned,  Exp.  99, 
pour  a  few  teaspoonfuls  of  a  solution  of  blue-litmus,  and  shake  the  bot- 
tle. The  litmus  solution  will  first  become  red,  as  it  would  if  any  other 
acid  than  sulphurous  were  present,  and  will  then  be  decolorized. 

The  same  property  may  be  illustrated  by  holding  a  red  rose  in  the 
fumes  of  burning  sulphur,  or  by  immersing  the  rose  in  an  aqueous  solu- 
tion of  sulphurous  acid  (Exp.  100),  and  leaving  it  for  a  few  minutes  until 
it  has  become  white. 

In  the  same  way  the  stains  of  fruit  or  wine  can  be  removed  from 
clothing.  A  bit  of  sulphur  is  burned  beneath  a  small  open  cone  of 
paper,  which  serves  as  a  chimney,  and  the  stain,  having  first  been 
slightly  moistened  with  water,  is  held  in  the  fumes  at  the  top  of  this 
chimney.  The  cloth  should  finally  be  carefully  washed  with  water  at 
the  place  where  it  has  been  exposed  to  the  sulphurous  acid. 

In  the  arts,  the  process  of  bleaching  is  usually  conducted  in  large 
chambers,  in  which  the  slightly  moistened  articles  are  hung  while  sul- 
phur is  burned  below.  The  damp  goods  absorb  the  sulphurous  acid 
and  gradually  become  white.  The  presence  of  water  is  essential ; 
perfectly  dry  sulphurous  acid  will  not  bleach. 

222.  The  manner  in  which  sulphurous  acid  acts  as  a  bleach- 
ing agent  is  not  clear.  It  is  remarkable  that,  as  a  general  rule, 
it  does  not  actually  destroy  the  coloring  matter ;  and  that  upon 
many  coloring  matters,  it  has  little  or  no  action.  Most  of  the 
yellows,  and  the  green  coloring  matter  of  leaves,  are  in  this  lat- 
ter category,  and  upon  litmus,  cochineal,  and  logwood  the  acid 
does  not  act  very  readily.  In  the  few  instances  where  it  really 
destroys  the  color,  as  in  the  case  of  the  red-beet  and  the  garden 
amaranthus,  it  appears  to  act  as  a  deoxidizing  agent.  But,  in 
most  cases,  it  appears  to  enter  into  combination  with  the  color- 
ing matters  and  to  form  colorless  compounds.  These  colorless 
compounds  of  sulphurous  acid  and  coloring  matter  can  be  broken 
up,  with  restoration  of  color,  by  exposing  them  to  the  action  of 
various  chemical  agents  capable  of  expelling  sulphurous  acid. 

12 


178  OXIDATION    OF    SULPHUROUS    ACID. 

Exp.  102.  — Bleach  a  rose,  as  in  Exp.  101,  and  immerse  it  in  dilute 
sulphuric  acid.  Then  dry  and  warm  it,  so  that  the  volatile  sulphurous 
acid  may  be  driven  off.  The  color  of  the  rose  will  again  appear. 

In  many  cases  a  solution  of  caustic  soda  will  restore  the  color  as  well 
as  sulphuric  acid.  A  practical  illustration  of  this  action  of  alkaline 
solutions  is  seen  in  the  reproduction  of  the  original  yellow  color  of  the 
wool  when  new  flannel  is  washed  with  an  alkaline  soap. 

Sulphurous  acid  is  a  powerful  disinfecting  and  antiseptic  agent. 
It  retards,  to  a  remarkable  extent,  the  processes  of  putrefaction 
and  fermentation,  and  is  largely  employed  for  this  purpose  in 
wine-making ;  hops  and  compressed  vegetables  are  charged  with 
it  to  the  same  end,  and  it  has  been  successfully  employed  for 
preserving  meat.  It  has  often  been  employed  in  medicine,  in  the 
treatment  of  skin  diseases,  as  a  fumigation. 

223.  Although  sulphur  will  not  take  up  more  than  two  atoms 
of  oxygen  when  burned  in  the  air,  or  in  oxygen  gas,  it  is  never- 
theless a  matter  of  no  very  great  difficulty  to  cause  it  to  take  up 
a  third  atom. 

In  presence  of  water  it  gradually  absorbs  oxygen  from  the  air, 
and  is  converted  into  sulphuric  acid.  Hence  the  aqueous  solu- 
tion of  sulphurous  acid  (Exp.  100)  cannot  be  preserved  for  any 
great  length  of  time,  unless  it  be  kept  in  very  tight  vessels. 

224.  If  a  mixture  of  sulphurous  acid  gas  and  oxygen,  or  air, 
be  brought  in  contact  with  hot  platinum  sponge,  the  sulphurous 
acid  will  unite  with  oxygen,  and  sulphuric  acid  will  be  formed  ; 
the  same  union  occurs  when  the  mixed  gases  are  brought  in  con- 
tact with  various  other  substances,  such  as  pumice-stone,  clay, 
and  the  oxides  of  chromium,  iron,  and  copper.     Several  attempts 
have  been  made  to  put  these  methods  in  practice  for  manufactur- 
ing sulphuric  acid,  but  they  have  been  found  to  be  too  slow,  and, 
in  the  case  of  platinum  and  clay,  it  has  been  observed  that  these 
substances  soon  lose  their  power  and  cease  to  convert  the  mixed 
gases  into  sulphuric  acid. 

Sulphurous  acid  is,  indeed,  a  deoxidizing  agent  of  very  con- 
siderable power ;  and  is  much  employed  in  the  laboratory  as  a 
reducing  agent.  It  decomposes  iodic  acid  with  separation  of 
iodine,  and  nitric  acid  with  evolution  of  hyponitric  acid,  sulphuric 
acid  being  formed  in  both  cases. 


SULPHITES.  179 

I203  +  5H2O  +  5SO2  =  5(H2O,S03)  +  21. 
H20,N2O5  +     S°2  =      H2O,S03     +  2N°2- 

Exp.  103. —  Charge  a  dry  bottle,  of  the  capacity  of  a  FIG.  41. 
litre  or  more,  with  sulphurous  acid  gas,  by  burning  in  it  a 
bit  of  sulphur,  as  shown  in  Fig.  41.  Fasten  a  shaving,  or, 
better,  a  tuft  of  gun-cotton,  upon  a  glass  rod  or  tube  bent 
at  one  end  in  the  form  of  a  hook ;  wet  the  shaving  in  con- 
centrated nitric  acid,  and  hang  it  in  the  bottle  of  sulphu- 
rous acid.  Red  fumes  of  hyponitric  acid  will  immediately 
form  about  the  nitric  acid,  and  will  gradually  fill  the  bottle. 

In  presence  of  a  mixture  of  water  and  chlorine,  sulphurous 
acid  takes  up  an  atom  of  oxygen  from  the  water,  while  the  hy- 
drogen of  the  water  unites  with  chlorine. 

SO2  +  2H2O  +  2C1  =  H2O,SO8  +  2HC1. 
A  similar  reaction  occurs  between  iodine  and  sulphurous  acid,  if 
a  very  large  amount  of  water  be  present ;  in  spite  of  the  fact, 
already  mentioned,  §  140,  that  iodohydric  acid  is  readily  decom- 
posed by  concentrated  sulphuric  acid  with  liberation  of  iodine, 
sulphurous  acid,  and, water. 

225.  Sulphurous  acid,  though  a  weak  acid,  forms  numerous 
well-defined  salts  by  uniting  with  metallic  oxides.     These  salts, 
called  sulphites,  are  of  two  classes,  —  simple  or  normal  sulphites, 
such  as  the  sulphite  of  potassium  K2SO8  (or,  dualistic,  K20,S02), 
and  double  or  acid  sulphites,  such  as  the  acid  sulphite  of  potas- 
sium  KHSO8  (or,  dualistic,    KHO,SO2).      All  these  salts  are 
decomposed  by  strong  acids,  such  as  chlorhydric,  nitric,  or  sul- 
phuric, sulphurous  acid  being  expelled ;  but  they  are  not  decom- 
posed by  carbonic  acid.     On  the  contrary,  the  salts  of  carbonic 
acid  are  decomposed  by  sulphurous  acid,  and  hence  it  happens 
that  the  impure   sulphurous  acid  gas  obtained  by  heating  a  mix- 
ture of  charcoal  and  sulphuric  acid  can  be  used  for  preparing  the 
sulphites.     If,  for  example,  this  gas  be  conducted  into  an  aqueous 
solution  of  carbonate  of  sodium,  there  will  be  obtained  a  solu- 
tion of  sulphite  of  sodium,  and  carbonic  acid  will  be  set  free. 

226.  Besides  the  solution  of  sulphurous  acid,  such  as  was  pre- 
pared in  Exp.  100,  there  is  a  definite  crystalline  compound  of 
water  and  the  acid,  which  can  be  obtained  by  passing  a  current 
of  sulphurous  acid  gas  into  ice-water.     This  compound  is  very 


180  SULPHURIC    ACID. 

unstable,  and  is  destroyed  at  temperatures  but  little  above  0°  ; 
but  by  collecting  it  upon  a  cooled  filter  and  then  pressing  the 
crystals  repeatedly  between  folds  of  cold  blotting-paper,  it  has 
been  found  possible  to  remove  most  of  the  mother  liquor  which 
adheres  to  them  at  first,  and  to  obtain  the  compound  in  a  condi- 
tion of  tolerable  purity.  The  composition  of  the  crystals  appears 
to  be  SO2-fl5H2O. 

227.  Sulphuric  Acid.  The  term  sulphuric  acid  is  applied 
somewhat  indiscriminately  to  three  or  more  distinct  substances ; 
namely,  to  a  compound  of  one  atom  of  sulphur  and  three  atoms 
of  oxygen,  SO3 ,  which  we  shall  call  anhydrous  sulphuric  acid, 
and  to  certain  compounds  of  sulphur,  oxygen,  and  hydrogen, 
which  have  been  usually  regarded  as  compounds  of  the  an- 
hydrous sulphuric  acid,  just  mentioned,  and  water.  Of  these 
hydrates  the  most  important  are  those  of  the  composition  H2SO4 
(dualistic,  H2O,SO3)  [oil  of  vitriol],  and  H2S2O7  (dualistic, 
H2O,2S03)  [Nordhausen  or  fuming  sulphuric  acid].  The  body, 
whose  formula  is  H2SO4 ,  is  one  of  the  most  important  of  chemi- 
cal substances,  and  is  usually  the  thing  meant  when  sulphuric 
acid  is  spoken  of.  We  will,  therefore,  proceed  to  study  its  prop- 
erties before  touching  upon  those  of  the  other  substances  above- 
mentioned. 

Sulphuric  acid  is  one  of  the  most  important  products  of 
chemical  manufacture,  and  is  made  in  enormous  quantities.  In 
the  same  way  that  the  metal  iron  may  be  said  to  be  the  basis  of 
all  mechanical  industries,  sulphuric  acid  lies  at  the  foundation  of 
the  chemical  arts.  By  means  of  sulphuric  acid,  the  chemist 
either  directly  or  indirectly  prepares  almost  everything  with 
which  he  has  commonly  to  deal. 

Sulphuric  acid  might  be  prepared  by  passing  sulphurous  acid 
gas  into  boiling  nitric  acid,  until  all  of  the  latter  had  been  re- 
duced, and  finally  distilling  off  the  last  traces  of  the  lower 
oxides  of  nitrogen  which  would  be  formed.  Even  if  sulphur 
itself  were  boiled  in  concentrated  nitric  acid,  it  would  gradually 
be  oxidized  and  converted  into  sulphuric  acid.  But  neither  of 
these  processes  would  be  economical.  It  can  be  very  cheaply 
prepared,  however,  by  the  action  of  either  of  the  high  oxides  of 
nitrogen,  nitrous,  hyponitric,  or  nitric  acids,  upon  sulphurous  acid, 


MANUFACTURE    OF    SULPHURIC    ACID.  181 

in  presence  of  air  and  moisture ;  and  this  method  is  the  one  ac- 
tually followed  in  the  preparation  of  sulphuric  acid  on  the  large 
scale.  A  mixture  of  the  gases  above  mentioned  is  effected  in 
enormous  chambers  constructed  of  sheet  lead,  a  metal  upon  which 
cold  sulphuric  acid  has  little  or  no  action. 

228.  The  essential  points  of  the  process  are,  first,  that  SO2 , 
when  in  presence  of  much  moisture,  can  take  oxygen  from 
either  N2O  3  ,NO2 ,  or  N2O5 ,  and  reduce  them  to  nitric  oxide,  NO, 
while  it  is  itself  converted  into  sulphuric  acid  ;  and,  secondly,  that 
NO  can  take  oxygen  from  the  air  and  become  NO2 . 

In  practice,  the  sulphurous  acid  is  obtained  by  burning  crude  sulphur, 
or  more  commonly  a  compound  of  sulphur  and  iron,  known  as  iron- 
pyrites,  FeS2 ,  upon  a  grate ;  the  gas,  together  with  a  large  excess  of 
atmospheric  air,  is  then  conducted  into  the  first  of  a  series  of  leaden 
chambers  into  which  jets  of  steam  are  constantly  blowing.  Nitrous  fumes 
are  supplied  either  by  allowing  nitric  acid  to  fall  in  fine  streams  through 
the  incoming  current  of  sulphurous  acid  and  air,  or  from  the  decom- 
position of  a  mixture  of  salt,  nitrate  of  sodium,  and  sulphuric  acid,  as 
described  in  §  105,  or  by  placing  a  vessel  containing  nitrate  of. sodium, 
in  the  middle  of  the  sulphur  fire ;  as  sulphurous  acid  comes  in  contact 
with  this  salt,  the  nitrate  is  decomposed,  being  finally  converted  into 
sulphate  of  sodium,  and  the  nitrous  fumes  set  free  pass  forward  into 
the  leaden  chamber  in  company  with  the  sulphurous  acid. 

In  conformity  with  the  principles  above  stated,  the  sulphurous  acid, 
as  soon  as  it  comes  in  contact  wijh  the  steam,  reacts  upon  the  nitrous 
fumes ;  there  is  formed  sulphuric  acid,  which  condenses  upon  the  sides 
of  the  chamber  and  trickles  down  to  the  floor,  and  nitric  oxide.  But, 
as  there  is  present  in  the  chamber  an  excess  of  air,  the  nitric  oxide 
immediately  unites  with  a  portion  of  the  oxygen  therein  contained,, 
and  is  converted  into  hyponitric  acid.  This  hyponitric  acid  imme- 
diately reacts  upon  a  new  portion  of  sulphurous  acid,  and  the  process 
thus  goes  on  through  a  whole  series  of  leaden  chambers,  the  very  small 
portion  of  nitric  acid  at  first  taken  being  sufficient  to  prepare  a  large 
quantity  of  sulphuric  acid.  In  reality,  the  oxygen  employed  in  con- 
verting the  sulphurous  into  sulphuric  acid,  all  comes  from  the  air,  ex- 
cepting a  very  little  at  first ;  the  nitrous  fumes  serve  only  as  a  conveyer 
of  oxygen.  The  nitric  oxide  takes  oxygen  from  the  air  and  transfers 
it  to  the  sulphurous  acid,  which,  as  has  been  stated  in  §  223,  is,  by  itself 
and  unaided,  incapable  of  combining  with  oxygen.  It  will,  of  course, 
be  understood,  that  although  we  trace  out  these  reactions  as  if  they 
were  consecutive,  they  are  really,  so  far  as  we  know,  simultaneous. 


182  MANUFACTURE    OF    SULPHURIC    ACID. 

Theoretically,  a  single  portion  of  hyponitric  acid  would  be  sufficient 
to  effect  the  conversion  of  an  unlimited  amount  of  sulphurous  into  sul- 
phuric acid,  but  practically  this  power  is  qualified  by  a  variety  of  cir- 
cumstances. It  is  found  to  be  impossible,  for  example,  to  mix  new 
portions  of  air  with  the  mixture  of  sulphurous  acid  and  nitric  oxide 
for  an  indefinite  period,  for  at  a  certain  point  these  gases  become 
so  loaded  down  with  nitrogen  derived  from  the  air  already  consumed, 
that  they  are  as  good  as  lost  in  it.  In  general,  the  flow  of  gases  is  so 
regulated  that  all  the  sulphurous  acid  shall  be  oxidized,  and  that  noth- 
ing but  nitric  oxide  and  waste  nitrogen  shall  pass  out  of  the  last 
leaden  chamber. 

229.  The  process  of  manufacturing  sulphuric  acid  can  read- 
ily be  illustrated  upon  the  small  scale. 

A  large  glass  balloon,  or  receiver,  of  the  capacity  of  several  litres, 
placed  in  a  vertical  position,  is  closed  with  a  cork  pierced  with  five 
holes,  through  four  of  which  are  passed  small  glass  tubes.  All  of  these 
glass  tubes  reach  nearly  to  the  bottom  of  the  balloon,  and  are  bent*  at 
a  right  angle  above  the  cork  ;  one  of  the  tubes  is  connected  at  the  top 
with  a  flask  containing  copper-turnings  and  sulphuric  acid,  for  the  gen- 
eration of  sulphurous  acid  (see  Exp.  100)  ;  another  with  a  flask  contain- 
ing copper-turnings  and  furnished  with  a  thistle-tube,  through  which 
nitric  acid  can  be  poured,  for  the  generation  of  nitric  oxide  (see  Exp. 
37) ;  and  the  third  with  a  flask  containing  water  for  the  evolution  of 
steam ;  the  fourth  tube  and  the  fifth  hole  are  both  left  open. 

Everything  being  in  readiness,  nitric  oxide  is  generated  in  the  small 
flask  fitted  for  this  purpose ;  as  the  gas  passes  over  into  the  large  balloon 
it  unites  with  oxygen  from  the  air,  and  red  fumes  of  hyponitric  acid 
are  formed.  Sulphurous  acid  is  now  made  to  pass  into  the  balloon  ; 
this  will  have  no  action  upon  the  red  fumes,  so  long  as  there  is  no 
water  present,  but  the  moment  steam  is  thrown  in  from  the  third  small 
flask,  a  reaction  occurs,  the  hyponitric  acid  is  reduced,  and  the  sulphu- 
rous acid  oxidized.  By  means  of  bellows,  air  must,  from  time  to  time, 
be  blown  into  the  balloon,  through  the  fourth  glass  tube,  the  waste 
nitrogen  passing  off  through  the  fifth  hole  in  the  cork. 

If  but  little  steam  be  employed  in  this  experiment,  a  solid  compound, 
formed  by  the  union  of  nitrous  and  anhydrous  sulphuric  acids,  is  liable 
to  be  deposited  upon  the  walls  of  the  balloon ;  the  appearance  of  this 
body  always  indicates  that  the  supply  of  steam  is  insufficient ;  it  is  never 
formed  when  the  proper  proportion  of  moisture  is  present. 

230.  The  sulphuric  acid  which  collects  at  the  bottom  of  the 
leaden  chambers  is  necessarily  dilute,  because  of  the  large  amount 


PROPERTIES    OF    SULPHURIC    ACID.  183 

of  water  which  must  be  present,  in  order  that  the  reactions  above 
described  may  freely  occur ;  moreover,  it  would  not  be  advan- 
tageous to  allow  an  acid,  more  concentrated  than  that  of  specific 
gravity  1.6,  to  form  in  the  chambers,  since  a  stronger  acid  would 
absorb  and  retain  a  considerable  quantity  of  nitric  oxide.  To 
make  it  fit  for  the  purposes  for  which  sulphuric  acid  is  usually 
employed,  the  dilute  acid  of  the  chambers  must  be  concentrated 
by  expulsion  of  the  water ;  to  this  end,  it  is  run  off  into  shallow 
leaden  pans,  and  there  evaporated  until  it  is  of  specific  gravity 

1.71  to  1.75.     The  concentration  cannot  be  carried  beyond  this 
point  in  ordinary  leaden  vessels,  since  the  strong,  hot  acid  begins 
to  attack  the  metal,  and  the  temperature  at  which  the  liquid  boils 
is  so  high  as  to  approach  the  melting  point  of  lead.     This  acid  of 

1.72  specific  gravity  is  somewhat  extensively  employed,  for  a 
variety  of  purposes,  at  the  factories  where  it  has  been  prepared, 
but  is  still  too  dilute  for  transportation.     It  is,  therefore,  trans- 
ferred from  the  leaden  pans  to  large  glass  retorts  set  in  deep 
sand-baths,  or  to  platinum  stills,  and  there   evaporated  further, 
until  it  is  nearly  of  the  composition  H2SO4 . 

231.  The  acid  thus  boiled  down  is  the  concentrated  sulphuric 
acid,  or  oil  of  vitriol,  of  commerce  ;  its  specific  gravity  is  usually 
about  1.83,  that  of  the  absolutely  pure  acid  being  1.842.  Besides 
this  slight  excess  of  water,  it  contains  also,  in  solution,  a  certain 
quantity  of  sulphate  of  lead,  and  a  variety  of  other  impurities. 
For  most  purposes,  however,  it  will  answer  as  well  as  the  pure 
acid.  Like  the  latter,  it  is  a  heavy,  oily,  colorless,  and  odorless 
liquid,  boiling  at  about  330°. 

Since  a  comparatively  small  amount  of  heat  is  absorbed  in  the 
conversion  of  the  liquid  acid  to  the  condition  of  gas,  its  vapor  can  be 
very  easily  condensed ;  in  distilling  the  acid,  the  receiver  need  not 
even  be  placed  in  cold  water. 

From  the  same  cause,  combined  with  the  great  weight  of  the  liquid, 
the  acid  is  liable  to  boil  tumultuously,  the  act  of  ebullition  being 
irregular  and  attended  with  violent  blows  or  shocks.  The  bubbles  of 
vapor  formed  at  the  bottom  of  the  retort  condense  almost  as  soon  as 
they  are  formed,  and  the  heavy  liquid  above  suddenly  falls  back  to  fill 
the  vacuum. 

In  distilling  the  concentrated  acid,  it  is,  therefore,  best  to  heat  only 
the  upper  portions  of  the  liquid  in  the  retort ;  this  can  be  effected 


184  SULPHURIC    ACID    ABSORBS    WATER. 

either  by  placing  the  retort  upon  a  wire-grate  so  perforated  that  about 
half  the  body  of  the  retort  can  be  sunk  below  the  level  of  the  burning 
charcoal  upon  the  grate,  or  by  placing  a  layer  of  ashes,  or  of  some 
other  bad  conductor  of  heat,  beneath  the  very  bottom  of  the  retort, 
then  piling  sand  around  the  sides  of  the  retort,  outside  of  the  ashes, 
and  applying  heat  beneath  the  iron  pan  upon  which  the  whole  is  sup- 
ported. 

232.  The  common  acid  usually  freezes  at  about  — 34°,  but  it 
has  been  found  possible  to  lower  the  freezing  point  to  — 80°,  by 
adding  a  small  quantity  of  water  to  the  commercial  acid.     When 
once  frozen,  it  remains  solid  until  the  temperature  rises  to  about 
the  freezing  point  of  water.     Crystals  of  the  pure  acid  melt  at 
about  10°.     At  the  ordinary  temperature,  sulphuric  acid  does 
not  vaporize,  but,  on  the  contrary,  greedily  absorbs  water  from 
the  air  and  so  increases  in  bulk.     In   moist  weather,  its  bulk 
may  increase  to  the  extent  of  a  quarter  or  more,  in  the  course  of 
a  single  day,  and,  by  longer  exposure,  a  still  larger  quantity  of 
water  will  be  taken  up  ;  the  acid  must  always  be  kept,  therefore, 
in  tightly-stoppered  bottles. 

Exp.  104. —  Into  a  shallow  dish  of  about  200  c.  c.  capacity,  pour 
about  75  c.  c.  of  concentrated  sulphuric  acid ;  place  this  dish  of  acid 
upon  one  pan  of  a  balance,  and  upon  the  other  pan  put  enough  small 
shot,  or  clean,  dry  sand,  to  exactly  balance  the  acid.  Preserve  the  ma- 
terial of  the  counterpoise,  and  place  the  dish  of  acid  uncovered  in  the 
open  air ;  from  day  to  day  replace  it  upon  the  balance,  together  with 
the  counterpoise,  and  note  the  number  of  grammes  or  fractions  of  a 
gramme  that  it  has  increased  in  weight. 

If  the  acid  were  allowed  to  stand  for  a  week  or  two  in  a  damp  place, 
it  might  become  two  or  three  times  as  heavy  as  it  was  at  first.  From 
its  power  of  absorbing  aqueous  vapor,  sulphuric  acid  is  often  employed 
for  drying  gases.  (See  Appendix,  §  15.) 

233.  With  liquid  water  sulphuric  acid  unites  with  great  en- 
ergy, much  heat  being  evolved  at  the  moment  of  combination ; 
during  the  union  a  certain  amount  of  condensation  occurs,  the 
mixture,  when  cold,  occupying  less  space  than  was  previously  oc- 
cupied by  the  acid  and  the  water.     The  water  and  acid  may  be 
mixed  in  all  proportions,  being  mutually  soluble  one  in  the  other. 

In  mixing  water  and  sulphuric  acid,  the  acid  should  always  be  poured 
into  the  water,  in  a  fine  stream,  not  the  water  into  the  acid,  —  the  water 


MIXING    SULPHURIC    ACID    WITH    WATER.  185 

being  meanwhile  stirred.  In  this  way  the  heavy  acid  has  an  oppor- 
tunity to  mix  with  the  water  as  it  sinks  down  through,  it. 

If,  by  any  accident,  water  were  to  fall  upon  sulphuric  acid,  it  would 
float  on  top  of  it,  and  great  heat  would  be  developed  at  the  point  of 
contact  of  the  two  liquids;  if  the  quantities  of  acid  and  water  were 
large,  sudden  bursts  of  steam  would  be  occasioned  and  serious  damage 
might  arise  from  the  scattering  about  of  portions  of  the  acid. 

In  mixing  water  and  commercial  sulphuric  acid,  as  in  the  following 
experiment,  it  will  be  observed  that  the  solution  becomes  cloudy,  and 
that  a  white  powder  is  gradually  deposited  from  it.  This  precipitate  is 
sulphate  of  lead,  originally  derived  from  the  leaden  pans  in  which  the 
acid  was  concentrated ;  it  is  soluble  in  concentrated,  but  insoluble  in 
dilute  sulphuric  acid,  and  is  consequently  thrown  down  when  water  is 
added  to  the  commercial  acid. 

Exp.  105. —  Place  in  a  beaker  glass  of  about  250  c.  c.  capacity,  30 
c.  c.  of  water ;  in  accordance  with  the  directions  above  given,  pour  into 
the  water  120  grms.  of  concentrated  sulphuric  acid,  and  stir  the  mix- 
ture with  a  narrow  test-tube  containing  a  teaspoonful  of  water.  So 
much  heat  will  be  evolved  during  the  union  of  the  water  and  the  acid 
that  the  water  in  the  test-tube  will  boil. 

234.  If  sulphuric  acid  be  mixed  with  ice  or  snow,  the  latter 
will  be  immediately  liquefied.  If  the  proportion  of  ice  in  the 
mixture  be  small,  as  compared  with  that  of  the  sulphuric  acid, 
heat  will  be  evolved  much  as  is  the  case  with  liquid  water,  though 
to  a  less  extent.  But  when  a  large  proportion  of  ice  is  mixed 
with  a  comparatively  small  quantity  of  the  acid,  no  heat  will  be 
perceived,  but,  on  the  contrary,  intense  cold. 

Exp.  106.  —  Place  in  a  beaker  glass  of  about  half  a  litre  capacity 
1 20  grms.  of  snow,  or  finely  pounded  ice ;  pour  upon  it  30  grms.  of 
concentrated  sulphuric  acid,  and  stir  the  mixture  with  a  test-tube  con- 
taining a  small  quantity  of  water.  The  water  in  the  tube  will  be  frozen. 

Exp.  107.  —  Repeat  the  foregoing  experiment,  using  30  grms.  of 
snow  or  ice  and  120  grms.  of  sulphuric  acid.  A  very  considerable 
evolution  of  heat  will  occur,  as  may  be  seen  more  clearly  by  immersing 
a  thermometer  in  the  liquid. 

The  result  of  Exp.  106  seems,  at  first  sight,  inconsistent  with 
'.he  general  fact  that  heat  is  always  set  free  during  chemical  com- 
bination ;  for  though  chemical  union  between  the  acid  and  water 
has  evidently  occurred,  no  heat,  but  cold,  is  manifested.  The 
anomaly  is  only  a  seeming  one ;  a  certain  amount  of  heat  is  re- 


186  HYDRATES    OF    SULPHURIC    ACID. 

quired,  in  order  that  the  cohesive  force,  by  which  the  particles  of 
the  ice  are  held  together,  shall  be  overcome ;  hence  the  heat  which 
is  really  produced  by  the  chemical  combination  is  all  absorbed, 
together  with  much  more  taken  from  the  materials  and  the  ves- 
sel which  contained  them,  during  the  liquefaction  of  the  ice. 

235.  Besides  the  indefinite  mixture  or  solution  above  men- 
tioned, several  crystalline  compounds  of  anhydrous  sulphuric  acid 
and  the  elements  of  water,  of  fixed  composition  and  characteristic 
form,  can  be  prepared. 

If  the  commercial  acid  be  diluted  with  water  until  its  specific 
gravity  is  reduced  to  1.78,  and  the  liquid  be  then  cooled  strqngly, 
a  substance  of  composition  H4SO5  (dualistic,  2H2O,SO3)  will 
crystallize  out  in  the  form  of  large  rhombic  prisms.  These  crys- 
tals are  of  sp.  gr.  1.785 ;  they  melt  and  solidify  at  about  8°. 

A  second  hydrate,  of  composition  3H2O,SO8,  can  be  obtained 
by  evaporating  a  dilute  acid  in  a  vacuum  at  the  temperature  of 
100°,  until  it  ceases  to  lose  weight;  and  another  of  composition 
H20,2SO8,  will  be  described  below  when  we  come  to  speak  of 
fuming  sulphuric  acid. 

236.  Sulphuric  acid  is  one  of  the  most  powerful  acids  known. 
If  one  drop  of  it  be  diluted  with  a  thousand  times  as  much  water, 
it  is  still  capable  of  reddening  blue  litmus.     It  expels  most  of  the 
other  acids  from  their  compounds,  in  the  same  way  that  we  have 
seen  it  expel  nitric  acid  from  nitrate  of  sodium  in  Exp.  32.     At 
very  low  temperatures,  however,  as  at  — 80°,  it  loses  its  power  of 
reddening  litmus,  and  has  no  action  upon  the  carbonates,  though 
it  acts  violently  upon  these  salts  at  the  ordinary  temperature. 

It  is  intensely  caustic  and  corrosive,  and  quickly  chars  and 
destroys  most  vegetable  and  animal  substances. 

Exp.  108. —  Into  a  test-glass  pour  a  tablespoonful  of  sulphuric  acid 
and  immerse  in  it  a  splinter  of  wood.  The  wood  will  blacken  as  if 
charred  by  fire,  and  the  acid  will  become  dark-colored.  Wood  is  com- 
posed of  carbon,  hydrogen,  and  oxygen,  and  since  sulphuric  acid  unites 
with  compounds  of  hydrogen  and  oxygen,  rather  than  with  carbon,  a 
portion  of  the  latter  is  left  free  ;  some  carbonaceous  matter  is,  however, 
dissolved  by  the  acid  and  darkens  it.  The  acid  of  commerce  is  often 
dark-colored  from  fragments  of  straw  or  other  organic  matter  having 
accidentally  fallen  into  it. 


SULPHATES.  187 

The  action  of  the  acid  upon  organic  matter  is  more  rapid  when  moist- 
ure isspresent.  Thus,  if  a  few  drops  of  oil  of  vitriol  be  poured  upon 
dry  paper,  decomposition  will  take  place  only  slowly ;  but  if  a  little 
water  be  added  to  the  acid,  heat  will  be  developed  by  the  chemical 
union,  and  the  paper  will  be  at  once  decomposed  by  the  hot  acid. 

237.  When  heated  with  charcoal  or  with  any  organic  matter, 
sulphuric  acid  gives  up  oxygen,  as  has  been  shown  in  Exp.  100, 
and  is  itself  reduced  to  sulphurous   acid ;  by  sulphur,  also,  and 
by  several  of  the  metals,  such  as  copper  and  mercury,  it  is  re- 
duced in  a  similar  way.     (See  Exp.  100.)     Towards  some  metals, 
such  as  zinc  for  example,  its  behavior  is  various,  according  as  it 
is  concentrated  or  dilute.     If  zinc  be  treated  with  cold,  dilute 
sulphuric  acid,  the  zinc  simply  replaces  the  hydrogen  of  the  acid, 
sulphate  of  zinc  is  formed,  and  hydrogen  is  set  free. 

Zn  +  H2S04  =  ZnS04  +  2H. 

But  if  zinc  be  heated  with  concentrated  sulphuric  acid,  a  portion 
of  the  latter  is  reduced,  as  it  would  be  in  presence  of  copper  or 
mercury,  sulphurous  acid  is  evolved,  as  well  as  hydrogen,  and 
these  gases,  reacting  upon  each  other,  produce  sulphydric  acid  and 
a  deposit  of  sulphur,  in  accordance  with  the  following  formulae :  — 

SO2  +  6H  =  H2S  +  2H20 . 
SO2  +  4H  =  S      +  2H2O. 

As  a  general  rule,  concentrated  sulphuric  acid  acts  but  feebly 
upon  the  metals  in  the  cold,  though,  when  boiled  upon  them,  it 
often  behaves  as  an  oxidizing  agent. 

238.  With  the  oxides  of  the  metals,  sulphuric  acid  unites 
directly  to  form,  the  very  important  salts  called  sulphates,  water 
being  simultaneously  eliminated.     Oil  of  vitriol,  H2SO4,  may,  in 
fact,  be  itself  regarded  as  a  salt,  in  the  composition  of  which,  hy- 
drogen fills   the  same  place  that  sodium  does  in    sulphate   of 
sodium,  Na2SO4 ,  and  it  might  well  be  called  sulphate  of  hydro- 
gen, were  it  not  that  usage  has  assigned  to  it  another  name. 
Besides  the  normal  sulphates,  in  which  all  the  hydrogen  has  been 
replaced  by  a  metal,  as  above,  —  or,  on  the  dualistic  hypothesis, 
in  which  all  the  water  has  been  replaced  by  a  metallic  oxide, 
there  is  another  class  of  sulphates,  often  called  bi  or  acid  sul- 
phates, in  which  only  half  of  the  hydrogen  has  been  thus  re- 


188  FUMING    SULPHURIC    ACID. 

placed ;  as  an  example  of  these,  the  student  will  recall  the  acid 
sulphate  of  sodium,  NaHSO4,  mentioned  in  §  101. 

Acids,  which,  like  sulphuric  acid,  contain  two  replaceable  atoms 
of  hydrogen,  and  are,  therefore,  capable  of  forming  two  series  of 
salts,  are  called  bibasic,  in  contradistinction  to  the  monobasic 
acids,  like  nitric  acid,  which  form  but  one  series  of  salts.  There 
is  but  one  nitrate  of  sodium,  for  example,  NaNO3 .  It  is  for  this 
reason  that  many  chemists  object  to  the  doubled  formula  for 
nitric  acid  H2N2O6 ,  in  spite  of  its  convenience,  because  this  for- 
mula suggests,  what  is  not  true,  that  one  or  both  of  the  atoms  of 
hydrogen  might  be  replaced  by  any  metal  which,  like  sodium, 
potassium,  or  silver,  replaces  hydrogen  atom  for  atom. 

239.  Fuming  Sulphuric  Acid.  The  acid  H2SO4,  above  de- 
scribed, has  been,  for  nearly  a  century,  the  most  important  of  the 
several  varieties  of  sulphuric  acid,  but  long  previous  to  the  dis- 
covery of  the  process  of  making  it  in  leaden  chambers,  there 
was  manufactured  another  variety,  now  known  as  fuming  sul- 
phuric acid. 

This  fuming  acid,  or  Nordhausen  acid,  as  it  is  often  called, 
from  the  name  of  a  German  town  in  which  large  quantities  of 
it  were  formerly  prepared,  was  at  first  obtained  by  distilling  in 
earthen  retorts  the  salt  now  known  as  sulphate  of  iron,  formerly 
called  green  vitriol.  Hence  the  origin  of  the  name  oil  of  vitriol, 
which,  in  England  and  this  country,  has  come  to  be  applied 
solely  to  the  common  acid  H2SO4 ,  though  it  is  still  used  as  a 
synonyme  for  the  fuming  acid  by  German  writers. 

When  dry  sulphate  of  iron  is  exposed  to  a  full  red  heat,  it 
suffers  decomposition  ;  a  considerable  quantity  of  sulphuric  acid 
is  given  off  and  can  be  collected  in  receivers.  The  distillate 
thus  obtained,  which  is  a  dense,  fuming  liquid  of  about  1.9  spe- 
cific gravity,  is  the  acid  now  in  question.  Though  of  far  less 
importance  than  was  formerly  the  case,  considerable  quantities  of 
the  fuming  acid  are  still  prepared  for  the  purpose  of  dissolving 
indigo  and  for  other  special  uses,  where  an  acid  stronger  than 
the  commor*  acid  is  needed.  It  may  be  regarded  as  a  solution 
of  varying  quantities  of  the  anhydrous  acid  S03  in  the  common 
acid  H2SO4 ;  if  it  be  gently  heated,  all  of  the  anhydrous  acid  will 
be  expelled,  and  common  sulphuric  acid  will  remain.  So,  too,  if 


ANHYDROUS    SULPHURIC    ACID.  189 

it  be  exposed  to  the  air,  the  anhydrous  acid  will  be  given  off,  and, 
coming  in  contact  with  the  moisture  of  the  air,  will  combine 
therewith  to  form  common  sulphuric  acid,  which,  falling  as  a 
cloud,  occasions  the  appearance  of  fumes. 

When  the  fuming  acid  is  cooled  to  about  —5°,  a  crystalline 
compound  of  composition  H2S2O7  (dualistic,  H2O,2S03)  separates 
out.  After  having  been  freed  from  liquid  acid,  these  crystals 
melt  at  35°.  When  pure,  the  fuming  acid  is  colorless,  but  the 
commercial  article  is  often  brown,  from  having  been  in  contact 
with  organic  matter.  It  is  an  excessively  corrosive  liquid,  and 
destroys  most  organic  matters,  even  more  rapidly  than  the  com- 
mon acid.  On  being  dropped  into  water,  a  noise  is  emitted  as 
if  a  red-hot  bar  of  metal  had  been  touched  to  the  water. 

240.  Anhydrous  Sulphuric  Acid  (SO3).  As  has  been  men- 
tioned in  §  224,  this  substance  can  be  obtained  by  passing  a  mix- 
ture of  sulphurous  acid  gas  and  oxygen  over  hot,  finely-divided, 
metallic  platinum,  or  over  various  oxides  and  other  porous  sub- 
stances. 

Exp.  109. —  Prepare  a  small  quantity  of  platinized  asbestos  as  fol- 
lows :  dissolve  about  0.25  grm.  of  metallic  platinum  in  aqua  regia  (§  104), 
and  soak  in  this  solution  as  much  soft,  porous  asbestos  as  will  form  a 
loose  ball  of  1.5  c.  m.  diameter;  heat  the  wet  asbestos  gently  until  it 
has  become  dry,  and  then  ignite  it  in  the  flame  of  the  gas-lamp.  The 
chloride  of  platinum,  which  was  formed  by  the  solution  of  the  metal,  is 
decomposed  by  heat,  and  metallic  platinum,  in  a  finely  divided  condi- 
tion, is  left  adhering  to  the  asbestos. 

Select  a  tube  of  hard  glass,  No.  3,  about  30  c.  m.  long,  and  at  a  dis- 
tance of  about  10  c.  m.  from  one  end,  bend  it  to  an  obtuse  angle,  so 
that  when  the  tube  is  supported  upon  a  ring  of  the  iron  stand  above 
the  gas-lamp,  the  shorter,  bent  portion  can  be  thrust  into  the  neck  of  a 
receiver ;  in  the  centre  of  the  longer  portion  of  the  glass  tube  pack  the 
platinized  asbestos  loosely ;  then  force  into  and  through  the  tube  a  cur- 
rent of  mixed  sulphurous  acid  and  oxygen  ;  at  the  same  time,  heat  over 
the  gas-lamp  that  portion  of  the  tube  which  contains  the  platinized 
asbestos,  and  collect  the  anhydrous  sulphuric  acid,  which  is  formed  in  a 
perfectly  dry  test-tube  or  U-tube  immersed  in  ice,  or,  better,  in  a  freez- 
ing mixture  of  ice  and  salt. 

The  mixture  of  sulphurous  acid  and  oxygen  may  be  made  before  the 
experiment  in  a  small  gas-holder,  or,  as  well,  during  the  progress  of 


190  MAKING    ANHYDROUS    SULPHURIC    ACID. 

the  experiment  in  a  bottle  behind  the  asbestos  tube.  This  bottle,  which 
should  be  of  at  least  half  a  litre  in  capacity,  is  fitted  with  a  cork  car- 
rying three  glass  tubes,  and  is  connected  with  the  asbestos  tube  by  one 
of  these  tubes,  which  reaches  HO  lower  than  the  cork ;  by  the  other 
tubes,  which  pass  nearly  to  the  bottom  of  the  bottle,  and  dip  beneath  the 
surface  of  a  layer  of  common  sulphuric  acid,  which  has  been  placed 
in  it,  the  bottle  is  connected  with  a  flask  in  which  sulphurous  acid  is 
being  generated  (Exp.  100),  and  with  a  gas-holder  containing  oxygen. 
The  sulphuric  acid  in  the  bottle  serves  to  dry  the  gases,  and  the  rapid- 
ity with  which  the  bubbles  of  gas  pass  through  the  liquid,  enables  the 
operator  to  judge  of  the  proportions  in  which  the  gases  are  being 
mixed ;  the  flow  of  oxygen  having  been  fixed  at  a  moderate  rate,  once 
for  all,  the  sulphurous  acid  will  alone  need  attention. 

The  action  of  the  platinum  in  this  experiment  is  obscure ;  it  will  be 
treated  of  under  the  metal  platinum. 

Instead  of  the  platinized  asbestos,  oxide  of  iron,  oxide  of  copper,  or 
oxide  of  chromium,  or,  better,  a  mixture  of  the  last  two  can  be  heated 
in  the  tube  through  which  the  mixed  gases  are  passing.  These  pro- 
cesses of  preparing  sulphuric  acid  are  interesting,  from  a  scientific  point 
of  view,  but,  as  has  been  already  stated  (§  224),  they  do  not  admit  of 
commercial  application. 

Anhydrous  sulphuric  acid  can  readily  be  prepared  by  heating  the 
Nordhausen  acids  (see  §  239)  :  — 

20  or  30  grms.  of  fuming  sulphuric  acid  are  poured  into  a  perfectly 
dry,  small  glass  retort ;  the  neck  of  the  retort  is  thrust  into  a  dry,  cold 
receiver,  and  the  acid  is  slowly  heated  until  it  boils  moderately.  The 
vapor  of  the  anhydrous  acid  will  condense  and  solidify  in  the  receiver. 

The  anhydrous  acid  may  also  be  obtained  by  distilling  dry  bisulphate 
of  sodium.  The  bisulphate  is  prepared  by  heating  a  mixture  of  3  parts, 
by  weight,  of  dry  sulphate  of  sodium  and  2  parts  of  common  sulphuric 
acid,  until  the  mixture  fuses.  All  the  water  of  the  acid  is  thus  elimi- 
nated :  — 

Na^ASO,  +  H2O,SO3  =  Na2O,2SO3  -f  H2O. 

The  bisulphate  of  sodium,  on  being  distilled  in  an  earthen  retort,  will 
give  up  one  molecule  of  anhydrous  sulphuric  acid,  and  a  residue  of 
normal  sulphate  of  sodium  will  remain  in  the  retort :  — 

Na2O,2SO3  =  Na2O,SO3  -f  SO3. 

241.  As  thus  prepared,  anhydrous  sulphuric  acid  is  a  glisten- 
ing white  solid  mass  of  silky,  crystalline  fibres,  somewhat  resem- 
bling asbestos ;  it  is  tough  and  ductile,  and  can  be  moulded  with 
the  fingers  like  wax.  So  long  as  no  water  is  present,  it  can  be 


PROPERTIES    OF    ANHYDROUS    SULPHURIC    ACID.  191 

handled  without  danger ;  when  perfectly  dry,  it  is  not  corrosive, 
nor  does  it  even  react  upon  blue  litmus.  It  unites  with  water, 
however,  with  great  avidity,  and  is  converted  into  common  sul- 
phuric acid.  It  rapidly  absorbs  water  from  the  air  and  deli- 
quesces ;  at  the  same  time,  it  forms  dense  fumes,  for  it  is  volatile, 
to  a  very  considerable  extent,  at  the  ordinary  temperature,  and 
its  vapor  combines  with  the  moisture  of  the  air.  On  being 
brought  in  contact  with  a  small  quantity  of  water,  it  combines 
with  it  with  explosive  violence,  and  much  heat  is  evolved.  If  a 
bit  of  it  be  thrown  into  a  large  quantity  of  water,  the  water 
hisses  as  if  a  hot  iron  had  been  thrust  into  it.'  Owing  to  its 
great  tendency  to  deliquesce,  the  solid  acid  can  only  be  preserved 
in  dry  tubes  sealed  at  the  lamp. 

The  specific  gravity  of  anhydrous  sulphuric  acid  is  1.97.  It 
melts  readily  upon  being  heated,  but  it  has  been  noticed  that 
some  samples  melt  far  more  easily  than  others.  There  appear 
to  be  two  distinct  varieties  of  the  acid,  for  in  some  cases  a  tem- 
perature of  18°  is  sufficient  to  render  the  mass  fluid,  while,  in 
others,  the  heat  must  be  carried  even  to  100°.  The  easily  fusible 
modification  appears  to  change  gradually,  by  keeping,  into  that 
which  is  more  difficultly  fusible,  and  the  latter  seems  to  be 
changed  to  the  former  by  distillation.  The  melted  acid  boils  at 
about  35°,  and  evolves  a  colorless  and  transparent  vapor,  three 
times  as  heavy  as  air,  which,  upon  coming  in  contact  with  the 
air,  unites  with  moisture  and  forms  dense  white  fumes.  When 
brought  in  contact  with  hot  lime  or  baryta  (oxide  of  calcium  and 
oxide  of  barium),  it  unites  with  them  directly;  intense  heat  is 
evolved,  and  there  is  formed  sulphate  of  calcium  or  sulphate  of 
barium :  — 

CaO  +  S03  =  CaS04  =  CaO,SO3. 

242.  On  being  exposed  to  a  strong  red  heat,  the  vapor  of  an- 
hydrous sulphuric  acid  splits  up  into  oxygen  and  sulphurous 
acid ;  two  volumes  of  it  yielding  two  volumes  of  sulphurous  acid 
and  one  volume  of  oxygen.  As  has  been  shown  in  §  218,  two 
volumes  of  sulphurous  acid  gas  contain  one  volume  of  sulphur 
vapor  and  two  volumes  of  oxygen ;  hence  it  follows  that  the 
volumetric  composition  of  anhydrous  sulphuric  acid  is  one  vol- 


192  HYPOSULPHUROUS    ACID. 

ume  of  sulphur  vapor  and  three  volumes  of  oxygen,  the  whole 
condensed  to  two  volumes.  The  specific  gravity  of  sulphur 
vapor  is  32,  that  of  oxygen  is  16,  and  the  proportions,  by  weight, 
in  which  sulphur  and  oxygen  are  united  in  anhydrous  sulphuric 
acid,  are  consequently  32  and  16  X  3  =  48;  the  combining 
weight  of  sulphuric  acid  being  32  -[-  48  =  80.  The  combining 
weight  of  sulphuric  acid  can  also  be  readily  determined  in  a 
manner  analogous  to  that  employed  in  the  case  of  nitric  acid 
(§  73),  by  saturating  with  the  common  acid  a  known  quantity  of 
oxide  of  lead,  evaporating  off  the  water  and  excess  of  acid,  and 
then  determining  the  weight  of  the  dry  sulphate  of  lead  which 
is  formed.  By  subtracting  from  the  latter,  the  weight  of  the 
original  oxide  of  lead,  we  obtain  the  weight  of  the  sulphuric  acid 
which  has  combined  with  it.  Experiment  will  show  that  the 
weight  of  this  sulphuric  acid  is  to  that  of  the  oxide  of  lead  in 
the  ratio  of  80  to  223. 

The  facility  with  which  sulphuric  acid  is  decomposed  at  a  red 
heat  (§  242)  is  the  basis  of  a  very  economical  method  of  prepar- 
ing oxygen  gas  in  large  quantities  for  manufacturing  purposes. 
Commercial  sulphuric  acid  is  allowed  to  drop  upon  fragments  of 
red-hot  porcelain,  there  to  be  decomposed,  in  accordance  with 
the  formula, 

H2SO4  =  S02  +  O  +  H2O , 

and  the  products  of  the  decomposition  are  then  washed  with 
water,  so  that  the  sulphurous  acid  may  be  absorbed,  the  steam 
condensed,  and  the  oxygen  left  free.  Here  again,  as  in  our  ear- 
lier experiments  (§  10),  the  oxygen  has  really  been  obtained  from 
the  air  ;  and  if  it  were  desirable,  the  solution  of  sulphurous  acid 
obtained  in  washing  this  oxygen,  might  be  placed  in  the  leaden 
chambers  and  again  be  converted  into  sulphuric  acid  by  the 
addition  of  oxygen  from  the  air. 

243.  Hyposulphurous  Acid  (S2O2),  has  never  been  obtained 
in  the  free  state,  nor  is  any  compound  of  it  with  water  known  ; 
but  there  are  numerous  saline  compounds  of  which  it  makes 
part,  and  some  of  these  are  of  considerable  importance  in  the 
arts.  These  salts,  called  hyposulphites,  can  be  prepared  in 
various  ways ;  for  example,  by  digesting  sulphur  in  a  hot,  but 


CHLORIDES    OF    SULPHUR.  193 

not  boiling,  concentrated  solution  of  an  alkaline  sulphite ;  if  sul- 
phite of  sodium  be  taken,  the  reaction  can  be  thus  formulated, 

Na2O,SO2  +  S  =  Na20,S2O2. 

Another  method  of  preparing  the  hyposulphites  is  to  pass  a  cur- 
rent of  sulphurous  acid  gas  through  the  solution  of  an  alkaline 
sulphide,  until  no  further  precipitation  of  sulphur  occurs :  — 

2CaS  +  3SO2  =  2(CaO,S2O2)  +  S. 

When  a  hyposulphite  is  treated  with  a  strong  acid,  decomposi- 
tion immediately  ensues.  S202  breaks  up  into  SO2  -j-  S  ;  hence- 
our  inability  to  isolate  the  acid. 

Some  of  the  hyposulphites  will  be  more  fully  described  when 
we  come  to  treat  of  the  metals. 

244.  Other  Compounds  of  Sulphur  and  Oxygen.     With   the 
exception  of  hyposulphuric  acid,  S2O5,  none  of  these  compounds 
(see  §   216)  have  been  very  thoroughly  studied  ;  any  detailed 
description  of  the  methods  of  preparing  them  would  be  out  of 
place  in  an  elementary  manual. 

245.  Compounds  of  SulphUr  and  Chlorine.     Chlorine    and 
sulphur  combine  with  one  another  directly  and  readily,  forming 
several  different  compounds  whose  properties  vary  in  accordance 
with   the   varying   proportions   of  chlorine    and  sulphur  which 
they  respectively  contain. 

246.  bichloride  of  Sulphur  (SCI),  is  the  best  known  of  the 
compounds  of  chlorine  and  sulphur,  and  is  often  called  simply 
chloride  of  sulphur. 

It  can  be  prepared  by  passing  a  current  of  dry  chlorine  through  a 
flask  or  tubulated  retort  containing  flowers  of  sulphur.  The  chlorine 
is  rapidly  absorbed  by  the  sulphur,  and  care  must  be  taken  lest  the 
mass  become  too  hot.  The  reddish-yellow  liquid  obtained,  as  the  result 
of  the  reaction,  is  a  solution  of  sulphur  in  dichloride  of  sulphur ;  by 
distilling  it,  the  excess  of  sulphur  can  be  separated. 

Dichloride  of  sulphur  is  a  yellowish-brown  liquid  of  1.68  spe- 
cific gravity,  and  boiling,  without  decomposition,  at  144°.  It 
emits  a  peculiar  odor  which  has  been  compared  with  that  of 
sea-plants  ;  its  vapor  excites  tears,  and  its  taste  is  acid,  acrid,  and 
bitter.  It  fumes  strongly  in  the  air,  being  decomposed  by  the 

moisture   of  the  air  with  evolution   of  chlorhydric  acid.     It  is 
13 


194  CHLORIDE  OF  SULPHUR. 

decomposed  by  water,  but  can  be  mixed  with  bisulphide  of  car- 
bon and  with  benzine.  It  is  remarkable  as  a  powerful  solvent 
of  sulphur  ;  100  parts  of  dichloride  of  sulphur  can  take  up  about 
70  parts  of  sulphur  at  the  ordinary  temperature ;  on  slowly  cool- 
ing the  hot  saturated  solution,  beautiful  crystals  of  sulphur  are 
deposited.  Dichloride  of  sulphur  is  used  in  a  process  of  vulcan- 
izing caoutchouc,  known  as  the  cold  process. 

247.  Chloride  of  Sulphur  (SC12).     This  compound  is  formed 
when  sulphur  is  treated  with  in  excess  of  dry  chlorine,  or  when 
a  current  of  chlorine  is   passed  into  dichloride  of  sulphur ;  the 
dichloride  requires  some  278  times  its  own  bulk  of  chlorine  gas, 
and  absorbs  it  very  slowly.     Chloride  of  sulphur  is  a  red  liquid, 
of  1.625  specific  gravity.     It  exhales  suffocating  and  irritating 
fumes  of  chlorine  and  the  dichloride,  since  it  slowly  decomposes 
when  kept ;  the  decomposition  is  particularly  rapid  in  a  strong  light, 
and  so  much  gas  is  evolved  that  a  tightly-stoppered  bottle,  con- 
taining chloride  of  sulphur,  will  explode,  after  a  time,  if  it  be 
placed  in  sun-light.     On  being  heated,  the  liquid  gives   off  so 
much  chlorine  at  50°  that  it  seems  to  boil,  but  the  temperature 
gradually  rises  to  64°,  which  appears  to  be  the  real  boiling  point 
of  the  liquid.     It  is  slowly  decomposed  by  water. 

The  density  of  its  vapor  has  been  found  to  be  53 ;  admitting 
that  the  gas  is  composed  of  one  volume  of  sulphur  vapor  and 
two  volumes  of  chlorine,  condensed  to  two  volumes  of  vapor,  the 
calculated  specific  gravity  of  its  vapor  would  be  51.5.  In  view 
of  the  instability  of  the  compound,  the  experimental  result  is 
sufficiently  near  coincidence  with  the  calculated  number  to  make 
it  certain  that  the  composition  of  the  gas  is  really  as  above  stated. 

248.  The  other  compounds  of  sulphur  with  chlorine,  and  with 
chlorine  and  oxygen,  need  not  here  be  discussed  ;  and  the  same 
remark  applies  to  the  compounds  formed  by  the  union  of  sulphur 
with  iodine,  bromine,  fluorine,  and  nitrogen.     The  compounds  of 
sulphur  with  carbon,  phosphorus,  arsenic,  and  the  metals,  will  be 
treated  of  hereafter. 


SELENIUM.  195 


CHAPTER    XIV. 

SELENIUM    AND    TELLURIUM. 

249.  These  elements  are  rare,  and  of  little  or  no  industrial 
importance ;  but  to  the  chemist  they  are  exceedingly  interesting 
on  account  of  the  close  resemblance  they  bear  to  sulphur.     The 
three  elements,   sulphur,  selenium,  and  tellurium,  constitute    a 
group  which  is  equally  remarkable  with  that  formed  by  chlorine, 
bromine,  and  iodine.     (See  §  152.) 

250.  Selenium,  Se,  is  never  found  in  any  considerable  quan- 
tity in  any  one  place.     Traces  of  it  occur  in  many  varieties  of 
native  sulphur,  and  in  various  metallic  sulphides.     It  is  now  ob- 
tained   chiefly   from    the  sulphides  of   iron,  copper,   and   zinc. 
These  sulphides  often  contain  minute  traces  of  selenium,  though 
the  quantity  is  sometimes  so  small  that  it  can  hardly  be  detected 
by  the  ordinary  methods  of  analysis.     When  these  sulphides  are 
burned  for  the  purpose  of  manufacturing  sulphuric  acid,  or  in 
metallurgies!  operations,  the  selenium  goes  off  with  the   sulphu- 
rous acid  produced  by  the  combustion  and  is  deposited  either  in 
the  dust  flues  of  the  furnaces  or  upon  the  floors  of  the  leaden 
chambers  at  the  sulphuric  acid  works.     In  this  way  the  selenium 
from  hundreds  of  tons  of  the  pyritous  ores  is  collected  and  con- 
centrated in  a  comparatively  small  bulk.     The  deposit  taken 
from  the  leaden  chambers  of  some  sulphuric  acid  works  contains 
as  much  as  from  2  to  10  per  cent,  of  selenium.     The  methods 
of  obtaining  pure  selenium  from  these  deposits  are  founded  upon 
the  facts   that  by  treatment  with  nitric  acid  or  aqua  regia,  the 
selenium  can  all  be  oxidized  and  converted  into  selenious  acid, 
SeO2 ;  that  selenious  acid  is  soluble  in  water,  and   that  when  a 
solution  of  it  is  treated  with  sulphurous  acid,  the   selenious  acid 
is  reduced  and  pure  selenium  deposited. 

SeO2  +  2SO2  =  2SO3  +  Se  . 

251.  In  its  properties  and  in  its  chemical  behavior,  selenium 
resembles  sulphur  in  many  respects,  while,  in  others,  it  is  like 
tellurium.     Like  sulphur  and  oxygen,  it  occurs  in  distinct  allo- 


196  PROPERTIES    OF    SELENIUM. 

tropic  modifications  (§§  162,  197).  The  precipitate  obtained  by 
mixing  solutions  of  sulphurous  and  selenious  acids  is  of  a  deep 
red  color,  almost  like  that  of  cinnabar.  But,  after  having  been 
fused  and  suddenly  cooled,  selenium  appears  as  a  brilliant  black 
mass,  amorphous  like  glass,  and  of  4.3  specific  gravity.  When 
fused  selenium  has  been  slowly  cooled,  it  appears  as  a  dark -gray, 
very  brittle,  crumbling  mass  of  crystalline  or  granular  structure, 
and  a  metallic  lustre  like  that  of  lead ;  the  specific  gravity  of 
this  variety  is  4.81.  The  amorphous  or  vitreous  modification  of 
selenium  does  not  conduct  electricity,  but  the  granular  or  crystal- 
line variety  conducts  it,  and  the  more  readily  in  proportion  as  it 
is  hotter.  The  specific  heat  of  selenium,  at  the  ordinary  tem- 
perature, is  0.0746,  being  the  same  for  both  the  vitreous  modifi- 
cation and  that  with  metallic  lustre.  The  vitreous  variety  is 
soluble  in  bisulphide  of  carbon,  but  the  granular  variety  is  in- 
soluble in  that  liquid. 

Selenium  melts  readily  upon  being  heated,  and  the  liquid  thus 
obtained  boils  at  about  700°,  being  converted  into  a  dark-yellow 
vapor,  the  specific  gravity  of  which  has  been  found  to  be  82.3. 
The  atomic  weight  of  selenium  has  been  determined  to  be  79.5. 
Of  itself,  selenium  has  neither  taste  nor  odor.  When  heated  in 
the  flame  of  a  lamp,  it  burns  with  a  beautiful  blue  flame  and 
exhales  a  peculiarly  offensive  odor,  like  that  of  putrid  horse-rad- 
ish, selenious  acid,  SeO2  ,  being  the  chief  product  of  the  reaction. 

Selenium  combines  with  most  of  the  elements,  usually  in  the 
same  way  as  sulphur,  though  not  always,  since  it  is  a  weaker 
chemical  agent'  than  sulphur ;  its  compounds  are  as  a  rule  some- 
what less  stable  than  the  corresponding  sulphur  compounds. 
With  oxygen  it  forms  selenious  acid,  SeO2,  and  selenic  acid, 
SeO8 ,  —  analogous  to  sulphurous  and  sulphuric  acids  respec- 
tively. Besides  these,  there  is  a  lower  oxide,  SeO(?);  it  is  a 
colorless  gas,  having  the  strong  and  disagreeable  odor  like  horse- 
radish, before-mentioned. 

252.  Both  selenious  and  selenic  acids  form  numerous  salts, 
which  closely  resemble  the  corresponding  sulphites  and  sulphates, 
in  composition  and  in  many  of  their  properties.  Normal  seleniate 
of  potassium,  for  example,  K2SeO4  cannot  be  distinguished,  by  its 
external  appearance,  from  sulphate  of  potassium,  K2SO4 ;  the 


ISOMORPHISM.  197 

crystalline  form  of  the  two  bodies,  as  well  as  their  texture,  color, 
and  lustre,  being  identical.  If  solutions  of  these  two  salts  be 
mixed,  neither  of  the  salts  can  subsequently  be  crystallized  out 
by  itself,  when  the  solution  is  evaporated;  the  crystals  obtained 
will  be  composed  of  sulphate  of  potassium  and  seleniate  of  po- 
tassium mixed  in  the  most  varied  proportions.  Bodies  which 
are  thus  capable  of  crystallizing  together  in  all  proportions,  with- 
out alteration  of  the  crystalline  form,  are  said  to  be  isomorphous 
(like-formed).  The  formula?  of  the  two  isomorphous  salts,  just 
mentioned,  differ  only  in  this,  —  that  the  one  contains  the  atom 
Se,  where  the  other  contains  the  atom  S.  It  is,  therefore,  pos- 
sible to  replace  32  parts  by  weight  of  sulphur  by  79.5  parts  of 
selenium,  or  79.5  of  selenium  by  32  of  sulphur,  without  chang- 
ing the  crystalline  form  of  the  salts ;  it  follows,  that  32  parts  by 
weight  of  solid  sulphur,  and  79.5  parts  of  solid  selenium,  occupy 
the  same  space.  That  this  is  actually  the  case,  may  be  shown, 
by  comparing  the  quotients  obtained,  by  dividing  the  atomic 
weights  of  the  two  elements  by  their  specific  gravities  ;  these 
quotients  will  be  found  to  be  equal,  or  as  nearly  equal  as  the 
limits  of  error  of  the  physical  determinations  involved  will  per- 
mit. The  specific  gravity  of  prismatic  sulphur  is  1.91,  or,  in  other 
words,  one  cubic  centimetre  of  solid  sulphur  weighs  1.91 
grammes;  the  specific  gravity  of  crystalline  selenium  is  4.81,  or 
one  cubic  centimetre  of  selenium  weighs  4.81  grammes;  32 
grammes  of  sulphur  will,  therefore,  occupy  T3^T  =  16.75  cubic 
centimetres  ;  79.5  grammes  of  selenium  will  occupy  |%f  =  1 6.53 
cubic  centimetres.  What  is  true  of  grammes,  is  true  of  any  parts 
by  weight,  and  ultimately  of  the  atoms.  This  quotient,  obtained 
by  dividing  the  atomic  weight  of  an  element  by  its  specific  grav- 
ity, is  called  the  atomic  volume  of  the  element ;  it  must  be  borne 
in  mind  that  the  standard  of  specific  gravity  for  liquids  and  solids 
is  water ;  for  gases,  hydrogen  ;  and  that,  therefore,  the  atomic  vol- 
ume of  a  solid  or  liquid  must  not  be  directly  compared  with  that 
of  a  gas.  Two  elements,  whose  atomic  volume  is  the  same,  can 
be  exchanged  in  their  compounds  without  alteration  of  crystalline 
form,  precisely  as  a  brick  or  stone  taken  out  of  a  wall  can  be 
replaced  by  another  of  the  same  size  and  shape  without  changing 
the  form  of  the  wall. 


198  TELLURIUM. 

253.  With  chlorine,  selenium  forms  two  compounds,  SeCl  and 
SeCl4,  the  first  of  which  is  analogous   to  dichloride   of  sulphur. 
With  hydrogen  it  forms  a  compound,  H2Se,  called   selenhydric 
aqid,  or  seleniuretted  hydrogen,  which  is  perfectly  analogous  to 
sulphuretted  hydrogen,  H2S,  but  possesses  a  still  more  disagree- 
able odor.     In   its   action   upon   solutions  of  the  metallic  salts, 
upon  metals  and  metallic  oxides,  selenhydric  acid  behaves  like 
sulphydric  acid,  a  selenide  of  the  metal  being  always  formed. 

254.  Tellurium  (Te),  occurs  in  nature  even  more  rarely  than 
selenium  ;  sometimes  it  is  found  in  the  free  state,  but  more  gen- 
erally in  combination  with  the  heavy  metals,  such  as  gold,  silver, 
lead,  copper,  and  bismuth.     It  is  one   of  the  few  elements,  with 
regard  to  which  chemists  have,  at  times,  been  in  doubt,  whether 
or  no  it  should  be  classed  as  a  metal.     Many  of  its  physical  prop- 
erties are  similar  to  those  of  the  metals,  and  it  particularly  resem- 
bles the  metal  antimony,  but  it  is  so  intimately  related  to  sulphur 
and  selenium,  in  its  chemical  properties,  its  crystalline  form  and 
mode   of  occurrence  in  nature,   that  it  is  now  almost   always 
studied  as  a  member  of  the  sulphur  group. 

255.  Tellurium  is  of  a  silver-white  color  and  glittering  me- 
tallic lustre.     It  is  hard  and  brittle,  and  crystallizes  very  easily 
in  rhombohedrons.     It  is  a  bad  conductor  of  heat  and  electricity. 
Its  specific  gravity  is  6.2  ;  its   specific  heat  is   0.04737,  and  its 
atomic  weight,  128.     It  melts  at  a  temperature  somewhat  above 
the  melting  point  of  lead,  and  is  volatile  at  a  full  red  heat,  the 
vapor  being  c\f  a  yellow   color,  like  that  of  selenium.     When 
heated  in  the  air,  it  takes  fire,  and  burns  with  a  greenish-blue 
flame,  copious  fumes  of  tellurous  acid,  TeO2 ,  being  at  the  same 
time  evolved. 

256.  The  compounds  of  tellurium  and  oxygen,  tellurous  acid, 
TeO2 ,  and  telluric  acid  Te08 ,  are  analogous  to  sulphurous  and 
sulphuric  acids.     By  uniting  with  metallic  oxides,  they  form  nu- 
merous salts,  analogous  to,  and  isomorphous  with,  the  correspond- 
ing compounds  of  sulphur  and  selenium.     So,  too,  the  hydrogen 
compound,  H2Te ,  is  analogous  to  sulphuretted  and  seleniuretted 
hydrogen,  in  composition  and  properties.     With   the  metals  it 
unites  directly  to  form  tellurides.     There  are  chlorine  compounds 
also,  TeCland  TeCl4. 


THE    SULPHUR    GROUP.  199 

257.  The  close  relationship  which  subsists  between  sulphur 
and  oxygen,  has  been  already  alluded  to,  as  well  as  the  many 
points  of  resemblance  between  sulphur,  selenium,  and  tellurium, 
the  student  is,  therefore,  now  prepared  to  recognize  the  fact  that 
in  oxygen,  sulphur,  selenium,  and  tellurium,  we  have  another 
group  or  family  of  elements,  as  intimately  and  naturally  related 
to  each  other  as  are  the  members  of  the  chlorine  group.  (See 
§  152.)  It  will  be  seen,  at  a  glance,  that  in  passing  from  oxygen, 
at  one  end  of  the  series,  to  tellurium,  at  the  other,  we  meet  with 
the  same  progression  of  physical  and  chemical  properties  that 
was  so  noticeable  in  passing  from  chlorine  to  iodine.  The  prop- 
erties of  the  various  compounds  formed  by  the  union  of  the 
members  of  the  sulphur  group  with  other  elements,  exhibit  the 
same  kind  of  progression ;  that  these  compounds  are  of  analo- 
gous composition,  has  been  shown  in  the  preceding  paragraphs. 

With  hydrogen  the  members  of  the  sulphur  group  unite  in 
the  proportion  of  two  atoms  of  hydrogen  to  one  atom  of  the 
other  element ;  thus,  H2O,  H2S,  H2Se,  H2Te.  This  peculiar  rela- 
tion to  hydrogen  is  an  important  characteristic  of  the  group. 

In  this  sulphur  group,  precisely  as  in  the  chlorine  group,  the 
relative  chemical  power  of  each  element  in  the  family  is  great 
in  proportion  as  its  atomic  weight  is  low  (§  153)  ;  oxygen  is,  as 
a  rule,  stronger  than  sulphur,  sulphur  than  selenium,  and  selenium 
than  tellurium,  their  atomic  weights  being  respectively :  — 

O  =  16,  S  =  32,  Se  =  79.5(80?).     Te  =  128. 


CHAPTER    XV. 

COMBINATION     BY      VOLUME. 

258.  A  comparison  of  the  formulae  representing  the  volumetric 
composition  of  all  the  well-defined  compound  gases  and  vapors 
which  have  been  thus  far  studied,  will  bring  into  clear  view  some 
of  the1  general  facts  relating  to  combination  by  volume. 


200  COMBINATION    BY    VOLUME. 

It  has  been  established,  by  experiment,  that  the  following  com- 
pounds are  formed  by  the  chemical  union,  without  condensation, 
of  equal  volumes  of  the  two  elements  which  enter  into  each 
compound :  — 


Hydrogen   , 
1vol. 

Chlorine 
1  vol. 

Chlorhvdric  Acid 
2  vols.             '  °r 

H  ,     Cl 

1     '    35.5 

IIC1 

:    36.5 

Hydrogen   , 

Bromine 
1  vol. 

Bromliydric  Acid 
2  vols.              '  Or 

H,     Br 

1  ""    80 

HBr 

81 

Hydrogen   . 
1  vol. 

Iodine 
1  vol. 

lodohvdric  Acid 
2  vols.              '°r 

II,       I 

1     '    127 

HI 

128 

Nitrogen     , 

Oxvgen 

Nitric  Oxide 

N    ,     O 

NO 

1  vol. 

1  vol. 

2  vols. 

14    i"    16 

30 

It  has  further  been  demonstrated  that  the  following  compounds 
of  two  elements  contain  two  volumes  of  one  element  and  one 
volume  of  the  other,  but  that  these  three  volumes  are  condensed, 
during  the  act  of  combination,  into  two  volumes  :  — 


Hydrogen  , 
2  vols.  ' 

Oxygen 
1  vol. 

Steam 
2  vols. 

,  or 

H2 

2 

_L    ° 

~  16 

H20 

•  18 

Hydrogen  , 
2  vols.  ' 

Sulphur 
1  vol. 

__  Sulphydric  Acid 
2  vols. 

,  or 

H2 

2 

_L    S 
i     32 

H2S 

'  34 

Hydrogen  , 
2  vols.  ' 

Selenium 
1  vol. 

_  Selenhydric  Acid 
2  vols. 

,or 

Ho 

2 

~^~79.5 

HoS 

~  81.5 

Hydrogen  , 
2  vols.  ' 

Tellurium 
1  vol. 

Tellurhvdric  Acid 
2  vols. 

,  or 

H2 

2 

L   Te 
i~]28 

H.,Te 

~  130 

Chlorine  , 
2  vols.  "t" 

Oxygen 
1  vol. 

Hvpochlorous  Acid 
2  vols. 

,  or 

n2 

0 

"   16 

C1,O 

:  87 

Chlorine      , 
.  1  vol. 

Nitrogen  , 
2  vols.  ~r 

Oxvgen 
2  vols. 

Oxygen 
1  vol. 

_  Hypoehloric  Acid 
2  vols. 

Nitrous  Oxide 
2  vols. 

,or 
,or 

Cl 
35.5 

N2 
28 

,            Oo 

+  ^2 

+  S 

_C1O2 
~"  67.5 

N20 

'  44 

Nitrogen  , 
1  vol. 

Oxygen 
2  vols. 

Hyponitric  Acid 
2  vols. 

,  or 

N 
14 

,         Oo 

~T~  32 

46 

Sulphur  , 
1  vol. 

Oxygen 
2  vols. 

_  Sulphurous  Acid 
2  vols. 

,  or 

S 
32 

,     02 
'     32 

:  64 

Selenium  , 
1  vol. 

Oxygen 
2  vols. 

Selenious  Acid 
2  vols. 

,  or 

Se 
79.5 

,     02 
'     32 

SeO2 
~111.5 

Tellurium  , 
1  vol. 

Oxygen 
2  vols. 

Tellurous  Acid 
2  vols. 

,or 

Te 
128 

,   o2 

~"~  23 

Te02 

"  160 

1     Lastly,  still  a  third  mode  of  combination  by  volume  with  con- 
densation of  four  volumes  to  two,  has  been  thoroughly  studied  in 


CONDENSATION-RATIOS.  201 

the  case  of  ammonia,  and   has  been  further  illustrated  in  the 
composition  of  anhydrous  sulphuric  acid:  — 

Nitrogen     ,       Hydrogen  Ammonia  N  ,    H3     _  NH3 

1vol.  3  vols.  2  vols.  '0rl4~T3        ~    17 

Sulphur       .        Oxygen  Sulphuric  Acid  S    •    O3     _  SO3 

1vol.  3  vols.  2  vols.  'Or32~t~48    ==    80 

Throughout  these  tables  the  unit-volume  is,  of  course,  the 
same  for  every  element  and  compound.  What  the  absolute  bulk 
of  this  unit-volume  may  be,  is  not  an  essential  point,  for  the  rela- 
tions remain  the  same,  whatever  the  unit  of  measure ;  some 
chemists  have  thought  that  an  advantage  was  gained  by  using 
the  bulk  of  one  gramme  of  hydrogen  at  the  ordinary  pressure 
and  temperature,  viz.,  11.2  litres,  as  the  unit-volume,  while  others 
prefer  to  use  the  litre  itself  as  the  unit. 

Three  condensation-ratios  are  exhibited  in  these  tables  ;  in 
the  first,  the  condensation  is  0 ;  in  the  second,  it  is  ^,  and  in  the 
third,  it  is  l.  The  typical  character  of  the  three  compounds, 
chlorhydric  acid,  water,  and  ammonia,  is  also  clearly  brought 
out;  each  of  these  bodies  represents  a  group  of  compounds 
which  obey  the  same  structural  law.  The  tables  also  show  very 
clearly  the  fact  that  very  unequal  weights  of  the  compounds 
tabulated  occupy  equal  spaces,  under  the  same  conditions  of  tem- 
perature and  pressure.  The  space  occupied  by  the  compound 
molecule  is,  in  each  case,  exactly  twice  the  unit-volume. 

259.  The  symbols  H,  C1,~O,  and  N,  represent  the  relative 
weights  of  the  same  volume  of  four  elements  which  are  gaseous 
at  common  temperatures  and  pressures ;  the  symbols  Br,  I,  S, 
and  Se,  represent  the  relative  unit-volume  weights  of  four  other 
elements  which  are  not  gases  under  the  ordinary  atmospheric 
conditions,  but  which  can  be  converted  into  gases  at  a  higher 
temperature.  At  this  higher  temperature  their  unit-volume 
weights  have  been  experimentally  determined,  and  from  these 
observed  volume-weights,  the  unit-volume  weights  which  they 
would  possess  at  the  ordinary  pressure  and  temperature  have 
been  deduced.  The  symbols  of  these  eight  elements,  therefore, 
represent  at  once  the  combining  weights  and  the  relative  weights 
of  equal  volumes  (specific  gravities)  of  these  substances  in  the 
gaseous  state.  In  the  present  state  of  the  science,  these  eight 


202  COMBINING    WEIGHT    AND    VOLUME-WEIGHT. 

symbols  are  the  only  ones  of  which  this  can  be  affirmed  ;  tellu- 
rium would  undoubtedly  make  a  ninth,  if  the  relative  size  of  its 
combining  weight  had  been  experimentally  determined,  but  until 
this  determination  has  been  made,  the  symbol  Te  represents  only 
the  combining  weight  of  the  element,  and  not  its  equal-volume 
weight  as  well. 

The  relative  sizes  of  the  combining  weights  of  four  other  ele- 
ments in  the  state  of  vapor,  have  been  experimentally  ascer- 
tained.    These  four  elements  are  arsenic,  phosphorus,  cadmium, 
and  mercury.     When  we  come  to  study  these  elements,  we  shall 
find  that  the  symbols  of  arsenic  and  phosphorus,  namely,  As  and 
P,  represent  only  the  half-volume  weights  of  these   two  bodies, 
while  the  symbols  of  cadmium  and  mercury  represent  the  two- 
volume  weights  of  these  volatile  metals.     Coincidence   of  the 
combining  weight  and  the  volume-weight  has  been  established 
for  eight  elements ;  discrepancy  between  the  combining  weight 
and  the  volume-weight  has  been  proved  for  four  elements  ;  of  the 
remaining  elements,   constituting  more   than   four-fifths    of  the 
whole  number,  the  equal- volume  weights  are  wholly  unknown, 
inasmuch  as  these  elements  have  never  been  converted  into  vapor 
under  conditions  which  permit  the  experimental  determination  of 
the   equal-volume  weights  of  their  vapors.     For  example,  the 
symbols  Na  and  K  represent  the  combining  weights  of  these  two 
metals  ;  but  they  can  be  held  to  represent  the  weights  of  the  unit- 
volumes  of  these  metals  only  by  pure  assumption,  or,  at   best, 
on  the  uncertain  evidence   of  analogies,  since   the  unit-volume 
weights  of  these  metals,  when  converted  by  intense  heat  into 
gases,  have  never  yet  been  determined.     As  the  great  majority 
of  the  known  elements  cannot  be  volatilized,  or  made  gaseous, 
by  the  highest  temperatures  as  yet  at  our  command,  under  con- 
ditions which  permit  the  chemist  to  experiment  with  the  gases 
produced,  it  is  plain  that  composition  by  weight  is,  in  the  present 
state  of  chemistry,  of  far  greater  practical  importance  than  coin- 
position  by  volume.     The  symbols  of  all  the  elements  represent 
their  combining  weights,  as  determined  by  ponderal  analysis  ;  the 
symbols   of  eight    elements   represent    also    the    equal- volume 
weights  of  the  substances  they  stand  for.     These  eight  elements, 
though  few  in  number,  are  nevertheless  the  leading  elements  in 
norganic  chemistry. 


DOUBLE    OR    PRODUCT-VOLUME.  203 

260.  The  volume  of  the  molecule  of  every  compound  gas  in 
the  above  tables  is  twice  that  occupied  by  the  atom  of  hydrogen. 
Two  volumes  of  compound  gas  invariably  result  from  the  chemi- 
cal combination  of  one  volume  of  hydrogen  with  one  volume  of 
chlorine,  of  two  volumes   of  hydrogen  with  one  of  oxygen,  of 
three  volumes  of  hydrogen  with  one  of  nitrogen,  and  these  in- 
stances are  but  types  of  large  classes  of  chemical  reactions.     In 
organic  chemistry  the  same  law  holds  good  for  a  great  multi- 
tude of  complicated  compounds  of  carbon  ;  the  molecule  of  every 
organic  compound  in  the  state  of  vapor  occupies  a  volume  twice 
as  large  as  that  occupied  by  an   atom  of  hydrogen,  or,  in  other 
words,  twice  the   unit-volume.     This   doubled  volume  is  often 
called  the  normal,  or  product- volume  of  a  compound  gas.     Since 
the  combining  weight  of  a  compound  gas  or  vapor  occupies  two 
unit-volumes,  it  is  obvious  that  the  weight  of  one  volume,  which 
is  the  specific  gravity  of  the  gas  or  vapor,  is  deduced  from  the 
combining  weight  by  dividing  the  latter  by  two.     The   specific 
gravity  of  a  compound  gas  or  .vapor  is,  therefore,  one  half  its 
combining  weight. 

261.  Molecular  condition   of  elementary  gases.     Bearing   in 
mind  our  definitions   of  atom   and   molecule  (§§  38,  39),  let  us 
inquire  what  inferences   concerning  the  molecular  condition  of 
simple  gases  in  a  free  state  can  be  legitimately  drawn  from  our 
knowledge  of  the  molecular  condition  of  compound  gases.     To 
give  definiteness  to  our  conceptions,  let  us  assume  the  unit-volume 
of  the  elements  to  be  one   litre ;  the  product-volume  of  a  com- 
pound will  then  be  two  litres.     Two  litres,  the   product-volume, 
of  chlorhydric  acid  gas  are  made  up  of  one   litre   of  hydrogen 
and  one  litre  of  chlorine,  united  without  condensation,  and  each 
molecule  of  chlorhydric  acid  must  contain  at   least  one  atom  of 
hydrogen  and  one  of  chlorine.    In  these  two  litres  of  chlorhydric 
acid  there  must  be  some  definite  number  of  molecules ;  the  num- 
ber is,  of  course,  indeterminable,  but  let  us   assign  to  it   some 
numerical  value,  say  1000,  in  order  to  give  clearness  to  our  rea- 
soning.    One  litre   of  chlorhydric   acid   will   then   contain    500 
molecules,  and  since  equal  volumes  of  all  gases,  whether  simple 
or  compound,  are  assumed  to  contain,  under  like  conditions,  the 
same  numbers  of  molecules  (§  39),  one  litre  of  hydrogen  or  of 


204 


MOLECULES    OF    ELEMENTARY    GASES. 


chlorine  will  also  contain  500  molecules.  But  the  one  litre  of 
hydrogen  arid  the  one  litre  of  chlorine,  which,  by  uniting,  pro- 
duced 2  litres  =  1000  molecules  of  chlorhydric  acid,  must  each 
have  contained  1000  atoms  of  hydrogen  and  of  chlorine  respec- 
tively, for  each  molecule  of  chlorhydric  acid  demands  an  atom  of 
hydrogen  and  an  atom  of  chlorine.  The  litre  of  hydrogen,  or  of 
chlorine,  then  contains  500  molecules,  but  1000  atoms,  each 
molecule  of  the  simple  gas  being  made  up*  of  two  atoms  of  the 
single  element,  just  as  each  molecule  of  the  compound  gas  under 
review  is  composed  of  two  atoms,  one  of  hydrogen  and  one  of 
chlorine.  It  is  clear  that  this  train  of  reasoning  is  independent 
of  the  particular  numerical  value  assumed  as  the  number  of 
molecules  in  two  litres  of  chlorhydric  acid.  If,  therefore,  the 
molecule  of  chlorhydric  acid  is  represented  by  the  formula  HC1, 


and  the  diagram, 


HC1 


there  is  good  reason  to  assign  to  free  hydrogen  and  free  chlorine 
the  formula?  HH  and  C1C1,  and  to  represent  the  constitution  of 
all  uncombined  gases  by  such  diagrams  as 


HH 


C1C1 


Upon  these  models  the  molecular  formulae  of  all  the  elements 
witli  which  we  have  become  acquainted  might  readily  be  written. 
It  is  only  in  a  free  state  that  the  elementary  gases  and  vapors 
are  thus  conceived  to  exist  as  molecules ;  when  they  enter  into 
combination,  it  is  by  atoms  rather  than  by  molecules.  An  atom 
of  hydrogen  unites  with  an  atom  of  chlorine ;  three  atoms  of 
hydrogen  combine  with  one  of  nitrogen. 

If  this  view  of  the  molecular  structure  of  free  elementary 
gases  and  vapors  be  correct,  perfect  consistency  would  require 
that  no  equation  should  ever  be  written  in  such  a  manner  as  to 
represent  less  than  two  atoms,  or  one  molecule,  of  an  element  in  a 
free  state  as  either  entering  into  or  issuing  from  a  chemical  re- 
action. Thus,  instead  of  H2  -f-  O  =  H2O,  N  -f  311  =  NH3 , 
HC1  -\-  Na  =  NaCl  -|-  H,  it  would  be  necessary  to  write 


PHOSPHOKUS.  205 

2HH  +  00  =  2H20,  NN  +  3HH  =  2NH8, 

2HC1  -f  NaNa  =  2NaCl  +  HH. 

We  have  not  heretofore  conformed  to  this  theoretical  rule,  aifdl 
do  not  propose  to  in  the  succeeding  pages,  and  this  for  two  rea- 
sons ;  first,  because  many  equations,  representing  chemical  reac- 
tions, must  be  multiplied  by  two,  in  order  to  bring  them  into  con- 
formity with  this  hypothesis  concerning  molecular  structure  ;  the 
equations  are  thus  rendered  unduly  complex  ;  —  secondly,  because, 
in  undertaking  to  make  chemical  equations  express  the  molecular 
constitution  of  elements  and  compounds,  as  well  as  the  equality 
of  the  atomic  weights  on  each  side  of  the  sign  of  equality,  there 
is  imminent  danger  of  taking  the  student  away  from  the  sure 
ground  of  fact  and  experimental  demonstration,  into  an  uncer- 
tain region  of  hypotheses  based  only  on  definitions  and  analogies. 
The  symbol  Na  represents  23  proportional  parts  by  weight  of  the 
metal  sodium  ;  of  the  molecular  symbol  NaNa  ,  the  most  that  can 
be  said  is,  that  some  strong  analogies  justify  us  in  assuming,  for 
the  present,  in  default  of  any  experimental  evidence  on  the  sub- 
ject, that  a  molecule  of  free  sodium  gas,  if  we  could  get  at  it, 
would  be  found  to  consist  of  two  least  combining  parts  by  weight 
of  sodium.  We  know  as  much,  at  least,  of  the  molecular  struc- 
ture of  sodium  as  we  do  of  four-fifths  of  the  recognized  chemical 
elements.  For  the  present,  the  biatomic  structure  of  the  mole- 
cule of  a  simple  gas  or  vapor  in  the  free  state  must  take  place 
in  an  elementary  manual,  as  an  ingenious  and  philosophical  hy- 
pothesis, rather  than  as  a  general  and  indubitable  fact. 


CHAPTER    xl. 

PHOSPHORUS. 

262.  Phosphorus  occurs  somewhat  abundantly  and  very 
widely  diffused  in  nature.  It  is  never  found  in  the  free  state,  but 
almost  always  in  combination  with  oxygen  and  some  one  of  the 
metals.  The  most  abundant  of  its  compounds  is  phosphate  of 
calcium ;  small  quantities  of  this  mineral  are  found  in  most  rocks 


206  ORDINARY    PHOSPHORUS. 

and  soils,  and  in  several  localities  it  occurs  in  large  beds.  Phos- 
phate of  calcium  is  the  chief  mineral  constituent  of  the  bones  of 
animals ;  it  contains  one-fifth  of  its  own  weight  of  phosphorus. 
The  proportion  of  phosphorus  present  in  most  of  the  ordinary 
rocks,  and  in  the  soils  which  have  resulted  from  their  disintegra- 
tion, is  usually  very  small,  and  phosphorus  would  be  an  exceed- 
ingly costly  substance  if  we  were  compelled  to  collect  it  directly 
from  this  source ;  but  it  so  happens  that  the  phosphorus  com- 
pounds are  important  articles  of  food  for  plants  and  animals,  and 
it  is  easy  to  obtain  through  their  intervention  the  phosphorus 
which  was  before  widely  diffused,  but  has  been  by  them  concen- 
trated. Growing  plants  seek  out  and  collect  the  traces  of  phos- 
phorus-compounds which  exist  in  the  soil ;  the  herbiverous 
animals  in  their  turn  consume  the  phosphorus  which  has  been 
accumulated  by  the  plants,  and  from  the  bones  of  animals  chem- 
ists and  manufacturers  derive  the  phosphorus  of  which  they  stand 
in  need. 

Like  oxygen  and  sulphur,  phosphorus  occurs  in  several  dis- 
tinct allotropic  modifications.  Of  these,  the  best  known  are 
called  respectively  ordinary  phosphorus  and  red  phosphorus. 

263.  Ordinary  phosphorus,  when  perfectly  pure,  is  a  trans- 
parent, colorless,  wax-like  solid  of  1.8  specific  gravity,  which, 
when  freshly  cut,  emits  an  odor  like  garlic,  though  under  ordi- 
nary conditions  this  odor  is  overpowered  by  the  odor  of  ozone, 
which,  as  has  been  previously  stated  (§  164),  is  developed  when 
phosphorus  is  exposed  to  the  air.  It  unites  with  oxygen  readily, 
even  at  the  ordinary  temperature  of  the  air,  and  with  great  en- 
ergy at  somewhat  higher  temperatures  (above  60°)  ;  when  in 
contact  with  air  it  is  all  the  while  undergoing  slow  combustion. 

Exp.  110.  —  Thoroughly  wash  a  piece  of  phosphorus  by  rinsing  it  in 
successive  large  quantities  of  water ;  place  it,  for  a  moment,  upon  a 
sheet  of  filter  paper,  in  order  that  a  portion  of  the  water  adhering  to 
it  may  be  removed,  then  lay  it  upon  a  clean  porcelain  capsule,  and  at 
short  intervals  press  against  it  a  slip  of  blue  litmus  paper.  In  a  very 
few  moments  the  color  of  the  paper  will  be  changed  to  red,  for  the  pro- 
ducts of  the  oxidation  of  phosphorus  are  acid,  and  they  are  formed 
with  great  rapidity. 

If  the   temperature   of    the    slowly  burning   phosphorus   be 


FRICTION    MATCHES.  207 

slightly  increased  in  any  way,  the  mass  will  burst  into  flame  and 
be  rapidly  consumed.  On  account  of  this  extreme  inflammabil- 
ity, phosphorus  must  always  be  kept  under  water  ;  it  is  best 
also  to  cut  it  under  water,  lest  it  become  heated  to  the  kindling 
point  by  the  warmth  of  the  hand  or  by  friction  against  the  knife. 
When  wanted  for  use,  the  phosphorus  is  taken  from  the  water 
and  dried  by  gently  pressing  it  between  pieces  of  blotting-paper. 

Phosphorus  must  always  be  handled  with  great  caution,  for 
when  once  on  fire,  it  is  exceedingly  difficult  to  extinguish  it,  and 
in  case  it  happens  to  burn  upon  the  flesh,  painful  wounds  are 
inflicted,  which  are  exceedingly  difficult  to  heal.  Whenever 
phosphorus  is  cut  or  broken,  care  must  likewise  be  taken  that  no 
small  fragments  of  it  fall  unobserved  into  cracks  of  the  table  or 
floor  where  they  might  subsequently  take  fire. 

Exp.  111. — Nip  a  piece  of  phosphorus,  as  large  as  a  small  pea,  be- 
tween two  bits  of  wood,  in  such  manner  that  a  part  of  the  phosphorus 
shall  project  below  the  wood ;  rub  the  phosphorus  strongly  upon  a  sheet 
of  coarse  paper ;  it  Avill  take  fire  at  the  temperature  developed  by  the 
friction. 

264.  On  account  of  this  easy  inflammability  by  friction,  phos- 
phorus is  extensively  employed  for  making  matches.  The 
matter  upon  the  end  of  an  ordinary  friction-match  usually  con- 
tains a  little  phosphorus,  together  with  some  substance  capable 
of  supplying  oxygen,  such  as  red-lead,  black  oxide  of  manganese, 
saltpetre,  or  chlorate  of  potassium.  The  phosphorus  and  the 
oxidizing  agent  are  kneaded  into  a  paste  made  of  glue  or  gum, 
and  the  wooden  match-sticks,  the  ends  of  which  have  previously 
been  dipped  in  melted  sulphur,  are  touched  to  the  surface  of  the 
phosphorized  paste,  so  that  the  sulphured  points  shall  receive  a 
coating  of  it.  The  sulphur  serves  merely  as  a  kindling  material 
which,  as  it  were,  passes  along  the  fire  from  the  phosphorus  to  the 
wood.  By  rubbing  the  dried,  coated  point  of  the  match  against 
a  rough  surface,  heat  enough  is  developed  to  bring  about  chemi- 
cal action  between  the  phosphorus  and  the  oxygen  of  the  other 
ingredient,  combustion  ensues,  the  sulphur  becomes  hot  enough 
to  take  Oil  oxygen  from  the  air,  and  finally  the  wood  is  involved 
in  the  play  of  chemical  force. 

Exp.  112.  —  Put  a  piece  of  phosphorus,  as  big  as  a  grain  of  wheat 


208  PHOSPHORESCENCE. 

upon  a  filter-paper,  and  sprinkle  over  it  some  lamp-black,  or  powdered 
bone-black.  The  phosphorus  will  melt  after  a  time,  and  will  finally 
take  fire.  As  will  be  more  fully  explained  hereafter,  under  carbon, 
the  porous,  finely-divided  lamp-black  has  the  power  of  absorbing  and 
condensing  within  its  pores  much  oxygen  from  the  air  ;  heat  is  devel- 
oped by  the  act  of  condensation,  and,  at  the  same  time,  oxygen  js 
brought  into  very  intimate  contact  with  the  phosphorus,  particularly 
with  the  vapor  of  phosphorus  which  is  condensed  by  the  lamp-black 
together  with  the  oxygen,  so  that  chemical  action  soon  results,  and  ulti- 
mately fire.  Both  the  lamp-black  and  the  paper  are  bad  conductors 
of  heat ;  they  prevent  the  phosphorus  from  losing  the  heat  developed 
by  the  condensation  and  by  the  slow  action  of  oxygen. 

It  is  remarkable  that  when  dry  phosphorus,  in  very  thin  slices,  is 
laid  upon  fine  feathers,  wool,  lint,  flannel,  dry  wood,  or  other  non-con- 
ducting substances,  it  quickly  melts,  and  readily  inflames  upon  the 
slightest  friction,  heat  enough  being  produced  by  the  slow  combustion 
of  the  phosphorus  to  fuse  it,  if  only  this  heat  can  be  retained  by  some 
bad  conductor. 

265.  At  the  ordinary  temperature  of  the  air,  and  still  more 
at   somewhat  higher    temperatures,  phosphorus    shines   with    a 
greenish-white  light,  as  may  be  seen  by  placing  the  phosphorus 
in  the  dark;  hence  the  name,  phosphorus,  from  Greek  words  sig- 
nifying light-bearing.     This   phosphorescence  is  seen  when  an 
ordinary  friction-match  is  rubbed  against  any  surface  in  a  dark 
room.     Although    the   phenomena   of  phosphorescence   and    of 
oxidation,  or  slow  combustion,  occur  simultaneously  when  phos- 
phorus is  exposed  to  the  air,  it  does  not  appear  that  the  phos- 
phorescence is   a  consequence  of  the  oxidation,  for  phosphorus 
shines  not  only  in  the  air,  but  also  when  placed  in  an  atmosphere 
of  pure  hydrogen,  or  nitrogen,  or  carbonic  acid,  or  even  in  a 
vacuum,  though  the  light  emitted  by  phosphorus  in  these  inert 
gases  is  of  different  appearance  from  that  developed  in  presence 
of  oxygen. 

266.  In  warm  weather  phosphorus  is  soft  and  somewhat  flexi- 
ble ;  it  may  then  be  bent  without  breaking ;  can  be  scratched 
with  the   nail  and  cut  with  a  knife   like  wax,  but  at  0°  it  is 
brittle,  and  exhibits  a  crystalline  fracture  when  broken.     It  melts 
at  44°,  forming  a  viscid  oily  liquid,  which  boils  at  about  290°, 
and  is  converted  into  colorless  vapor.     Phosphorus  can  readily  be 


SOLUTIONS    OF    PHOSPHORUS.  209 

distilled  in  a  retort  filled  with  some  inert  gas,  like  hydrogen, 
nitrogen,  or  carbonic  acid.  The  specific  gravity  of  its  vapor  has 
been  found  to  be  G2.1.  Contrary  to  all  our  previous  experience, 
the  density  of  phosphorus  is,  however,  not  identical  with  its 
atomic  weight,  a  point  which  will  be  discussed  when  the  com- 
pounds of  phosphorus  and  hydrogen  are  treated  of. 

On  being  heated  to  about  230°,  out  of  contact  with  air,  it  is 
converted  into  red-phosphorus.  (See  Exp.  115.)  By  exposure 
to  light,  also,  phosphorus  undergoes  a  certain  amount  of  change, 
hence  it  is  rarely  seen  in  the  perfectly  colorless,  transparent 
condition  which  it  exhibits  when  recently  prepared  and  perfectly 
pure.  The  phosphorus  of  commerce  is  usually  of  a  light  amber 
color.  When  kept  for  some  time  under  water,  phosphorus  be- 
comes covered  with  a  white  opaque  coating,  which  appears  to  be 
a  result  of  the  oxidizing  action  of  air  held  in  solution  by  the 
water ;  the  surface  of  the  phosphorus  is  irregularly  corroded  by 
this  dissolved  oxygen,  and  is  thus  roughened  and  made  opaque, 
in  much  the  same  way  that  the  transparency  of  glass  is  destroyed 
by  grinding  one  of  its  surfaces.  It  is  noticed,  for  that  matter, 
that  the  water  in  which  phosphorus  is  kept  soon  becomes  strongly 
acid  ;  for  it  dissolves  the  acid,  oxygenated  compounds  which  are 
produced  by  the  action  of  the  dissolved  air  upon  the  phosphorus. 
The  specific  heat  of  solid  phosphorus  is  0.1788 ;  of  liquid  phos- 
phorus, 0.2045.  It  is  a  non-conductor  of  electricity,  both  in 
the  solid  and  in  the  liquid  state. 

Phosphorus  is  insoluble  in  water,  but  is  somewhat  soluble  in 
ether,  petroleum,  benzine,  oil  of  turpentine,  and  other  oils  ;  it 
dissolves  abundantly  in  bisulphide  of  carbon,  in  chloride  of  sul- 
phur, and  in  sulphide  of  phosphorus. 

Exp.  113.  —  Pour  into  a  phial  of  the  capacity  of  80  or  90  c.  c.,  10 
or  12  c.  c.  of  bisulphide  of  carbon,  and  threw  into  this  liquid  a  bit  of 
phosphorus  as  large  as  a  pea.  Cork  the  phial,  and  shake  its  contents, 
at  intervals,  until  the  phosphorus  has  dissolved.  Preserve  the  solution 
for  use  in  subsequent  experiments. 

267.    From  the  solution  in  chloride  of  sulphur  and  from  that 

in  sulphide  of  phosphorus,  crystals  of  phosphorus,  usually  in  the 

form  either  of  regular  octahedrons  or  of  rhombic  dodecahedrons, 

can  be  obtained  ;  but  owing   to   the  slowness  with  which  phos- 

14 


210  PHOSPHORUS    A    POISON. 

phorus  passes  from  the  liquid  to  the  solid  state,  distinct  crys- 
tals cannot  readily  be  prepared  by  the  method  of  fusion,  unless  a 
comparatively  large  quantity  of  phosphorus  be  operated  upon. 

268.  When  a  solution  of  phosphorus  in  ether,  or,  betterj  in 
bisulphide  of  carbon,  is  poured  upon  the  surface  of  any  bad  con- 
ductor of  heat  and  left  to   evaporate  in   the   air,   the   volatile 
solvent  will  quickly  escape,  leaving  the  phosphorus  behind  in  a 
very  finely  divided  condition.     In  proportion  as  a  substance  is 
more  finely  divided,  the  greater  will  be  the  surface  which  it  pre- 
sents to  the  oxygen  of  the  air,  and  the  more  readily  will  it  com- 
bine with  this  oxygen.     In  the  case  before  us,  the  comminuted 
phosphorus  absorbs  oxygen  very  rapidly,  and  this  chemical  action 
is  attended  with  the  evolution  of  so  much   heat  that  the  phos- 
phorus takes  fire. 

Exp.  114. —  Pour  some  of  the  solution  of  phosphorus  obtained  in 
Exp.  113,  upon  a  sheet  of  filter-paper,  and  hang  the  paper  upon  the 
iron  stand  in  such  manner  that  the  bisulphide  of  carbon  may  freely 
evaporate.  The  paper  will  soon  burst  into  flame.  It  will  be  noticed 
that  the  vapor  is  not  completely  consumed,  but  that  a  very  considerable 
residue  of  carbon  remains  unburned.  This  depends  upon  the  fact  that 
the  product  of  the  combustion  of  the  phosphorus,  phosphoric  acid, 
quickly  covers  the  paper  with  a  varnish  which  is  not  only  incombustible 
in  itself,  but  is  quite  impervious  to  air. 

In  lack  of  bisulphide  of  carbon,  this  experiment  can  be  performed 
with  the  ethereal  solution  of  phosphorus,  prepared  in  the  manner  de- 
scribed in  Exp.  113,  excepting  that  common  ether  is  substituted  for  the 
bisulphide,  and  that  the  mixture  is  left  to  digest  for  a  day  or  two. 

269.  Ordinary  phosphorus  is  a  violent  poison  ;  a  few   deci- 
grammes of  it  being  sufficient  to  destroy  human  life.     It  is  the 
efficient  ingredient  of  many  preparations  used  for  poisoning  rats, 
cockroaches,  and  other  vermin.     Phosphorus  evaporates  rather 
freely  at  the  ordinary  temperature  of  the  air,  and  the  vapor  has 
been  found  to  be  exceedingly  injurious  to  persons  constantly  ex- 
posed to  it.     The  makers  of  friction  matches  are  subject  to  a 
horrible  wasting  disease,  one  of  the  symptoms  of  which  is  the 
destruction  of  the  bones  of  the  jaws. 

270.  When  phosphorus  burns  with  flame   in  free   air,   two 
atoms    of  it    unite    with   five   atoms   of   oxygen,    and  there   is 
formed  the  compound  of  P2O5 ;  this  highest  oxide  of  phosphorus 


MANUFACTURE    OF    PHOSPHORUS.  211 

is  called  phosphoric  acid.  This  compound  occurs  in  bones,  and 
from  it  phosphorus  is  prepared.  Bone-earth,  that  portion  of 
bones  which  remains  after  all  the  organic  matter  has  been  burnt 
off  in  the  fire,  consists  mainly  of  triphosphate  of  calcium,  Ca3P2O8 . 
In  order  to  obtain  phosphorus  from  bone-earth,  the  calcium  and 
the  oxygen  must  both  be  removed ;  the  calcium  is  removed  by 
means  of  sulphuric  acid,  and  the  oxygen  by  means  of  hot  char- 
coal. Bones  are  burnt  to  a  white  ash,  —  calcined,  as  the  term 
is,  —  then  finely  powdered  and  mixed  with  a  quantity  of  dilute  sul- 
phuric acid.  The  sulphuric  acid  removes  two  of  the  atoms  of 
calcium  and  forms  sulphate  of  calcium,  while  there  remains 
monophosphate  of  calcium  (superphosphate  of  lime)  in  accord- 
ance with  the  following  reaction  :  — 

Ca3P2O8  +  2H2SO4  =  CaH4P2O8  +  2CaS04. 

It  will  be  remembered  that  one  atom  of  calcium  replaces  two 
atoms  of  hydrogen.  (See  p.  76.) 

The  solution  of  monophosphate  of  calcium  is  then  filtered  off 
from  the  insoluble  sulphate  of  calcium,  and  evaporated  to  the 
consistence  of  syrup  ;  the  syrup  is  mixed  with  powdered  char- 
coal, and  the  mixture  dried  at  a  dull  red  heat;  by  this  means  a 
quantity  of  water  is  expelled  from  the  monophosphate  :  — 

CaH4P208  =  CaPA  +  2H20. 

The  porous  dry  mixture  is  finally  placed  in  retorts  of  fire-clay 
and  intensely  ignited.  At  high  temperatures,  charcoal  is  a  pow- 
erful deoxidizing  agent;  it  takes  away  oxygen  from  the  phosphate 
of  calcium,  and  forms  carbonic  oxide,  which  goes  off  as  a  gas  ; 
phosphorus  is  thus  set  free,  and  distilling  over  into  an  appropriate 
receiver,  is  condensed  under  cold  water,  a  quantity  of  triphos- 
phate of  calcium  is  at  the  same  time  reproduced  and  remains-  in 
the  retort :  — 

3  CaP200  +  IOC  =  10CO  +  4P-+  Ca3P2O8. 

If  a  quantity  of  sand  (silicic  acid)  be  added  to  the  mixture  of 
charcoal  and  monophosphate,  the  whole  of  the  phosphorus  can 
be  expelled,  —  the  phosphate  of  calcium,  which  would  otherwise 
escape  decomposition,  being  entirely  converted  into  silicate  of 
calcium, 


212  RED    PHOSPHORUS. 

2CaP2O6  +  2Si02  +  IOC  =  10CO  +  4P  +  2CaSi03. 
Another  proposed  method  is  to  pass  chlorhydric  acid  gas  over  a 
mixture  of  bone   phosphate  and  charcoal,  maintained  at  a  red- 
heat,  in  a  cylinder  of  fire-clay.     By  this  means  all  of  the  phos- 
phorus is  set  free  and  chloride  of  calcium  remains:  — 

Ca3P208  +  8C  +  6HC1  =  3CaCl2  +  8CO  +  6H  +  2P . 

The  crude  phosphorus  thus  obtained  is  remelted  and  purified  by 
filtration,  redistillation,  and  by  chemical  treatment  with  a  mixture 
of  bichromate  of  potassium  and  sulphuric  acid,  which  oxidizes 
the  principal  contaminations.  The  purified  phosphorus  is  finally 
remelted  and  cast  into  the  sticks  or  cakes  in  which  it  is  found  in 
commerce. 

271.  Red  Phosphorus.  This  remarkable  allotropic  modifica- 
tion of  phosphorus  is  a  body  as  unlike  ordinary  phosphorus  in 
most  respects  as  could  well  be  conceived.  It  is  of  a  scarlet-red 
color,  has  neither  odor  nor  taste,  is  not  poisonous  so  far  as  is 
known,  is  not  phosphorescent,  does  not  take  fire  at  ordinary  tem- 
peratures, is  insoluble  in  bisulphide  of  carbon,  and  in  general 
behaves  altogether  differently  from  the  ordinary  modification. 
Yet  it  is  no  difficult  matter  to  change  one  of  these  modifications 
into  the  other.  For  example,  if  red  phosphorus  be  heated  to 
about  260°,  in  an  atmosphere  of  nitrogen,  or  other  inert  gas,  it 
will  pass  into  the  condition  of  ordinary  phosphorus  without 
undergoing  any  alteration  of  weight,  or,  in  other  words,  without 
absorbing  or  disengaging  anything. 

Exp.  115. —  In  a  narrow  glass  tube,  No.  6,  about  30  c.  m.  long  and 
closed  at  one  end,  place  a  quantity  of  red  phosphorus  as  large  as  a  small 
pea ;  heat  the  phosphorus  gently  over  the  gas-lamp,  and  note  that  a 
sublimate  of  a  light-colored  substance  is  quickly  deposited  upon  the  cold 
walls  of  the  tube  a  short  distance  above  the  heated  portion.  This  light- 
colored  sublimate  is  ordinary  phosphorus,  as  may  be  shown  by  cutting 
off  the  tube  just  below  the  sublimate,  after  the  glass  has  been  allowed 
to  cool,  and  then  scratching  the  coating  with  a  piece  of  wire ;  the  coat- 
ing will  take  fire.  The  air  in  the  narrow  tube  employed  is  deprived 
of  its  oxygen  by  the  combustion  of  a  small  portion  of  the  phosphorus 
at  the  moment  of  its  transformation  from  the  red  to  the  ordinary  con- 
dition ;  the  remaining  phosphorus  is  thus  enveloped  in  nitrogen  and  so 
protected  from  further  loss. 


CRYSTALLIZED    RED    PHOSPHORUS.  213 

272.  Red  phosphorus  is  itself  neither  volatile  nor  inflamma- 
ble ;  it  neither  rises  as  vapor  nor  inflames  at  temperatures  lower 
than  260°,  the  point  at  which  it  changes  into  ordinary  phosphorus  ; 
at  250°  it  suffers  no  alteration.     As  compared  with  ordinary 
phosphorus,  it  may  be  said  that  red  phosphorus  can  be  handled 
without  danger,  and  that  it  may  be  kept  in  bottles  without  special 
precautions,  since  it  is  not  liable  to  take   fire  by  moderate  fric- 
tion ;  but  by  powerful  friction  heat  enough  may  be  evolved  to 
convert  it  into  ordinary  phosphorus,  and  if  it  be  even  moder- 
ately heated;  by  friction,  or  in  any  other  way,  in  contact  with 
oxidizing  agents,  it  instantly  bursts  into  flame. 

Exp,  116.  — In  order  to  observe  the  comparative  difficulty  of  inflam- 
ing red  phosphorus,  lay  an  inverted  cover  of  a  porcelain  crucible  upon 
an  iron  triangle  upon  the  lamp-stand;  place  upon  the  cover,  which 
may  be  15  c.  m.  wide,  a  small  bit  of  ordinary  phosphorus,  and  at  a 
distance  of  1 2  c.  m.,  the  same  quantity  of  red  phosphorus ;  heat  the 
cover  gently  and  gradually  over  the  gas-lamp.  The  ordinary  phos- 
phorus will  soon  inflame  and  burn  away,  but  a  considerable  space  of 
time  will  elapse  before  the  red  phosphorus  takes  fire. 

By  operating  in  vessels  filled  with  nitrogen,  or  some  other  gas  which 
has  no  chemical  action  upon  phosphorus,  the  precise  temperature  at 
which  the  red  phosphorus  ceases  to  exist  can  be  noted,  and  the  ordi- 
nary phosphorus  obtained  from  it  can  be  distilled  over  and  collected. 

273.  Red  phosphorus  has  been  obtained  in  crystals  by  dissolving 
common  phosphorus  in  melted  lead,  and  subjecting  the  fluid  mass 
to  a  high  temperature  for  several  hours  in   closed  tubes ;  when 
the  lead  cools,  the  phosphorus  separates  in  thin  crystals,  which 
have  a  metallic  lustre  and  a  black  color ;  the  crystals,  however, 
transmit  a  yellowish-red  light,  and  the  thinnest  of  them  appear, 
not  black,  but  red.     These  crystals  of  red  phosphorus  are  gener- 
ally enveloped  in  the  lead,  but  the  lead  may  be  mostly  dissolved 
away  by    dilute  nitric  acid,  and   the  phosphorus  crystals    may 
thus  be  obtained  in  a  condition  of  comparative  purity.     They 
are  not  affected  by  exposure  to  the  air.     These  crystals  are  seen 
under  the  microscope  to  be  rhombohedrons,  so  that  phosphorus, 
like  the  succeeding  members  of  the  family  of  elements  to  which  it 
belongs,  is  dimorphous,  presenting  forms  both  of  the  monometric 
and  hexagonal  systems. 

The  red  variety  of  phosphorus  has  been  not  inaptly  called 


214          PREPARATION  OP  RED  PHOSPHORUS. 

metallic  phosphorus,  crystallized  in  the  crystals  just  described, 
and  amorphous  in  the  usual  form  of  red  phosphorus.  The  crys- 
tallized metallic  phosphorus  is  less  volatile,  and  has  a  higher  spe- 
cific gravity  than  the  amorphous.  The  power  of  the  so-called 
metallic  phosphorus  to  conduct  electricity  is  small,  if  compared 
with  that  of  the  common  metals,  but  it  is  very  much  greater  than 
the  conducting  power  of  colorless  phosphorus,  for  this  latter 
substance  is  generally  classed  with  the  insulators. 

The  specific  gravity  of  amorphous  red  phosphorus  is  2.14;  its 
specific  heat  is  0.1698.  When  dry,  it  undergoes  no  change  at 
the  ordinary  temperature  of  the  air ;  but  in  moist  air  it  oxidizes 
very  slowly.  It  is  easily  soluble  in  nitric  acid,  which  oxidizes  it ; 
and  since  it  is  much  more  readily  dissolved  than  ordinary  phos- 
phorus, the  latter  can  be  purified  from  any  contamination  of  red 
phosphorus,  by  digesting  it  at  a  gentle  heat  in  dilute  nitric  acid. 

274.  Amorphous  red  phosphorus  is  prepared  by  maintaining 
ordinary  phosphorus,  for  some  time,  at  a  temperature  of  230°  to 
235°,  either  under  water  in  an  air-tight  vessel,  or  in  an  atmos- 
phere of  some  gas  which  has  no  chemical  action  upon  phospho- 
rus. It  is  manufactured  upon  the  large  scale  by  heating  ordinary 
phosphorus  in  a  cast-iron  vessel  provided  with  a  gas  delivery-tube 
dipping  into  mercury  outside  the  vessel,  in  such  manner  that  while 
the  expanded  air  and  some  escaping  vapors  of  phosphorus  can 
pass  out,  no  air  can  enter  the  vessel.  About  200  kilos,  of  phospho- 
rus are  taken  for  a  single  charge  ;  this  quantity  of  phosphorus  is 
maintained  during  ten  days  or  more,  as  nearly  as  may  be,  at  the 
temperature  of  240°,  care  being  taken  that  the  heat  shall,  at  no 
time,  much  exceed  this  limit.  Under  these  conditions,  the  ordi- 
nary phosphorus  slowly  changes  into  the  red  variety.  After  the 
phosphorus  has  been  exposed  during  the  time  which  the  previous 
experience  of  the 'manufacturer  has  shown  to  be  most  advan- 
tageous, the  apparatus  is  allowed  to  become  cold,  and  the  trans- 
muted phosphorus  is  found  adhering  to  the  sides  of  the  vessel,  in 
the  shape  of  a  hard,  brittle,  brick-colored  coating,  which  can  be 
removed  by  means  of  hammer  and  chisel,  after  covering  it  with 
water.  It  is  ground  to  powder  under  water,  and  any  particles 
of  ordinary  phosphorus  which  have  escaped  change  are  removed 
from  it  by  means  of  bisulphide  of  carbon,  or  by  a  solution  of 


SAFETY-MATCHES.  215 

caustic  soda,  which  dissolves  ordinary  phosphorus  without  acting 
upon  the  red  modification. 

275.  Red  phosphorus  is  employed,  to  a  certain  extent,  as  an 
adjunct  to  the  so-called  safety -matches.  Such  matches  contain 
no  phosphorus  in  themselves,  and  will  not  take  fire  readily  by 
friction  upon  an  ordinary  rough  surface,  though  they  burst  into 
flame  at  once  when  rubbed  upon  a  surface  specially  prepared 
with  red  phosphorus.  The  matter  upon  the  tips  of  safety- 
matches  is  usually  a  mixture  of  chlorate  of  potassium  and  sul- 
phide of  antimony,  made  into  a  paste  by  means  of  glue ;  the 
surface  upon  which  the  match  is  to  be  rubbed  is  composed  of  red 
phosphorus,  black  oxide  of  manganese,  and  glue.  In  favor  of 
the  use  of  red  phosphorus  for  matches  are  the  facts,  that,  unlike 
ordinary  phosphorus,  it  is  not  deleterious  to  the  workmen  who 
have  to  deal  with  it,  and  that  it  is  far  less  liable  to  be  set  on  fire 
by  accidental  friction.  For  these  reasons,  the  manufacture  of 
safety-matches  has  been  encouraged  by  the  governments  of  sev- 
eral European  countries,  and  such  matches  are  now  much  used 
in  France  and  upon  other  parts  of  the  continent,  though  they 
are  manifestly  less  convenient,  in  several  respects,  than  the  ordi- 
nary matches  which  can  be  ignited  by  friction  upon  any  rough 
surface. 

27 G.  Phosphorus  combines  readily  with  many  other  elements 
besides  oxygen.  The  ordinary  modification  of  phosphorus  com- 
bines violently  with  sulphur  at  temperatures  near  the  melt- 
ing point  of  sulphur,  the  act  of  combination  being  attended  with 
vivid  combustion  and  loud  explosion.  Red  phosphorus,  on  the 
other  hand,  does  not  combine  with  sulphur  at  temperatures  lower 
than  230°,  and  the  combustion,  though  rapid,  is  not  explosive. 
With  chlorine,  bromine,  and  iodine,  ordinary  phosphorus  unites 
directly  at  the  ordinary  temperature  of  the  air,  the  combination 
being  rapid  and  attended  with  inflammation.  Red  phosphorus 
also  unites  with  chlorine,  bromine,  and  iodine,  at  the  ordinary 
temperature,  and  much  heat  is  evolved  during  the  act  of  com- 
bination, though  the  amount  of  heat  is  usually  insufficient  to 
produce  ignition. 

Phosphorus  unites  directly  with  most  of  the  metals  also,  and 
several  of  the  compounds  thus  formed  closely  resemble  the  so- 


216  PHOSPHORUS    WITH    OXIDIZING    AGENTS. 

called  alloys,  or  compounds  of  one  metal  with  another.  With 
hydrogen  it  forms  several  interesting  compounds,  which  will  be 
described  directly.  From  the.  remarkable  facility  with  which  it 
combines  with  oxygen  (§§  263,  264),  it  follows  necessarily  that 
phosphorus  is  a  powerful  reducing  agent.  Many  oxygen  com- 
pounds^ can  be  decomposed  by  means  of  it.  When  immersed  in 
the  vapor  of  anhydrous  sulphuric  acid,  phosphorus  takes  fire 
after  a  time,  and  combines  with  the  oxygen  of  the  acid,  while 
sulphur  is  deposited.  Monohydrated  sulphuric  acid  is  reduced 
to  sulphurous  acid,  while  phosphorous  acid  is  formed :  — 
3H2S04  +  2P  =  2H3P03  +  3S02. 

A  solution  of -sulphurous  acid,  on  being  heated  with  phosphorus, 
yields  phosphorous  and  sulphydric  acids,  as  follows :  — 

S02  +  4H20  +  2P  =  2H3P03  +  H2S . 

When  gently  heated  with  chlorate  or  with  nitrate  of  potassium, 
or  with  other  highly  oxygenated  bodies,  like  the  peroxides  of 
lead  and  manganese,  phosphorus  combines  with  their  oxygen  so 
rapidly  that  an  explosion  ensues ;  heat  enough  to  bring  about  the 
reaction  can  be  developed  by  gentle  friction,  as  when  the  phos- 
phorus and  the  other  ingredient  are  rubbed  together  upon  some 
hard  surface.  (Compare  §  264.) 

Exp.  117.  —  Provide  a  bit  of  ordinary  phosphorus,  as  large  as  a  pin's 
head,  also  an  equal  quantity  of  red  phosphorus ;  add  to  each  of  these 
portions  enough  finely  powdered  chlorate  of  potassium  to  cover  the 
phosphorus ;  fold  up  each  of  the  mixtures  tightly  and  separately  in  a 
small  piece  of  writing-paper ;  place  the  parcels,  one  after  the  other, 
upon  an  anvil  and  strike  them  sharply  with  a  hammer.  They  will 
explode  with  violence. 

277.  Compounds  of  Phosphorus  and  of  Hydrogen.  There 
are  three  compounds  of  phosphorus  and  hydrogen,  one  gaseous, 
PH3,  one  liquid,  PH2,  and  one  solid,  P2H  ,  at  ordinary  tempera- 
tures. The  gaseous  compound,  or,  rather,  the  gaseous  compound 
charged  with  the  vapor  of  the  liquid  compound,  is  somewhat 
interesting,  from  the  fact  that  it  takes  fire  spontaneously,  imme- 
diately on  coming  into  contact  with  the  air. 

Exp.  118. —  In  a  thin-bottomed  flask  of  about  140  c.  c.  capacity,  put 
1  gramme  of  phosphorus  and  115  c.  c.  of  potash-lye  of  1.27  specific 


PHOSPHURETTED    HYDROGEN.  217 

gravity,  obtained  by  dissolving  40  grins,  of  hydrate  of  potassium  in 
110  c.  c.  of  water.  Pour  two  or  three  drops  of  ether  upon  the  liquid 
in  the  neck  of  the  flask,  then  close  the  flask  with  a  cork  carrying  a  long 
gas  delivery-tube  of  glass,  No.  5.  Place  the  flask  over  the  gas-lamp, 
upon  the  Avire-gauze  ring  of  the  iron  stand,  and  immerse  the  end  of 
the  delivery-tube  in  the  water  pan,  then  gently  heat  the  flask.  The 
ether  is  added  to  the  contents  of  the  flask,  in  order  that  the  last  traces 
of  air  may  be  expelled  from  the  flask  by  the  vapor  which  arises  from 
this  highly  volatile  liquid  so  soon  as  it  is  warmed. 

As  the  potash-lye  becomes  hot,  small  bubbles  of  gas  will  be  seen  to 
arise  from  the  surface  of  the  phosphorus,  and  in  a  short  time  large 
bubbles  of  gas  will  escape  from  the  delivery-tube  ;  each  of  these  bub- 
bles, as  it  comes  in  contact  with  the  air  at  the  surface  of  the  water, 
will  spontaneously  burst  into  flame,  and  burn  with  a  vivid  light  and 
the  formation  of  beautiful  rings  of  white  smoke,  if  the  air  be  not  dis- 
turbed by  draughts.  In  burning,  the  pho.sphuretted  hydrogen  is  con- 
verted into  phosphoric  acid  and  water,  or,  rather,  into  hydrated 
phosphoric  acid,  and  of  this  product  the  white  smoke  is,  of  course, 
composed. 

2PH3  +  80  =  H6P208. 

Exp.  119.  —  Place  a  small  inverted  bottle  full  of  water  over  the  end 
of  the  delivery-tube  from  which  the  phosphuretted  hydrogen  is  escap- 
ing, as  in  Exp.  118,  and  collect  50  or  100  c.  c.  of  this  gas.  By  single 
bubbles  pass  the  gas  thus  collected  into  a  litre  bottle  half-full  of  oxygen 
standing  inverted  upon  a  shelf  in  the  water-pan.  The  phosphuretted 
hydrogen  will  burn  much  more  vividly  in  oxygen  than  in  the  air.  In 
case,  however,  several  successive  bubbles  of  the  gas  should  fail  to  in 
flame  on  coming  in  contact  with  the  oxygen,  the  experiment  must  be  in- 
terrupted and  the  oxygen  thrown  away,  for  the  introduction  of  another 
bubble  of  phosphuretted  hydrogen  into  this  explosive  mixture  might 
set  fire  to  it  and  so  shatter  the  bottle. 

278.  The  reaction  which  occurs  during  the  preparation  of 
phosphuretted  hydrogen  is  chiefly  between  water  and  phosphorus. 
Phosphorus  and  water  by  themselves  do  not  react  upon  each 
other,  but  when  in  presence  of  powerful  bases,  like  soda,  potash, 
lime,  or  baryta,  water  is  decomposed  by  phosphorus  with  forma- 
tion both  of  oxygenated  and  hydrogenized  phosphorus  com- 
pounds :  — 
3(K2O,II2O)  +  8P  +  6HaO  =  2PH3  +  3(KaO,2HaO,lV)). 

Hypopkospliite  of  Potassium.. 


218  PREPARATION    OF    PHOSPHURETTED    HYDROGEN. 

Another  method  of  obtaining  phosphuretted  hydrogen  is  by 
decomposing  phosphide  of  calcium  with  water : 

Exp.  120. —  Prepare  a  number- of  small  balls  or  sticks  of  quick  lime 
by  moulding  moistened  slaked  lime  into  these  forms  and  then  drying 
and  calcining  the  product.  Select  a  tube  of  hard  glass,  No.  1,  close  it 
at  one  end,  place  a  few  cubic  centimetres  of  phosphorus  at  the  closed  end ; 
fill  the  tube  with  the  pellets  of  quick  lime,  and  place  it  in  a  sheet  iron 
trough  above  a  wire-gauze  gas-lamp,  in  the  manner  depicted  in  Fig.  8. 
Heat  to  redness  the  portion  of  the  tube  which  contains  the  lime,  and 
then  cause  the  vapor  of  phosphorus  to  pass  over  it  by  cautiously  heat- 
ing the  closed  end  of  the  tube  with  an  ordinary  gas-lamp.  After  the 
phosphorus  has  all  been  driven  forward  from  the  closed  end  of  the 
tube,  the  open  end  of  the  tube  should  be  stopped  with  a  cork  and  the 
lamps  should  be  extinguished;  the  tube  is  then  left  at  rest  until  it  has 
become  cold. 

When  a  piece  of  the  impure  phosphide  of  calcium  thus  obtained  is 
thrown  into  water,  it  slowly  decomposes  with  formation  of  hypophos- 
phate  of  calcium  and  disengagement  of  phosphuretted  hydrogen ;  the 
bubbles  of  gas  take  fire  as  they  reach  the  surface  6*f  the  water. 

279.  Besides  the  spontaneously  inflammable  gas,  there  is 
another  variety  of  phosphuretted  hydrogen  which  does  not  take 
fire  of  itself  in  the  air.  It  can  be  prepared  in  various  ways, 
for  example,  by  heating  hypophosphorous  or  phosphorous  acids 
(§§  286,  287),  these  acids  being  resolved  by  heat  into  phosphoric 
acid  and  phosphuretted  hydrogen  :  — 

Hypophosphorous  j  Empirical:  2H3P(X  =  PH3  -f  H3PO4. 

Acid.  |Dualistic:  6H2O,2P2O  =  2PH3  -f  3H2O,P2O5. 

Phosphorous     (  Empirical:  4H3PO5  =  PH3  -f-  3H,PO4. 

Acid.  I  Dualistic :  4(3H2O,P2O3)  =  2PH3-f-  3(3H2O,P2O5). 

The  non-inflammable  gas  is  regarded  as  pure  phosphuretted  hy- 
drogen, the  property  of  spontaneously  inflaming  possessed  by  the 
other  variety  being  supposed  to  depend  upon  the  presence  of 
minute  portions  of  some  foreign  substance ;  the  vapor  of  liquid 
phosphuretted  hydrogen,  PH2,  produces  this  effect;  on  adding 
to  the  non-inflammable  gas  so  small  a  quantity  as  riJtk<nyth  °f 
its  bulk  of  nitric  oxide,  it  acquires  the  property  of  inflaming 
spontaneously. 

Pure  phosphuretted  hydrogen  gas  is  colorless  and  highly  in- 
flammable ;  its  odor  is  fetid,  and  has  been  compared  with  that  of 


ANALYSIS    OF    PHOSPHURETTED    HYDROGEN. 


219 


tainted  fish  ;  it  is  slightly  soluble  in  water,  and  can  be  liquefied, 
but  has  not  yet  been  solidified.  Neither  the  gas  nor  its  solu- 
tions have  any  action  on  red  or  blue  litmus.  It  is  a  powerful 
deoxidizing  agent,  and  is,  in  general,  easily  decomposed.  Most 
of  the  metals,  when  heated  in  the  gas,  combine  with  its  phos- 
phorus and  liberate  its  hydrogen,  just  as  we  have  seen  the  metal 
potassium  set  free  hydrogen  from  ammonia.  (Exp.  46.)  This 
ready  decomposition  of  the  gas  by  hot  metals  is  the  basis  of  the 
method  of  determining  its  decomposition  by  weight. 

280.  Phosphuretted  hydrogen  is  resolved  into  its  two  ele- 
ments, and  the  proportional  weights  of  the  elements  which  enter 
into  its  composition  are  simultaneously  determined  by  the  follow- 
ing process  :  —  The  gas  is  passed  through  a  hard-glass  tube  (-4, 
Fig  42),  filled  with  copper  turnings  and  heated  to  redness ;  the 

FIG.  42. 


copper  retains  all  the  phosphorus  and  the  hydrogen  becomes  free. 
This  last  gas  is  carried  forward  through  a  second  tube,  B,  filled 
with  oxide  of  copper  heated  to  redness ;  the  hydrogen  combines 
with  the  oxygen  of  the  oxide  of  copper,  and  the  steam  thus 
formed  is  condensed  and  absorbed  in  a  third  tube,  0,  filled  with 
pumice-stone  soaked  in  sulphuric  acid.  (Appendix,  §  15).  The 
tubes  A  and  G  are  weighed  both  before  and  after  the  experi- 
ment, and  the  augmentation  of  weight  gives  the  phosphorus  in 
A  and  the  water  in  G ;  from  the  weight  of  the  water  is  calcu- 
lated the  weight  of  the  hydrogen  required  to  produce  it.  Care 
must  be  taken  that  the  tube  A  be  heated  so  moderately  as  not 
to  distort  it,  and  that  nothing  be  added  to  its  weight  by  deposi- 
tions from  the  lamp-flames  used  to  heat  it.  It  is  also  necessary 
to  fill  the  tubes  with  nitrogen  gas,  before  beginning  the  actual 


220  COMPOSITION    OF    PHOSPHURETTED    HYDROGEN. 

analysis,  and  to  sweep  them  out  with  nitrogen  at  the  end.  This 
operation  is  easily  performed  by  the  aid  of  a  small  gas-holder 
full  of  nitrogen.  It  has  thus  been  experimentally  proved  that 
any  given  weight  of  phosphuretted  hydrogen  contains  8.57  per 
cent,  of  hydrogen  and  91.43  per  cent,  of  phosphorus.  Now,  it 
has  been  determined,  as  the  result  of  many  experiments  and  of 
a  careful  collation  of  the  formulae  of  all  known  compounds  of 
phosphorus,  that  the  least  proportional  weight  of  this  element 
which  enters  into  combination  is  31,  that  of  hydrogen  being  1. 
The  proportion, 

91.43:  8.57  ==  31:  x, 

gives  as  'the  value  of  x,  2.905  ;  the  nearest  whole  number  is  3, 
and  the  discrepancy  may  be  attributed  to  defects  of  the  analyti- 
cal process,  always  specially  to  be  feared  in  cases,  like  the  pres- 
ent, where  the  quantity  of  one  ingredient  is  many  times  larger 
than  that  of  the  other.  A  loss  of  matter,  or  error  in  weighing, 
which  would  amount  to  only  1  per  cent,  of  90  centigrammes, 
would  cause  an  error  of  more  than  11  per  cent,  on  8  centi- 
grammes. The  analysis  clearly  points  to  the  formula  PH8,  as 
representing  the  composition  of  phosphuretted  hydrogen,  inas- 
much as  for  every  31  parts  by  weight  of  phosphorus,  the  gas 
contains  three  parts  by  weight  of  hydrogen.  This  result  is  par- 
tially corroborated  by  volumetric  analysis.  If  the  hydrogen  lib- 
erated from  any  measured  quantity  of  phosphuretted  hydrogen 
by  passing  the  gas  through  a  tube  filled  with  hot  metal,  be  accu- 
rately measured,  it  will  be  found  that,  for  every  two  volumes  of 
the  compound  gas,  three  volumes  of'  hydrogen  are  set  free. 

Thus  far  the  composition  of  phosphuretted  hydrogen  has 
seemed  to  be  completely  analogous  to  that  of  ammonia  gas ;  but, 
at  this  point,  the  analogy  fails.  In  ammonia,  three  parts  by 
weight  of  hydrogen  are  combined  with  fourteen  of  nitrogen,  and 
three  volumes  of  hydrogen  are  united  with  one  volume  of  nitro- 
gen to  form  two  volumes  of  the  compound  gas.  If  the  par- 
allelism between  NH3  and  PH3  were  perfect,  one  volume  of 
phosphorus-vapor  ought  to  be  united  with  the  three  volumes  of 
hydrogen  which  two  volumes  of  phosphuretted  hydrogen  invari- 
ably contain.  The  densities  of  phosphorus-vapor  and  of  phos- 


AMMONIA    AND    PHOSPHURETTED    HYDROGEN.. 


221 


phuretted  hydrogen,  as  experimentally  determined,  prove  that 
this  is  not  the  case.  The  unit-volume  being  that  volume  of 
hydrogen  which  weighs  1, 

From  the  weight  of  2  unit-volumes  of  PH3  (Sp.  Ur.  =  17.09),  34.18 
Subtract  the  weight  of  3  unit-volumes  of  hydrogen,  .  .  3.00 

And  there  remains  for  the  weight  of  the  phosphorus-vapor,  31.18 

The  specific  gravity,  or  relative  weight  of  one  unit-volume  of 
phosphorus-vapor,  is  62.1,  as  has  been  already  mentioned.  Two 
volumes  of  phosphuretted  hydrogen,  therefore,  contain,  not  one 
volume,  but  only  half  a  volume  of  phosphorus-vapor.  The  atom 
of  phosphorus  weighing  31,  combines  with  the  same  quantity  of 
hydrogen  by  weight,  as  the  atom  of  nitrogen  weighing  14,  but 
the  volume  of  the  phosphorus  atom  is  only  one-half  the  volume 
of  the  nitrogen  atom.  The  combining  weights  and  the  unit-vol- 
ume weights  of  all  the  elements  previously  studied  have  been 
identical,  but  the  combining  weight  of  phosphorus  must  be 
doubled,  in  order  to  bring  it  into  coincidence  with  its  unit-volume 
weight.  The  volumetric  and  the  ponderal  composition  of  phos- 
phuretted hydrogen  are  both  exhibited  in  the  annexed  diagram. 

281.  This  difference  between 
ammonia  and  phosphuretted 
hydrogen  is  completely  out- 
weighed by  the  essential  like- 
ness in  composition  of  these 
two  gases  and  by  the  other 
striking  analogies  which  exist 
between  them.  When  one 
or  more  of  the  hydrogen  atoms  in  phosphuretted  hydrogen 
are  replaced  by  certain  groups  of  elements,  which  in  organic 
chemistry  play  the  part  of  elements,  compounds  are  obtained 
which,  like  ammonia,  neutralize  acids  and  are  strongly  alkaline. 
Phosphuretted  hydrogen  itself  combines  with  certain  of  the  acids 
in  definite  proportions.  With  bromhydric  and  iodohydric  acids, 
for  example,  it  forms  crystalline  compounds  whose  composition  i£ 
represented  by  the  formulas  PII4Br  and  PH4T,  —  formula?  which 
are  evidently  comparable  with  NIT4Br  and  NH4T. 

282.    Liquid  Phosphuretted  Hydrogen  (PH2)  ma7  he  obtained 


222  PHOSPHORUS    AND    OXYGEN. 

by  passing  the  spontaneously  inflammable  gaseous  compound, 
obtained  in  Exp.  118,  through  a  U-tube  surrounded  by  a  mix- 
ture of  ice  and  salt.  Under  these  conditions,  the  vapor  of  the 
liquid  compound,  which  was  diffused  in  the  gas,  condenses  and 
separates.  Liquid  phosphuretted  hydrogen  is  colorless,  has  a  high 
refracting  power,  and  is  not  miscible  with  water.  It  does  not 
solidify  at  — 20°  ;  when  heated  to  30°  or  40°  it  decomposes.  It 
is  exceedingly  inflammable,  and  bursts  into  flame  when  brought 
in  contact  with  the  air ;  when  a  small  quantity  of  its  vapor  is 
mingled  with  combustible  gases,  such  as  carbonic  oxide,  hydro- 
gen, or  carburetted  hydrogen,  these  gases  acquire  the  property 
of  inflaming  spontaneously.  When  exposed  to  sunlight,  it  is 
resolved  into  gaseous  and  solid  phosphuretted  hydrogen :  — 

5PH2  =  P2H  +  3PH3 . 

283.  Solid  Phosphuretted  Hydrogen  (P2H  ?)  is  formed  by  ex- 
posing liquid  phosphuretted  hydrogen  to  sunshine,  or  by  acting 
upon  the  liquid  with  chlorhydric  acid,  or  by  dissolving  phosphide 
of  calcium  in  strong  chlorhydric  acid.     It  is  a  compound  insolu- 
ble in  water  or  alcohol,  but  soluble  in  warm  potash-lye  with  lib- 
eration   of  gaseous    phosphuretted    hydrogen.     It  takes  fire  at 
about   150°,  and  is  of  a  yellow   color,  but  becomes  red  when 
exposed  to  light. 

284.  Compounds  of  Phosphorus  and  of  Oxygen.    Phosphorus 
unites  with  oxygen  in  four  different  proportions,  as  follows  :  — 

Oxide  of  Phosphorus,    P4O  . 
Hypophosphorous  Acid,  P20. 
Phosphorous  Acid,  P2O3 . 
Phosphoric  Acid,  P2O5 . 

All  of  these  compounds  exhibit  a  more  or  less,  distinct v acid 
character,  especially  when  combined  with  water,  and  the  one 
containing  most  oxygen,  phosphoric  acid,  is  a  very  important  acid. 

285.  Oxide  of  Phosphorus  (P4O).     When  ordinary  phospho- 
rus is  burned  in  a  confined  volume  of  air  or  oxygen,  insufficient 
for  its  complete  combustion,  there  will  be  found  mixed  with  the 
unconsumed  phosphorus,  after  the  chemical  action  has  ceased,  a 
certain  quantity  of  a  red  powder,  which  is  the  oxide  of  phos- 
phorus now  in  question. 


RED    OXIDE    OF    PHOSPHORUS.  223 

Exp.  121. —  Repeat  Exp.  13,  and  examine  the  red  mass  which  re- 
mains in  the  porcelain  capsule  after  it  has  been  sunk  in  the  water-pan 
and  thoroughly  cooled. 

Since  the  red  oxide  of  phosphorus  is  insoluble  in  bisulphide  of  car- 
bon, it  can  readily  be  obtained  in  a  state  of  purity  by  dissolving  in  this 
liquid  the  free  phosphorus  with  which  it  is  contaminated. 

Although  the  red  oxide  is  not  spontaneously  inflammable  by 
itself,  a  mixture  of  it  with  free  phosphorus,  such  as  the  residue 
from  the  preparation  of  nitrogen  (Exp.  13),  takes  fire  with  great 
ease,  being  even  more  readily  inflammable  than  phosphorus  alone. 
Such  residues  must  be  handled  with  special  care. 

Red  oxide  of  phosphorus  can  be  obtained  in  larger  quantities 
by  bringing  a  stream  'of  oxygen  gas  into  contact  with  phosphorus 
melted  under  hot  water. 

Exp.  122.  —  Place  about  a  cubic  centimetre  of  ordinary  phosphorus 
in  the  bottom  of  a  conical  test-glass,  or  wine-glass,  and  pour  upon  it 
hot  water  enough  to  half  fill  the  glass ;  the  phosphorus  will  melt,  but 
cannot  burn,  since  the  water  protects  it  from  contact  with  the  air,  and 
since  phosphorus  by  itself  is  incapable  of  decomposing  water.  By 
means  of  a  narrow  gas  delivery-tube  of  glass,  conduct  a  slow  stream  of 
oxygen  from  a  gas-holder  to  the  bottom  of  the  test-glass,  so  that  the 
oxygen  shall  come  into  immediate  contact  with  and  bubble  through  the 
melted  phosphorus.  The  phosphorus  will  burn  with  a  vivid  light  be- 
neath the  water ;  red  oxide  of  phosphorus  will  be  formed,  and  will 
float  about  in  the  water,  from  which  it  may  be  separated  by  filtration. 

In  the  lack  of  oxygen,  air  may  be  forced  down  upon  the  phosphorus, 
—  even  the  impure  air  blown  from  the  mouth  will  answer,  but,  with  air, 
the  reaction  is  less  intense  than  with  oxygen ;  hence,  when  it  is  em- 
ployed, the  experiment  had  better  be  performed  in  a  dark  room. 

Oxide  of  phosphorus  has  neither  taste  nor  smell.  On  being 
heated  to  *JdO°  to  400°,  it  splits  up  into  phosphoric  acid  and  free 
phosphorus,  the  latter,  of  course,  taking  fire  in  case  oxygen  be 
present. 

286.  Hypophosphorous  Acid  (H6P204-=  2H3P02).  This 
compound  has  usually  been  classed  among  the  oxides  of  phos- 
phorus, on  the  supposition  that  it  might  be  possible  to  obtain 
from  it  an  anhydrous  oxide,  of  the  composition  P2O ;  the  oxide 
in  question  has,  however,  never  as  yet  been  obtained. 

When  ordinary  phosphorus  is   boiled  in  a  solution  of  caustic 


224  HYPOPHOSPHITES. 


••• 

potash,  soda,  lime,  or  baryta,  water  is  decomposed,  a  compound 
of  phosphorus  and  hydrogen  (§  278)  is  formed,  and  a  hypophos- 
phite  of  the  alkali  employed  remains  in  solution,  from  which  it 
may  be  separated  in  crystals  by  cautious  evaporation.  If  baryta 
be  employed,  the  reaction  may  be  formulated  as  follows:  — 

Empirical:  3BaH2O3  +  8P  +  6H,O  =  2PH3  -|-  3BaH4P2O4. 
Dualistic  :  3(BaO,H,O)  +  8P  +  6H2O  =  2PH3  -f  3(BaO,2lLO,P2O). 

By  cautiously  adding  sulphuric  acid  to  the  solution  of  the  barium 
salt,  sulphate  of  barium  is  precipitated  and  hypophosphorous 
acid  remains  in  solution:  — 

BaO,2H2O,P2O  +  H2O,S03  =  BaO,SO3  +  3II2O,P2O  . 

By  evaporating  the  aqueous  solution,  after  filtration,  hypo- 
phosphorous  acid  is  left  as  a  viscid,  uncrystallizable,  acid  liquid, 
which,  on  being  strongly  heated,  splits  up  into  phosphoric  acid 
and  phosphuretted  hydrogen.  It  unites  with  oxygen  readily, 
and  is  consequently  a  powerful  reducing  agent.  Sulphuric  acid, 
for  example,  is  reduced  by  it,  with  evolution  of  sulphurous  acid 
and  separation  of  sulphur. 

The  hypophosphites  are,  for  the  most  part,  crystallizable  salts, 
soluble  in  water  and  often  in  alcohol  also;  they  can  usually  be 
preserved  in  dry  air.  Several  of  them  have  recently  been  some- 
what extensively  employed  as  medicaments. 

287.  Phosphorous  Acid  (P2O8).  This  acid  is  a  product  of  the 
slow  combustion  of  phosphorus. 

When  phosphorus  is  gently  heated  in  a  very  slow  current  of  per- 
fectly dry  air,  it  takes  on  oxygen  enough  to  form  phosphorous  acid, 
which,  being  volatile,  condenses  upon  the  cold  walls  of  the  tube  beyond 
the  phosphorus  as  a  bulky  white  sublimate.  By  conducting  the  opera- 
tion in  a  tube  drawn  out  to  a  fine  point  at  one  end  and  almost  com- 
pletely closed  at  the  other  by  a  perforated  cork  carrying  a  narrow 
tube,  and  carefully  regulating  the  supply  of  air  which  is  admitted  into 
the  tube,  so  that  just  enough  oxygen  to  form  phosphorous  acid,  and  no 
more,  shall  come  in  contact  with  the  phosphorus,  a  tolerably  pure  prod- 
uct can  be  obtained.  For  purposes  of  illustration,  however,  a  simpler 
arrangement  of  the  apparatus  may  be  employed,  as  in  the  following 
experiment  :  — 

Exp.  123.  —  Place  a  bit  of  phosphorus,  as  big  as  a  pea,  in  the  mid- 
dle of  a  -piece  of  glass  tubing,  No.  2,  about  30  c.  m.  long,  and  open  at 


PHOSPHOROUS    ACID. 


both  extremities  ;  gently  heat  the  phosphorus  until  it  takes  fire,  and 
then  extinguish  the  lamp.  So  long  as  the  tube  is  held  in  a  horizontal 
position,  the  combustion  will  be  so  feeble  and  imperfect  that  some  red 
oxide  of  phosphorus  will  be  formed  as  well  as  phosphorous  acid.  On 
the  other  hand,  if  one  end  of  the  tube  be  inclined  upwards,  so  that  the 
products  of  combustion  can  pass  off  and  make  way  for  the  entrance  of 
fresh  air,  the  combustion  will  become  more  vivid,  and  there  will  be  pro- 
duced a  quantity  of  the  highest  oxide  of  phosphorus,  phosphoric  acid. 
If  the  tube  were  held  perpendicularly,  the  draught  of  air,  passing 
through  it  as  through  a  chimney,  would  be  so  powerful  that  all  the 
phosphorus  would  be  burned  completely  to  phosphoric  acid. 

It  is  evident,  from  the  foregoing,  that  if  it  were  only  possible  to  find 
out  the  precise  angle  at  which  the  tube  should  be  inclined,  and,  at  the 
same  time,  to  provide  means  for  continually  maintaining  a  suitable  tem- 
perature within  the  tube,  the  phosphorus  might  all  be  converted  into 
pure  phosphorous  acid,  instead  of  the  various  and  mixed  products 
which  are  actually  obtained. 

288.  Hydrated  phosphorous  acid,  H3PO3,  or  3H2O,P2QS,  is 
readily  obtained,  though  in  an  impure  condition,  by  exposing 
sticks  of  phosphorus  to  moist  air. 

Exp.  1  24.  —  Select  a  piece  of  glass  tubing,  the  diameter  of  which  is 
so  much  greater  than  that  of  an  ordinary  stick  of  phosphorus,  that  the 
latter  can  readily  be  slipped  into  it  ;  from  this  tubing  prepare  three  or 
four  short  tubes  3  or  4  c.  m.  long,  open  above  and  below,  but  drawn  in 
at  the  bottom  to  such  an  extent  that  a  stick  of  phosphorus  placed  in 
the  upper  part  of  the  tube  cannot  pass  the  narrowed  portion  and  fall 
out  of  the  tube.  In  each  of  these  short  tubes  put  a  stick  of  phos- 
phorus, and  place  them  all  in  a  glass  funnel  which  rests  upon  a  bottle 
standing  in  a  soup  plate  full  of  water  ;  over  the  funnel  and  bottle  place 
a  tall  tubulated  bell-jar,  from  which  the  stopper  has  been  removed, 
and  allow  the  apparatus  to  stand  at  rest  during  several  days  in  a  cool 
'place  where  no  damage  can  be  done  in  case  the  phosphorus  take  fire. 

Under  these  conditions,  the  phosphorus  will  slowly  oxidize  and  waste 
away,  —  if  time  enough  be  allowed  it  will  completely  disappear,  — 
and  the  mixture  of  phosphorous  and  phosphoric  acids  which  is  formed 
will  flow  down  through  the  tube  of  the  funnel  into  the  bottle  beneath. 
The  mixture  thus  obtained  is  often  technically  termed  phosphatic  acid: 

The  object  of  the  glass  tubes  employed  to  envelope  the  sticks  of 

phosphorus,  is  to  keep  the  several  pieces  of  phosphorus  from  touching 

one  another.     If  two  or  three  pieces  of  phosphorus  were  to  be  left  in 

contact,  in  the  air,  the  heat  generated  during  the  oxidation  of  each, 

15 


226  SPONTANEOUS    COMBUSTION. 

would  be  added  to  that  derived  from  the  others,  and,  after  a  time,  the 
mass  would  become  hot  enough  to  take  fire  spontaneously.  But  when 
each  stick  of  phosphorus  is  placed  within  a  glass  tube,  the  heat  gener- 
ated by  its  oxidation  passes  off  harmlessly,  and  a  dangerous  accumula- 
tion of  heat  is  very  much  less  likely  to  occur  than  if  no  such  system  of 
isolation  were  resorted  to. 

289.  The  fact  that  a  collection  of  fragments  of  phosphorus  is 
thus  liable  to  take  fire  so  well  illustrates  the  theory  of  spon- 
taneous combustion  in  general,  and  the  precautionary  measures, 
taken  in  the  foregoing  experiment  to  prevent  the  ignition  of  the 
phosphorus,  point  so  clearly  to  the  methods  which  must  often  be 
resorted  to,  in  order  to  prevent  the  spontaneous  inflammation  of 
many  highly  combustible  substances,  that  a  few  words  may  here 
be  appropriately  devoted  to  this  important  practical  subject. 

As  a  rule,  all  easily  oxidizable  substances,  when  finely  divided 
and  thrown  into  heaps,  are  liable  to  take  fire  spontaneously  in 
the  air.  Many  oils,  for  example,  particularly  the  so-called  dry- 
ing oils,  absorb  oxygen  from  the  air  and  enter  into  combination 
with  it.  Wherever  chemical  combination  occurs,  heat  is  devel- 
oped, and  in  case  the  oil  be  poured  upon  some  porous  substance 
which  is  both  combustible  and  a  non-conductor  of  heat,  like  wool 
or  cotton,  paper  or  cloth,  the  heat  developed  during  the  oxi- 
dation of  the  oil  may  very  readily  accumulate  to  the  extent 
necessary  to  produce  inflammation.  To  prevent  this  catastrophe, 
the  heap  of  greasy  wool  or  other  matter  should  be  broken  up  as 
soon  as  warmth  is  perceived  in  it,  and  its  particles  should  be 
scattered  about  so  that  air  may  have  free  access  to  them ;  the 
heat  will  then  pass  off  harmlessly  from  each  of  these  particles 
as  fast  as  it  is  generated. 

This  process  of  subdivision  will  prove  an  effectual  protection 
if  the  subdivision  be  carried  far  enough,  but  it  is  a  fact,  not  to 
be  lost  sight  of,  that  very  small  parcels  of  some  substances,  a 
hank  of  oiled  twine,  for  example,  or  a  handful  of  greasy  rags, 
may  take  fire  when  all  the  conditions  are  favorable,  and  it  is  a 
matter  of  the  first  importance  that  all  such  matters  should  be 
kept  in  places  where  no  harm  can  be  done  in  case  they  inflame. 

A  still  more  familiar  instance  of  the  accumulation  of  heat 
during  chemical  action  occurs  in  the  ordinary  process  of  hay- 


PROPERTIES    OF   PHOSPHOROUS    ACID.  227 

making,  as  when  a  cock  of  half-cured  hay  is  left  unopened  for 
any  length  of  time ;  the  green  hay  combines  with  oxygen  from 
the  air,  fermentation  sets  in,  and  heat  is,  of  course,  evolved ;  but 
when  the  hay  is  scattered  about  the  field,  this  heat  passes  off  into 
the  air  as  fast  as  it  is  generated,  and  we  cannot  perceive  it.  On 
the  other  hand,  if,  instead  of  the  usual  small  hay-cocks,  the  farmer 
were  to  throw  a  large  quantity  of  new-mown  hay  into  one  great 
stack,  this  stack  would  undoubtedly  take  fire  if  left  to  itself. 

In  large  heaps  of  many  kinds  of  bituminous  coal  also,  strong 
chemical  action  is  induced  under  very  various  conditions  as  re- 
gards temperature,  moisture,  and  mechanical  subdivision,  and 
the  heat  evolved  becomes  at  last  intense  enough  to  kindle  the 
coal.  Protection  from  the  weather,  exclusion  of  moisture,  free 
ventilation,  and  the  avoiding  of  too  large  heaps,  are  |he  most 
effectual  preventives  in  this  case. 

290.  Anhydrous  phosphorous  acid  is  a  white  amorphous  sub- 
stance, which  rapidly  absorbs  water  from  the  ^ir,  and  when 
sprinkled  with  water,  dissolves  rapidly  with  a  hissing  noise ;  it  is 
volatile,  and  may  easily  be  driven  from  one  place  to  another,  in 
a  tube  filled  with  nitrogen  (see  §  287),  by  applying  a  gentle  heat. 
Being  a  product  of  the  incomplete  combustion  of  phosphorus,  it 
is  necessarily  combustible  ;  when  heated  in  the  air,  it  undergoes 
vivid  combustion. 

Hydrated  phosphorous  acid  is  obtained  in  the  form  of  rec- 
tangular prisms,  when  the  aqueous  solution  is  evaporated  at 
temperatures  not  exceeding  200°.  The  crystals  are  deliquescent, 
and  they  gradually  absorb  oxygen  from  the  air ;  when  strongly 
heated,  they  are  dec6mposed  into  phosphoric  acid  and  phosphu- 
retted  hydrogen,  and  at  the  same  time  take  fire.  The  aqueous 
solution  of  phosphorous  acid,  exhibits  a  strong  acid  reaction ;  by 
absorbing  oxygen  from  the  air,  it  is  converted  into  phosphoric 
acid,  quickly  in  case  the  solution  is  dilute,  but  slowly,  if  it  be 
concentrated.  It  is  a  powerful  reducing  agent;  when  heated 
with  sulphurous  acid,  it  yields  phosphoric  and  sulphydric  acids. 
Though  a  very  weak  acid,  it  forms  salts  by  combining  with  those 
metallic  oxides  upon  which  it  exerts  no  reducing  action ;  the 
phosphites  of  the  alkalies  proper  are  easily  soluble  in  water,  but 
the  phosphites  of  calcium  and  barium  can  only  be  dissolved  with 


228  PHOSPHORIC    ACID. 

difficulty.     These  salts  are  more  stable  than  the  hypophosphites, 
but  are  all  decomposed  by  heat. 

291.  Phosphoric  Acid  (P2O5).  As  has  been  already  stated, 
this  highest  oxide  of  phosphorus  is  the  product  of  the  rapid 
combustion  of  phosphorus  in  an  excess  of  air  or  oxygen. 

Ex  p.  125.  —  Dry  thoroughly  a  large  porcelain  plate,  a  small  porce- 
lain capsule,  and  a  wide-mouthed  bottle  of  two  litres  capacity,  by 
warming  them  at  a  fire  ;  place  the  capsule  upon  the  plate  and  put  in 
the  capsule  a  bit  of  dry  phosphorus  of  the  weight  of  half  a  gramme 
or  thereabouts;  light  the  phosphorus,  and  cover  it  at  once  with  the  in- 
verted bottle.  The  phosphoric  acid,  formed  by  the  combustion  of  the 
phosphorus,  will  be  deposited  as  a  white  powder,  like  flakes  of  snow, 
upon  the  sides  of  the  bottle,  and  much  of  it  will  fall  down  upon  the 
plate  below. 

The  apparatus  employed  in  this  experiment  can  readily  be  arranged 
in  such  manner  that  fresh  portions  of  phosphorus  and  of  air  can  be 
introduced  into  the  bottle  as  fast  as  occasion  may  require  ;  the  process 
will  then  be  continuous,  and  any  desired  quantity  of  phosphoric  acid 
may  be  prepared  by  means  of  it. 

The  flocculent  amorphous,  odorless  powder,  thus  obtained, 
unites  with  water  with  remarkable  facility ;  if  it  be  left  in  the 
air  for  a  few  minutes,  it  deliquesces  completely,  and  upon  being 
thrown  into  water,  it  dissolves  with  a  hissing  noise  and  develop- 
ment of  much  heat.  In  order  to  preserve  it,  it  must  be  placed 
in  a  dry  tube,  and  the  tube  closed  by  sealing  it  in  the  lamp. 
When  touched  to  the  moist  tongue,  it  burns  as  if  it  we«re  red-hot 
metal.  On  account  of  this  strong  affinity  for  water,  it  is  fre- 
quently employed  by  chemists  to  withdraw  the  elements  of  water 
from  other  substances:  anhydrous  sulphuric  acid,  for  example, 
can  be  prepared  from  oil  of  vitriol  by  heating  the  latter  with 
anhydrous  phosphoric  acid.  On  being  heated  with  various  or- 
ganic substances,  such  as  some  of  the  alcohols  and  essential  oils, 
composed  of  carbon,  hydrogen,  and  oxygen,  it  decomposes  them 
in  such  manner,  that  the  oxygen  of  the  organic  substance  and  as 
much  of  its  hydrogen  as  is  necessary  to  form  water  by  uniting 
with  this  oxygen,  combine  with  the  phosphoric  acid,  while  a 
compound  of  carbon  and  hydrogen  (technically  called  a  hydro- 
carbon) is  set  free. 

After  the  anhydrous  acid  has  once  been  dissolved  in  water,  it 


HYDRATES    OF    PHOSPHORIC   ACID.  229 

cannot  again  be  completely  deprived  of  water  by  mere  evapora- 
tion or  ignition.  When  the  aqueous  solution  is  evaporated,  there 
is-  left,  not  the  anhydrous  powder,  but  a  transparent  glassy  mass, 
which  is  a  hydrate,  of  the  composition  H20,P2O5 .  This  hydrate 
is  often  called  glacial  phosphoric  acid.  It  is  extremely  deliques- 
cent, and,  at  a  bright  red  heat,  sublimes  in  dense  white  fumes. 
Besides  the  monohydrate,  there  are  two  other  hydrates  of  phos- 
phoric acid,  of  the  composition,  respectively,  2H2O,P2O5  and 
3H26,P2O5 .  Of  these,  the  terhydrate  is,  perhaps,  the  most  im- 
portant ;  it  is  the  substance  usually  meant  when  phosphoric  acid 
is  spoken  of. 

292.  Phosphoric  acid  can  be  prepared,  also,  by  the  oxidation 
of  phosphorus,  or  of  hypophosphorous .  or  phosphorous  acids,  or 
by  the  decomposition  of  some  one  of  its  salts,  such  as  the  phos- 
phate of  calcium  (bone-earth). 

When  phosphorus  is  heated  in  dilute  nitric  acid,  of  1.2  specific  grav- 
ity, the  nitric  acid  gives  up  oxygen  to  the  phosphorus,  nitric  oxide  and 
phosphoric  acid  are  formed,  and  the  latter  goes  into  solution.  When 
the  phosphorus  lias  all  disappeared,  the  solution  is  evaporated  to  dry- 
ness,  in  order  to  drive  off  the  nitric  acid,  which  was  employed  in  excess, 
and  there  is  obtained  a  quantity  of  the  monohydrated  glacial  acid. 

A  product  less  pure  than  the  acid  prepared  by  means  of  nitric  acid, 
is  obtained  by  neutralizing  a  solution  of  monophosphate  of  calcium, 
prepared  from  bones  in  the  manner  already  described  when  treating 
of  the  preparation  of  phosphorus  (see  §  270),  with  carbonate  of  am- 
monium, evaporating  the  filtered  solution  of  phosphate  of  ammonium 
to  dryness  and  heating  the  residue  to  low  redness.  Ammonia  is  ex- 
pelled and  glacial  phosphoric  acid  remains. 

293.  It  is  a  remarkable  fact  that  each  of  the  three  different 
hydrates  of  phosphoric  acid  possesses  properties  peculiar  to  itself, 
and  unlike  those  of  the  other  two ;  in  fact,  each  of  the  hydrates 
must  be  regarded  as  a  distinct   acid.     The  monohydrated,  or 
glacial  acid,  H2O,P2O5 ,  is   usually  called  metaphosphoric  acid  ; 
the  bihydrate,   2H20,P205,  is   called  pyrophosphoric  acid,  and 
the  terhydrate,  3H2O,P2O5,  is  commonly  spoken  of  as  ordinary 
phosphoric  acid,  or  simply  as  phosphoric  acid.     The  three  hy- 
drates are  sometimes  distinguished  as  a  phosphoric  acid  (meta), 
I  phosphoric    acid  (pyro),    and    c  phosphoric    acid    (ordinary). 
These  different  hydrates  of  the  acid  retain  their  peculiar  char- 


23U  META-   AND    PYROPHOSPHORIC    ACID. 

acteristics  for  a  considerable  time,  when  dissolved  in  water, 
though  the  mono-  and  bihydrates  change,  after  a  while,  to  the 
terhydrate,  and  in  combining  with  metallic  oxides  to  form  salts, 
they  unite  with  1,  2,  or  3  molecules  of  the  oxide,  accordingly  as 
they  themselves  contain  the  elements  of  1,  2,  or  3  molecules  of 
water.  There  are  thus  formed  three  distinct  series  of  salts,  each 
of  which  corresponds  to  one  of  the  hydrates,  as  is  seen  in  the 
following  formulae,  where  M  stands  for  any  metal  which  habitu- 
ally replaces  one  atom  of  hydrogen. 

Monohydrated  Acid.  Bihydrated  Acid.  Terliydrated  Acid. 

H2O,P2O5  2H2O,P2O5  3H,O,P2O5 

M26,P2O5  2M2O,P2O5  3M3O,P2O6 

Metaphosphate  of  M.          Pyroplwsphate  of  M.  Phosphate  of  M. 

This  behavior  is  very  different  from  that  of  the  hydrates  of  nitric 
or  of  sulphuric  acid ;  when  either  of  the  hydrates  of  nitric  acid, 
for  example,  is  made  to  combine  with  a  base,  like  soda,  there  is 
formed  always  one  and  the  same  salt,  nitrate  of  sodium.  In 
each  of  the  three  series  of  salts  formed  by  phosphoric  acid, 
the  acid  exhibits  peculiar  properties.  A  salt  of  the  formula 
3M2O,P2O5  will  behave  very  differently  towards  many  reagents 
from  a  salt  containing  the  same  metal,  but  in  the  proportions 
M2O,P2O5 ,  or  2M20,P2O5  .•  As  an  example  of  the  kind  of  dif- 
ferences here  alluded  to,  it  may  be  mentioned,  that  while  meta- 
phosphoric  acid,  on  being  added  to  a  solution  of  albumen,  will 
cause  the  albumen  to  coagulate,  no  such  coagulation  can  be 
brought  about  by  either  pyrophosphoric  acid  or  the  ordinary  ter- 
hydrate. Metaphosphoric  acid  gives  a  white  precipitate  when 
its  solution  is  mixed  with  a  solution  of  nitrate  of  silver,  but  no 
precipitate  is  produced  in  a  solution  of  nitrate  of  silver  by  either 
of  the  other  hydrates,  unless  they  are  first  neutralized  with  an 
alkali,  in  which  event  a  white  precipitate  is  produced  by  pyro- 
phosphoric acid,  and  a  yellow  precipitate  by  the  ordinary  acid. 
These  peculiarities  will  be  examined  in  detail  when  we  come  to 
treat  of  the  phosphates  of  sodium  in  the  chapter  upon  sodium 
and  its  compounds. 

From  the  formulae  given  in  the  above  table,  it  is  apparent  that 
metaphosphoric  acid  is  a  monobasic  acid,  that  pyrophosphoric  acid 
is  bibasic,  and  that  ordinary  phosphoric  acid  is  terbasic.  Since 


PROPERTIES    OF    PHOSPHORIC    ACID.  231 

each  of  the  atoms  of  M,  in  either  of  the  formulae,  can  be  re- 
placed by  an  equivalent  atom  of  any  other  metal,  or  by  hydrogen, 
it  follows  that  the  composition  of  some  of  the  salts  of  phosphor- 
ic acid  is  rather  complex,  thus  there  is  a  phosphate  of  potas- 
sium and  sodium  of  the  composition  (K2O,Na20,H2O)P205 ,  or 
KNaHP04 . 

Although  we  have  here  studied  the  acids  which  contain  phos- 
phorus, oxygen,  and  hydrogen,  as  if  they  were  in  reality  composed 
of  water  and  the  anhydrous  oxide  of  phosphorus,  as  the  manner  of 
their  derivation  would  suggest,  yet  it  must  not  be  forgotten  that 
we  have  absolutely  no  knowledge  of  the  actual  structure  of  the 
molecules  of  these  compounds,  and  that  the  empirical  formulae 
HPO8,  H4P2O7,  and  H3PO4,  express  all  that  is  absolutely 
known  of  their  composition. 

294.  In  whichever  way  prepared,  and  in  all  its  varieties, 
phosphoric  acid  is  a  very  strong  acid.  Although  a  less  powerful 
agent,  at  the  ordinary  temperature,  than  sulphuric  acid,  yet,  from 
being  much  less  volatile  than  sulphuric  acid,  it  can  expel  the 
latter,  and  most  other  acids,  from  their  compounds  on  being 
heated  with  them.  The  behavior  of  the  two  acids  towards  cal- 
cium, or  its  oxide,  furnishes  an  instructive  example  of  the  influence 
of  extrinsic  or  physical  circumstances  upon  the,  play  of  the  chemi- 
cal force.  When  triphosphate  of  calcium  (bone-earth)  is  treated 
with  dilute  sulphuric  acid  at  the  ordinary  temperature,  a  quan- 
tity of  phosphoric  acid  is  set  free  from  the  calcium  and  goes  into 
solution.  From  this  result  it  might,  at  first  sight,  be  thought 
that  the  calcium  was  removed  from  the  phosphate  of  calcium 
simply  perforce  of  the  superior  chemical  power  of  sulphuric,  as 
contrasted  with  phosphoric  acid,  but,  m  reality,  the  water  which 
is  present  plays  an  important  part  in  the  reaction.  Monophos- 
phate  of  calcium  is  readily  soluble  in  water,  sulphate  of  calcium, 
on  the  other  hand,  being  well-nigh  insoluble.  Hence  it  happens 
that  when  triphosphate  of  calcium  is  digested  in  dilute  sulphuric 
acid,  monophosphate  of  calcium  goes  into  solution,  while  sulphate 
of  calcium  is  deposited  as  an  insoluble  powder.  But  if  the  mix- 
ture of  solid  sulphate  of  calcium  and  of  dissolved  monophosphate 
of  calcium,  thus  obtained,  be  evaporated  to  dryness  and  the 
residue  be  strongly  heated,  all  the  sulphuric  acid  will  be  expelled 


232  TERCHLORIDE    OF    PHOSPHORUS. 

from  the  calcium ;  it  will  evaporate  and  pass  off  into  the  air,  and 
nothing  will  finally  be  left  in  the  vessel  but  triphosphate  of  cal- 
cium, precisely  similar  in  quality  and  quantity  to  that  with  which 
the  experiment  started.  In  the  same  way,  if  a  mixture  of  sul- 
phate of  calcium  and  glacial  phosphoric  acid  be  strongly  heated, 
the  sulphuric  acid,  being  readily  volatile,  as  compared  with  phos- 
phoric acid,  will  all  be  expelled  from  its  combination  with  the 
calcium:  — 

3CaS04  +  PA  =  Ca.P208  +  3S03 .  % 

9 

295.  Chlorides    of  Phosphorus.      Phosphorus    and    chlorine 
unite  readily  and  directly  even  at  temperatures  as  low  as  0°, 
the  act  of  combination  being  attended  with  evolution  of  light  and 
heat.     If  the  chlorine  be  in  excess,  as  regards  the  phosphorus, 
there  will  be  formed  a  solid  quinquichloride  of  phosphorus,  while, 
if  an  excess  of  phosphorus  be  present,  a  liquid  terchloride  of 
phosphorus  will  be  obtained. 

296.  Terchloride  of  Phosphorus  (PC13)  is  a  colorless  liquid 
of  about  1.5  specific  gravity,  which  boils  at  about  75°.     It  fumes 
in  the  air,  and  is  decomposed  by  moist  air.     When  heated  in  the 
flame  of  the  gas-lamp,  it  takes  fire  and  burns  with  a  bright  light. 
When  mixed  with  water  it  decomposes,  yielding  chlorhydric  and 
phosphorous  acids :  — 

2PC13+6H20  =6HC1+3H20,P203,  or,  PC13  +  3H2O=3HCl-fH3PO3 . 

This  reaction  is  particularly  interesting,  in  view  of  the  fact  that 
by  means  of  it  we  are  enabled  to  obtain  phosphorous  acid  in  a 
condition  of  purity.  It  will  be  remembered  that  by  the  method 
of  direct  oxidation  (§  287),  it  is  no  easy  matter  to  obtain  pure 
phosphorous  acid  from  phosphorus.  But  by  simply  treating  ter- 
chloride of  phosphorus  with  water  and  evaporating  the  solution, 
so  that  the  chlorhydric  acid  which  results  from  the  reaction  may 
be  expelled,  hydrated  phosphorous  acid  is  obtained  as  the  sole 
product.  Indirect  methods,  such  as  this,  are  frequently  employed 
by  -the  chemist  with  great  advantage. 

Terchloride  of  phosphorus  can  be  prepared  by  passing  a  slow 
stream  of  dry  chlorine  through  melted,  almost  boiling,  phospho- 
rus contained  in  a  tubulated  retort  which  has  previously  been 
filled  with  chlorine  in  the  cold,  and  condensing  the  chloride  in 


QUINQUICHLORIDE    OF   PHOSPHORUS.  233 

an  appropriate  receiver  as  fast  as  it  distils  over.  The  process, 
like  all  operations  with  phosphorus,  requires  special  care. 

297.  It  will  be  observed  that  the  formula  of  terchloride  of 
phosphorus  is  that  of  phosphuretted  hydrogen,  in  which  all  the 
hydrogen  has  been  replaced  by  chlorine.     The  two  substances 
have  a  perfectly  similar  volumetric  composition.     In  phosphu- 
retted hydrogen,  three  volumes  of  hydrogen  in  combination  with 
half  a  volume  of  phosphorus,  produce  two  volumes  of  the  com- 
pound gas  ;  if,  in  terchloride  of  phosphorus, 

Half  a  unit-volume  of  phosphorus- vapor,  weighing  .  .  31.00 
And  3  unit-volumes  of  chlorine,  weighing  (35.5  X  3)  .  106.50 

Produce  2  volumes  of  PCL,  vapor,  weighing  ....  137.50 
One  unit- volume  of  PC13  vapor,  should  weigh  .  .  .  68.75 

The  specific  gravity  of  the  vapor  of  terchloride  of  phosphorus 
has  been  found,  by  experiment,  to  be  69.12,  —  a  number  so 
nearly  identical  with  the  above  result  of  calculation  as  entirely 
to  confirm  the  assumption  on  which  the  calculation  rests,  viz., 
that  in  'terchloride  of  phosphorus  three  volumes  of  chlorine  are 
united  with  half  a  volume  of  phosphorus-vapor. 

298.  Quinquichloride    of  Phosphorus   (PC15)   is  a  white    or 
straw-colored  crystalline  solid,  which  volatilizes  at  a  temperature 
below  100°   without  previously  fusing,  but  when   subjected  to 
pressure,  it  melts  at  148°,  and  boils  at  a  temperature  somewhat 
higher.     It  burns  in  the  flame  of  a  candle  with  production  of 
phosphoric  acid  and  evolution  of  chlorine.     It  is  very  deliques- 
cent, and  is  decomposed  by  the  moisture  of  the  air ;  by  a  large 
excess  of  water  it  is  immediately  resolved  into  chlorhydric  and 
phosphoric  acids :  — 

PC15  -f  4H20  =  5HC1  +  H3PO4 ; 

with  a  smaller  quantity  of  water  it  yields  chlorhydric  acid  and 
oxychloride  of  phosphorus  ;  — 

PC15  -f-  H2O  =  2HC1  +  POC13 . 

Sulphydric  acid  decomposes  it  in  like  manner,  with  production  of 
chlorhydric  acid  and  sulphochloride  of  phosphorus :  — 

PC15  +  H2S  =  2HC1  +  PSC13 . 


234  QUINQUICHLORIDE    OF   PHOSPHORUS. 

Quinquichloride  of  phosphorus  reacts  upon  many  organic  com- 
pounds also,  with  formation  of  very  interesting  products,  and  is 
hence  an  important  agent  of  research  in  the  department  of 
organic  chemistry. 

299.  In  order  to  prepare   quinquichloride   of  phosphorus,  a 
current  of  dry  chlorine  may  be  passed  into  terchloride  of  phos- 
phorus  until   the    latter    has    been    completely   solidified ;    the 
product  is  then  distilled  in  a  current  of  chlorine.     The  quinqui- 
chloride may  be  obtained  directly  from  phosphorus  in  one  opera- 
tion, if  a  rapid  stream  of  chlorine  be  conducted  into  a  retort 
containing  phosphorus,  kept  so  cool  that  the  terchloride  of  phos- 
phorus at  first  produced  shall  not  distil  over.     Again,  if  powdered 
red  phosphorus  is  exposed  to  the  action   of  a  rapid  stream  of 
chlorine,  it  will  all  be  quickly  converted  into  the  solid  quinqui- 
chloride. 

300.  The  formula  above  given  for  quinquichloride  of  phos- 
phorus represents  the  following  ^composition  by  volume  :  — 

Half  a  unit-volume  of  phosphorus- vapor,  weighing  .  .  31.00 
And  5  unit-volumes  of  chlorine,  weighing  (35.5  X  5)  •  177.50 

Should  produce  2  vols  of  PC15  vapor,  weighing  .  .  N  .  208.50 
One  unit-volume  of  PC15  vapor  ought  then  to  weigh  .  .  104.25 

The  specific  gravity  of  the  supposed  vapor  of  quinquichloride  of 
phosphorus,  as  determined  by  experiment,  does  not  accord  with 
this  calculated  result;  it  is  52.81,  almost  exactly  one-half  of  the 
theoretical  unit-volume  weight  above  given.  If  four  volumes  of 
vapor,  instead  of  two,  resulted  from  the  union  of  half  a  volume 
of  phosphorus-vapor  with  five  volumes  of  chlorine,  the  calcu- 
lated and  the  actual  vapor-density  would  coincide.  But  hitherto 
we  have  never  found  a  single  compound  gas  in  which  the  prod- 
uct-volume was  four  unit-volumes ;  two  unit-volumes  have  in- 
variably resulted  from  the  union  of  the  constituent  volumes, 
whatever  the  character  and  number  of  the  constituents.  It 
would  be  necessary  to  admit  this  substance  as  presenting  an  ex- 
ceptional volumetric  composition,  were  it  not  for  a  well-founded 
distrust  of  the  experimental  determination  of  the  vapor-density 
of  this  compound.  It  not  infrequently  happens  that  all  attempts 
to  determine  the  vapor-density  of  volatile  compounds  of  two  or 


DISSOCIATION.  235 

more  elements  are  baffled  by.  their  splitting  up,  at  the  tempera- 
ture of  vaporization,  into  their  constituent  gases  or  vapors,  which, 
in  the  act  of  separating,  resume  their  own  proper  volumes,  how- 
ever much  they  may  have  been  condensed  during  combination. 
This  splitting  up  of  compound  vapors,  at  high  temperatures, 
into  less  complex  compounds,  or  into  the  elementary  constitu- 
ents, is  termed  dissociation.  Thus,  at  the  elevated  temperature 
necessary. to  convert  quinquichloride  of  phosphorus  into  vapor, 
it  is  probable  that  the  quinquichloride  splits  into  terchloride 
of  phosphorus  and  free  chlorine,  and  that  it  is  the  specific 
gravity  of  this  mixture  which  has  been  determined,  instead  of 
the  specific  gravity  of  the  real  unaltered  vapor  of  the  quinqui- 
chloride. 

Two  unit- volumes  of  PC13  weigh 138.24 

Two  unit- volumes  of  Cl  weigh 71. 

Four  unit-volumes  of  the  mixture  weigh  ....  209.2-4 
One  unit- volume  of  the  mixture  weighs  .  .  .  .  52.31 

The  specific  gravity  which  has  been  assigned  to  the  quinquichlo- 
ride is  52.81,  —  a  number  very  nearly  coincident  with  the  above 
calculated  density  of  the  mixture  of  terchloride-vapor  and  free 
chlorine.  At  the  high  temperature  of  vaporization  it  is,  there- 
fore, probable  that  quinquichloride  of  phosphorus  undergoes 
dissociation  into  terchloride  of  phosphorus  and  chlorine,  but  if 
this  be  the  case,  these  constituents  recombine  when  the  tempera- 
ture falls,  for  by  lowering  the  temperature  the  quinquichloride  is 
recovered. 

As  we  advance,  we  shall  meet  with  several  other  examples  of 
the  dissociation  of  compound  gases  and  vapors ;  for  the  present, 
it  will  be  sufficient  to  give  one  more  illustration  of  the  meaning 
of  this  term.  When  equal  volumes  of  dry  ammonia  and  dry 
chlorhydric  acid  gas  are  mixed  (Exp.  68),  the  two  gases  are 
completely  condensed  to  a  white  solid,  which  we  are  familiar 
with  as  chloride  of  ammonium.  Since  this  ammonium-salt  is 
readily  volatilizable,  there  would  be  no  difficulty  in  determining 
the  product-volume  of  the  compound  of  ammonia  and  chlorhy- 
dric acid,  were  it  not  for  the  fact  that  the  vapor  of  chloride  of 
ammonium  undergoes  dissociation  at  the  temperature  of  vaporiza- 


236 


BROMIDES    AND    IODIDES    OF    PHOSPHORUS. 


tion.     If  the  real  vapor  of  the  compound  could  be  measured,  the 
facts  would  undoubtedly  be  correctly  represented  by  the  diagram, 


but  the  vapor  of  the  compound  is  resolved  into  its  constituent 
gases  at  the  high  temperature  necessarily  employed,  so  that  the 
following  diagram  really  figures  the  actual  state  of  things :  — 


NIL 


When  the  dissociated  vapor  co'ols,  the  parted  gases  recombine  to 
form  solid  chloride  of  ammonium. 

It  is  obvious  that  the  phenomena  of  dissociation  interfere 
fatally  with  one  of  the  common  methods  of  arriving  at  the 
weight  and  structure  of  the  molecule  of  a  volatile  compound; 
the  indirect  method  of  getting  at  the  volumetric  composition  of 
a  substance  from  its  ponderal  composition  and  the  specific  grav- 
ity of  its  vapor  becomes  impracticable,  whenever  the  vapor  of 
the  compound  under  examination  is  liable  to  dissociation,  inas- 
much as  experiment  cannot  determine  beyond  a  doubt  the  real 
vapor-density  of  such  a  body. 

301,  Bromides  of  Phosphorus.     When  a  piece  of  phosphorus 
is  dropped  into  bromine,  the  two  elements  combine  with  explo- 
sive violence,  the  burning  phosphorus  being  thrown  about  in  a 
highly  dangerous  manner.     There  are  two  bromides  of  phos- 
phorus, PBr3  and  PBr5 ,   corresponding  to  the  two  chlorides. 
The  terbromide  is  liquid  at  ordinary  temperatures  and  the  quin- 
quibromide  solid. 

302.  Iodides  of  Phosphorus.     Iodine   and  phosphorus  unite 
directly,  when  brought  in  contact  with  one  another,  and  so  much 
heat  is  developed  by  their  union,  that  a  portion  of  the  phosphorus 
will  take  fire  if  the  mixture  be  in  contact  with  the  air.     There 
are  two  iodides  of  phosphorus,  both  of  them  solid  at  the  ordi- 
nary  temperature;   their   composition   is  respectively  PI2  and 
PI3 .     It  will  be  noticed  that,  while  the  teriodide  corresponds  to 


SULPHIDES    OF   PHOSPHORUS.  237 

the  terchloride  and  terbromide,  the  other  compound  is  a  biniodide, 
of  which  there  is  known  neither  a  bromine  nor  a  chlorine  ana- 
logue. The  fact  is  interesting,  as  illustrating  the  general  truth 
that,  when  in  any  group  or  family  of  elements,  we  compare  the 
behavior  of  its  several  members,  analogy  ceases  to  be  a  sure 
guide,  in  proportion  as  the  individuals  compared  are  more  widely 
separated  in  the  natural  series.  Chlorine  and  bromine  stand 
next  to  one  another  in  the  family  or  series  of  elements  to  which 
they  belong,  and  as  we  have  just  seen,  their  behavior,  as  regards 
phosphorus,  is  well-nigh  identical ;  but  iodine,  one  step  further 
removed  from  chlorine  than  bromine  is,  enters  into  new  combina- 
tions not  altogether  conformable  to  those  of  chlorine. 

303.  Sulphides  of  Phosphorus.  There  is  a  definite  sulphide 
of  phosphorus  corresponding  to  each  of  the  oxides,  and  in  addi- 
tion to  these  there  are  two  other  compounds,  which  may  be  repre- 
sented by  the  formulae  P4S3  and  P2S12  •  Sulphur  and  phosphorus 
may  also  be  melted  together  in  any  proportion.  The  sulphides 
of  phosphorus  are  exceedingly  inflammable,  taking  fire  even 
more  readily  than  phosphorus  itself,  and  they  are  all  more  readily 
fusible  than  either  of  the  two  elements  of  which  they  are  com- 
posed. They  may  be  prepared  by  heating  sulphur  under  water 
in  contact  with  melted  phosphorus.  The  union  of  the  two  ele- 
ments is  attended  with  development  of  much  heat,  and  sometimes 
with  dangerous  explosions.  It  is  well,  therefore,  to  operate  only 
upon  small  quantities,  and  to  add  the  sulphur  gradually  to  the 
phosphorus. 


CHAPTER    XVII. 

ARSENIC. 

304  Compounds  of  arsenic  have  been  known  from  very  early 
times.  The  element  is  sometimes  found  native,  but  much  more 
frequently  associated  with  other  metals  and  with  oxygen  and 
sulphur.  The  metals  in  connection  with  which  it  most  commonly 


238  PROPERTIES  OF  ARSENIC. 

occurs  are  iron,  cobalt,  nickel,  and  copper.  Ferruginous  ores 
and  deposits,  in  particular,  are  rarely  free  from  traces  of  arsenic. 
In  small  quantity,  arsenic  is  very  widely  distributed. 

The  greater  part  of  the  arsenic  of  commerce  is  prepared  from 
a  native  arsenide  and  sulphide  of  iron  (arsenical  pyrites)  corre- 
sponding to  the  formula  FeAsS  ,  and  from  the  arsenides  of  nickel 
and  cobalt.  Metallic  arsenic  is  obtained  directly  from  the  min- 
eral of  the  formula  FeAsS  by  heating  it  in  earthen  tubes  laid 
horizontally  in  a  long  furnace ;  a  tube,  made  by  rolling  up  a 
piece  of  thin  sheet  iron,  is  inserted  in  the  mouth  of  each  earthen 
retort,  and  an  earthen  receiver  is  luted  on  to  this  iron  tube. 
The  arsenic  condenses  principally  in  the  iron  tube,  in  the  form 
of  a  compact,  whitish,  crystalline  mass,  which  is  detached,  when 
cold,  by  unrolling  the  sheet-iron.  The  metal  is  also  indirectly 
obtained  by  reducing  the  arsenious  acid  (As203),  which  results 
from  roasting  (heating  in  a  current  of  air),  arsenides,  like  those 
of  cobalt  and  nickel ;  this  oxide  is  heated  with  charcoal  in  earthen 
crucibles,  covered  with  conical  iron  caps  or  inverted  crucibles, 
into  which  the  reduced  metal  sublimes.  The  metal  obtained  by 
this  second  process  is  gray  and  pulverulent,  instead  of  whitish 
and  coherent. 

^FeAsS  =  FeS  +  As ;  As3O8  +  3C  =  2As  +  SCO . 

305.  Arsenic  is  a  brittle  solid,  of  a  steel  gray  color  and  a  me- 
tallic lustre.  Its  specific  gravity  has  been  variously  given  at 
from  5.62  to  5.96.  Like  the  metals,  it  is  a  good  conductor  of 
electricity.  It  crystallizes  in  acute  rhombohedrons,  and  in  octo- 
hedrons  also,  thus  taking  on  forms  of  both  the  monometric  and 
hexagonal  systems,  as  do  phosphorus,  the  preceding  member,  and 
antimony,  the  succeeding  member  of  this  family.  At  a  dull  red 
heat  it  volatilizes  without  previous  fusion ;  the  vapor  is  colorless, 
and  possesses  a  characteristic  odor  resembling  that  of  garlic. 
The  specific  gravity  of  this  vapor  is  150,  while  the  atomic 
weight  of  the  element  is  75 ;  arsenic,  therefore,  resembles  phos- 
phorus, and  differs  from  all  the  other  elements  heretofore  studied, 
in  that  its  atomic  weight  is  not  identical  with  its  unit-volume 
weight ;  two  combining  proportions  by  weight  of  arsenic  occupy 
the  same  volume  as  one  combining  proportion  of  hydrogen ;  its 


ARSENIC    AND    HYDROGEN.  239 

symbol,  As,  represents  its  atomic  weight,  but  only  half  the  weight 
of  the  unit-volume  of  its  vapor.  At  the  ordinary  temperature 
the  compact  metal  does  not  tarnish  by  exposure  to  dry  air,  but  a 
moistened  powder  of  arsenic  is  slowly  converted  by  the  air  into 
a  mixture  of  arsenious  acid  and  metallic  arsenic.  At  a  red  heat 
the  metal  burns  w^th  a  whitish  flame,  producing  a  white  smoke  of 
arsenious  acid.  When  thrown,  in  fine  powder,  into  chlorine  gas, 
it  takes  fire  spontaneously  and  is  converted  into  chloride  of 
arsenic  (AsCl3).  Bromine,  iodine,  and  sulphur  also  combine 
readily  with  arsenic,  when  aided  by  a  gentle  heat.  Nitric  acid 
and  aqua  regia  convert  the  metal  into  arsenic  acid  (A?2O5)  ; 
chlorhydric  acid  has  little  action  upon  it.  Dilute  sulphuric  acid 
has  no  action  upon  the  metal,  but  the  concentrated  acid  has  the 
same  effect  upon  arsenic  as  upon  phosphorus  (§  276),  —  arsenious 
acid  is  formed  and  sulphurous  acid  escapes  :  — 

3H2SO4  +  2As  =  As2O3  +  3S02  +  3H2O . 

Some  fatty  oils  dissolve  arsenic,  to  a  slight  extent,  as  they  do 
phosphorus.  Metallic  arsenic  unites  by  fusion  with  most  metals, 
forming  alloys  which  the  arsenic  tends  to  make  hard  or  brittle. 
In  the  manufacture  of  shot  a  little  arsenic  is  added  to  the  lead 
to  facilitate  the  formation  of  regular  globules. 

306.  Arsenic  and  Hydrogen.     Arsenic  forms  two  combinations 
with  hydrogen ;  one  of  these  is  an  unstable,  brown  solid  of  un- 
certain composition ;  the  other  is  a  well-known  gas  whose  con- 
stitution is   represented  by  the  formula  AsH3 ,   and  which   is, 
therefore,  analogous  in  composition  to  ammonia  (NH3)  and  phos- 
phuretted  hydrogen  (PH3).     The  solid  hydride  is  so  obscure  a 
substance  that  nothing  need  here  be  said  of  it,  except  that  it  is" 
supposed  to  contain  two  atoms  of  hydrogen  and  one  of  arsenic 
(AsH2?). 

307.  Arseniuretted  Hydrogen.     This  very  dangerous  gas  may 
be  prepared  in  an  impure  state  by  decomposing  with  sulphuric 
acid  diluted  with  three  parts  of  water,  an   arsenide  of  zinc  ob- 
tained by  fusing  together  equal  weights  of  powdered  arsenic  and 
granulated  zinc :  — 

3H2SO4  +  Zn3As  =  3ZnHSO,  +  AsH3 . 
As  it  is  not  possible  to  prepare  the  precise  alloy  Zn3As ,  the 


240  ARSENIURETTED    HYDROGEN. 

arseniuretted  hydrogen  thus  obtained  is  always  mixed  with  hy- 
drogen. The  arsenide  of  sodium  can  be  decomposed  by  water 
with  evolution  of  arseniuretted  hydrogen  :  — 

3H2O  +  Na8As  =  3NaHO  +  AsH8. 

The  same  remark,  however,  applies  to  this  reaction  as  to  the  pre- 
ceding one ;  the  product  is  contaminated  with  an  indeterminate 
quantity  of  free  hydrogen.  Arseniuretted  hydrogen  seems  also 
to  be  formed  whenever  the  oxides  of  arsenic,  or  compounds  of 
these  oxides,  are  brought  in  contact  with  nascent  hydrogen.  A 
mixture  of  arseniuretted  hydrogen  and  hydrogen  may  be  readily 
obtained  by  acting  upon  zinc  by  dilute  chlorhydric  or  sulphuric 
acid  in  which  arsenious  acid  has  been  dissolved. 

308.  Arseniuretted  hydrogen  is  a  colorless  gas,  having  a  fetid 
odor ;  even  when  very  much  diluted  with  air,  it  is  intensely 
poisonous,  and  fatal  results  have  repeatedly  followed  its  acci- 
dental inhalation.  In  experimenting  with  this  deadly  gas,  the 
greatest  care  is  required  not  to  inhale  the  least  portion  of  it. 
It  has  been  condensed  at  — 40°  to  a  transparent  liquid,  but  it 
has  never  been  solidified.  The  gas  is  soluble  in  water  at  the 
ordinary  temperature  only  to  the  extent  of  one-fifth  of  its  vol- 
ume, and  neither  the  gas  nor  its  aqueous  solution  has  any  action 
upon  blue  or  red  litmus  paper.  In  spite,  therefore,  of  its  strong 
resemblance  to  ammonia  in  composition,  some  of  its  physical 
properties  are  strikingly  unlike  those  of  that  very  soluble  and 
intensely  alkaline  gas.  Arseniuretted  hydrogen  burns  in  the 
air  with  a  whitish  flame,  forming  water  and  a  white  smoke 
of  arsenious  acitf,  but  if  a  cold  body,  like  a  piece  of  porcelain, 
for  example,  be  introduced  into  a  jet  of  the  burning  gas,  the 
hydrogen  alone  will  burn,  and  the  arsenic  will  be  deposited  in 
the  metallic  state  upon  the  porcelain  surface,  forming  a  lustrous 
black  spot.  This  effect  is  precisely  similar  to  the  deposition  of 
soot  on  a  cold  body  held  in  the  flame  of  a  candle.  It  is  also  de- 
composed when  caused  to  pass  through  tubes  heated  to  dull  red- 
ness, metallic  arsenic  being  deposited  as  a  brown  or  blackish 
mirror,  while  hydrogen  gas  escapes.  Chlorine  in  excess  reacts 
violently  upon  it,  forming  terchloride  of  arsenic  (AsCl3)  and 
chlorhydric  acid ;  — 


ARSENIURETTED    HYDROGEN.  241 

AsH3  +  6C1  =  AsCl3  +  3HC1. 

When,  however,  chlorine  acts  on  an  excess  of  arseniuretted  hy- 
drogen, there  are  formed  chlorhydric  acid  and  metallic  arsenic ; 
flame  accompanies  this  reaction.  The  reactions  of  bromine  and 
iodine  are  similar  to,  but  less  violent  than,  those  of  chlorine. 
We  recall,  in  this  connection,  the  decomposition  of  ammonia  by 
chlorine,  with  formation  of  chlorhydric  acid  and  liberation  of 
nitrogen.  Arseniuretted  hydrogen  decomposes  the  solutions  of 
the  salts  of  many  of  the  heavy  metals,  but  the  products  are 
somewhat  various  ;  sometimes  a  metallic  arsenide  is  precipitated, 
sometimes  the  heavy  metal  is  precipitated  while  arsenious  acid 
remains  in  the  solution.  As  we  shall  shortly  see,  the  chemical 
properties  of  this  gas  are  of  great  importance  in  the  processes 
used  for^ietecting  arsenic  in  cases  of  poisoning. 

309.    Arseniuretted  hydrogen  may  be  analyzed  by  precisely 
the  same  method  which  was  used  for  the  analysis  of  phosphu- 
retted  hydrogen  (§  280),  but  the  results  can  only  be  approxi- 
mate, because  of  the  extreme  difficulty,  not  to  say  impossibility, 
of  obtaining  the  gas  in  a  state  of  tolerable  purity.     The  compo- 
sition of  the  analogous  gases,  ammonia  and  phosphuretted  hydro- 
gen, and  the  specific  gravity  of  the  gas,  lead  us  to  the  following 
statement  of  its  composition.     Two  volumes  of  the  gas  contain 
3  unit- volumes  of  hydrogen,  weighing      3X1      =3. 
^  unit- volume  of  arsenic  vapor,  weighing  ^  X  150  =  75. 

2  unit-volumes  of  arseniuretted  hydrogen  weigh          78. 
1  unit-volume  of  arseniuretted  hydrogen  weighs          39. 

The  actual  specific  gravity  of  arseniuretted  hydrogen,  as  deter- - 
mined  by  experiment,  is  as  nearly  as  possible  39,' — a  fact  which 
makes  it  certain  that  two  volumes  of  the  gas  do  not  contain  one 
volume  of  the  heavy  arsenic  vapor,  which  is  150  times  heavier 
than  hydrogen,  but  only  half  a  volume.  Herein  this  gas  differs 
from  ammonia,  but  resembles  phosphuretted  hydrogen.  The 
weight  of  the  quantity  of  arsenic,  which  combines  with  three 
atoms  of  hydrogen,  is  75,  just  as  the  weight  of  the  quantity  of 
nitrogen,  which  combines  with  three  atoms  of  hydrogen,  is  14; 
but  75  parts  by  weight  of  arsenic  vapor  only  occupy  one-half  the 

space  which  14  parts  of  nitrogen  fill. 
16 

r 


242  ARSENIOUS    ACID. 

310.  Arsenic  and   Oxygen.     Arsenic  forms   two  well-defined 
oxides,  arsenious  acid,  As2O3,  and  arsenic  acid,  As2O5.     The 
black  film  which  forms  on  the  surface  of  the  metal  when   ex- 
posed to  the  air,  is  by  many  supposed  to  be  a  suboxide,  while 
others  think  it  is  more  probably  a  mixture  of  metallic  arsenic 
with  arsenious  acid.     The  first  of  the  above-mentioned  acids  cor- 
responds with  nitrous  and  phosphorous  acids,  the   second  with 
nitric  and  phosphoric,  but  arsenious  acid  is  very  stable,  in  com- 
parison with  arsenic  acid,  while  the  reverse  is  true  of  the  analogous 
acids  containing  nitrogen  and  phosphorus.     The  element  arsenic 
possesses  many  properties  which  ally  it  to  the  metals,  but  in  its 
compounds  its  close  connection  with  nitrogen  and  phosphorus  is 
clearly  exhibited.     Its  oxides,  for  example,  are  both  acids,  and 
these  acids  unite  with  the  oxides   of  the  metals   proper  to  form 
stable,  crystallizable  salts,  which  are  in  many  cases  isomorphous 
with  the  corresponding  salts  containing  phosphorus. 

311.  Arsenious  Acid(As20B).      Arsenious  acid,  known  in  com- 
merce as  arsenic,  or  white  arsenic,  is  obtained  as  a  secondary  pro- 
duct in  the  roasting  of  arsenical  ores  of  nickel,  cobalt,  and  tin,  and  as 
a  principal  product  in  the  roasting  of  arsenical  pyrites.    The  vola- 
tile matters  which  escape  from  the  roasted  ores  consist  mainly  of 
sulphurous  and  arsenious  acids ;  the  first  is  allowed  to  pass  off 
into  the  atmosphere,  the  second  condenses  to  the  solid  state  in 
the  chambers  and  long  passages  through  which  the  vapors  are 
forced  to  pass  in   order  that  they  may  deposit  their  arsenious 
acid.     A  second  sublimation  purifies  the  raw  product.     Accord- 
ing to  the  temperature  at  which  the  arsenious  acid  is  sublimed 
and  condensed,  the  product  is  either  in  powder  or  in  transparent 
masses  ;  a  low  temperature  with  sudden  condensation  yields  a 
white   powder  of  minute   crystals,   a  higher  temperature  with 
gradual  solidification  produces  a  transparent  glass. 

312.  Arsenious  acid  is  a  white  solid  which  occurs,  not  only  in 
two  conditions,  one  amorphous  and  the  other  crystalline,  but  also 
in  two  distinct  crystalline  forms.     When  the  vapor  of  the  acid  is 
cooled  so  quickly  that  it  solidifies  at  once,  without  passing  through 
the  semi-fluid  state,  each  particle  of  the  solid  acid  assumes  more 
or  less  perfectly  the  octahedral  form.     A  hot  saturated  aqueous 
solution  of  the  acid  also  deposits  regular  octahedral  crystals  on 


ISOMERISM.  243 

cooling.  The  amorphous,  glassy  variety  of  the  acid  changes 
spontaneously,  when  kept  in  contact  with  the  air,  into  an  aggre- 
gation of  minute  octahedral  crystals,  thereby  becoming  opaque 
and  porcelain-like  in  appearance.  The  other  crystalline  form  of 
arsenious  acid  is  the  right  rhombic  prism ;  this  form  occurs  much 
less  frequently  than  the  first,  and  is  converted  into  the  octahedral 
form  by  sublimation  and  by  solution  in  hot  water. 

The  two  varieties  of  arsenious  acid,  the  vitreous  and  the  porce- 
laneous,  differ  decidedly  in  physical  and  chemical  properties,  yet 
they  have  precisely  the  same  chemical  composition,  and  when 
either  variety  changes  into  the  other,  no  alteration  of  weight,  no 
addition  or  subtraction  of  matter,  accompanies  the  change.  The 
two  varieties  contain  the  same  two  elements  in  precisely  the 
same  proportions.  When  two  or  more  compounds,  which  exhibit 
essential  differences  of  physical  and  chemical  properties,  are, 
nevertheless,  found  to  be  identical  in  respect  to  constituent  ele- 
ments and  their  proportions,  the  compounds  are  said  to  be  iso- 
meric  (equal  parts).  The  term  allotropism  (§  162)  properly 
applies  to  the  elements  only,  the  term  isomerism  to  compounds 
only ;  both  terms,  however,  refer  to  one  and  the  same  un- 
. questionable,  though  perplexing,  truth,  namely,  that  the  widest 
diversity'  of  properties  may  coexist  with  absolute  identity  of 
ultimate  chemical  constitution.  Two  allotropic  states  of  the 
same  element  not  infrequently  present  more  striking  differ- 
ences than  elements  recognized  as  distinct ;  and  among  the 
numerous  complex  compounds  of  carbon  with  which  organic 
chemistry  deals,  there  are  many  isomeric  compounds  which  are 
so  entirely  dissimilar  as  to  lead  almost  irresistibly  to  the  belief 
that  it  is  of  as  much  consequence  how  the  atoms  of  a  compound 
are  arranged  as  what  kind  of  atoms  they  are.  Arsenious  acid 
does  not  afford  a  very  striking  example  of  isomerism  ;  neverthe- 
less, the  properties  of  its  two  modifications  are  quite  diverse.  If 
it  be  true  that  the  different  arrangement  of  atoms  is  the  cause  of 
the  diversity  of  isomeric  compounds,  it  is  evident  that  the  differ- 
ences between  two  varieties  of  a  compound  of  only  two  kinds  of 
atoms,  united  in  the  simple  ratio  of  2  to  3,  cannot  be  expected  to 
be  so  marked,  as  the  differences  between  isomeric  compounds 
which  contain  four  or  five  elements  united  in  the  very  cornpli- 


244  PROPERTIES    OF    ARSENIOUS    ACID. 

cated  proportions  which  frequently  characterize  the  compounds 
of  carbon.  Nevertheless,  the  differences  between  the  two  iso- 
meric  compounds  of  arsenic  and  oxygen  are  sufficiently  distinct. 
The  glassy  acid  dissolves  much  more  rapidly  in  water  than 
the  porcelain-like  variety,  being  three  times  as  soluble  in  that 
liquid.  The  relation  of  the  two  varieties  to  heat  is  not  the  same, 
for  when  the  vitreous  acid  changes  into  the  opaque,  heat  is  disen- 
gaged. As  this  change  generally  takes  place  slowly,  from  the 
surface  towards  the  centre  of  any  fragment  of  the  vitreous  variety, 
the  heat  evolved  is  not  perceived ;  but  if  the  change  be  suddenly 
accomplished,  not  only  heat,  but  light  also  will  be  disengaged. 

Exp.  126. —  Dissolve  4  or  5  grms.  of  the  vitreous  acid  in  a  hot  mix- 
ture of  24  grms.  of  strong  chlorhydric  acid  and  8  c.  c.  of  water,  and 
let  the  solution  cool  slowly ;  the  arsenious  acid  will  crystallize  in 
transparent  octahedrons,  and  the  formation  of  the  crystals  will  be  ac- 
companied by  flashes  of  light. 

The  specific  gravity  of  the  vitreous  acid  is  3.738 ;  that  of  the 
porcelaneous  3.099.  The  opaque  variety  may  be  changed  into 
the  vitreous  by  long  boiling  with  water.  It  appears,  therefore, 
that  the  arrangement  of  atoms  which  may  be  supposed  to  fur- 
nish the  vitreous  acid  is  stable  only  at  high  temperatures,  and 
that  the  arrangement  of  atoms  which  is  peculiar  to  the  opaque 
acid  is  stable  only  at  low  temperatures. 

313.  Arsenious  acid  volatilizes  without  change  when  heated 
with  free  access  of  air ;  if  heated  in  contact  with  carbon,  it  gives 
up  its  oxygen,  and  metallic  arsenic  is  liberated.  Copper  and 
many  other  metals  reduce  arsenious  acid. 

Exp.  127.  —  Place  a  few  particles  of  arsenious  acid  in  an  open  tube 
of  hard  glass  (No.  5)  about  10  c.  m.  long,  and  heat  the  acid  over  the 
lamp,  holding  the  tube  in  a  sloping  position  ;  the  white  solid  will  be 
volatilized,  but  it  will  immediately  be  deposited  again  upon  the  cold 
part  of  the  tube.  By  the  aid  of  a  lens,  this  deposit  may  be  seen  to  be 
crystalline. 

FIG.  43.  Exp.   128. —  Drop  into  the  point 

of  a  drawn-out  tube  of  hard  glass, 
No.  5,  a  morsel  of  arsenious  acid, 
and  above  it  place  a  splinter  of 
charcoal  (Fig.  43) ;  heat  the  coal 
red-hot  in  the  flame  of  the  lamp,  and 


SOLUBILITY    OF    ARSENIOUS    ACID.  245 

then  volatilize  the  arsenious  acid.  The  acid  will  give  its  oxygen  to  the 
coal,  and  the  arsenic  will  be  deposited  in  a  ring  on  the  cold  part  of  the 
tube,  presenting  a  brilliant  metallic  appearance. 

Exp.  129.  —  Throw  a  particle  of  arsenious  acid  upon  a  piece  of  red- 
hot  charcoal ;  the  acid  will  be  partly  reduced,  and  the  peculiar  garlic 
odor  of  the  vapor  of  metallic  arsenic  will  be  perceived. 

Exp.  130.  —  Dissolve  a  few  centigrammes  of  arsenious  acid  in  5  or  6 
c.  c.  of  chlorhydric  acid  heated  in  a  test-tube ;  in  the  hot  solution  immerse 
a  narrow  strip  of  clean  copper;  an  iron-gray  film  will  be  deposited 
upon  the  copper.  This  coating  contains  metallic  arsenic  derived  from 
the  arsenious  acid ;  it  consists  of  an  alloy  of  arsenic  and  copper. 

314.  It  is  very  difficult  to  say  what  the  solubility  of  arsenious 
acid  in  water  really  is.     The  results  of  different  experimenters 
present  very  wide  discrepancies,  due  in  part  to  the  fact  already 
stated,  that  the  two  modifications  of  arsenious  acid  are  of  unlike 
solubility,  and  in  part  also  to  the  circumstance  that  the  acid  dis- 
solves with  extreme  slowness.     The  difficulty  of  the  determina- 
tion is  increased  by  the  readiness  with  which  either  modification 
passes  into  the  other  in  consequence  of  changes  of  temperature ; 
it  is  quite  possible  that  both  varieties  should  simultaneously  exist 
in  the  same  solution.     A  hot  aqueous  solution  usually  contains  1 
part  of  the  acid  in  10  or  12  parts  of  water;  on  cooling  this  solu- 
tion, a  portion  of  the  acid  separates,  leaving  a  solution  which 
contains  1  part  of  acid  in  20  to  30  parts  of  water.     The  aqueous 
solution  has  a  feebly  acid  reaction.     The  acid  is  much  less  solu- 
ble in  alcohol  than  in  water.     No  definite  hydrate  of  arsenious 
acid  is  known.     Hot  chlorhydric  acid  dissolves  it  with  facility, 
and  when  cold  retains  a  large  proportion  in  solution ;  other  acids, 
even  some  vegetable  acids,  dissolve  it  readily  when  hot,  though 
most  of  them  keep  but  little  in  solution  when  cooled.     When  tKe 
solution  of  arsenious  acid  in   chlorhydric  acid  is  evaporated,  a 
compound  of  chlorine  and    arsenic,  the    terchloride   of  arsenic 
(§  336)   is  volatilized,  and  the  solution  thus  loses  a  portion  of 
its  arsenic.     This  fact  is  of  importance  in  examinations  for  arsenic 
in  cases  of  suspected  poisoning. 

315.  Solutions  of  caustic  soda  and  potash  readily  dissolve  the 
acid,  a  soluble  arsenite  of  sodium  or  potassium  resulting  from  the 
reaction.     From  these    arsenites    of  sodium  and  potassium  the 
arsenites  of  other  metals   are  generally  obtained  by  the  way  of 


246  USES    OF    ARSENIOUS    ACID. 

double  decomposition.  The  arsenites  are  numerous,  but  they 
are  not  very  stable  and  have  been  but  little  studied. 

316.  Arsenious  acid  is  oxidized  and  converted  into  arsenic 
acid  by  digestion  with  nitric  acrd.  The  same  transformation  is 
brought  about,  but  quicker,  by  the  action  of  aqua  regia,  and  by 
chlorine,  bromine,  and  iodine,  in  presence  of  water.  When 
iodine  is  added  to  a  solution  of  arsenious  acid  mixed  with  a 
little  starch  paste,  the  whole  of  the  arsenious  acid  is  converted 
into  arsenic  acid,  before  any  blue  coloration  of  the  starch  is  pro- 
duced by  the  iodine.  Sulphuretted  hydrogen  colors  an  aqueous 
solution  of  arsenious  acid  yellow,  and  precipitates  a  yellow  sul- 
phide of  arsenic  (§  210)  from  a  solution  acidulated  with  chlor- 
hydric  acid. 

Arsenious  acid  is  a  violent  poison,  all  the  more  dangerous 
because  it  has  neither  taste  nor  odor  to  warn  the  victim  of  its 
presence  ;  two  decigrammes  of  it  will  cause  death.  All  the  solu- 
ble salts  of  arsenious  acid  are  likewise  horribly  poisonous.  The 
best  antidote  to  the  poison  is  a  mixture  of  moist,  freshly  precipi- 
tated, sesquioxide  of  iron  and  caustic  magnesia. 

317.  Arsenious  acid  is  largely  used  for  the  manufacture  of 
two   green  paints,   an  arsenite    of  copper  and  a  compound  of 
arsenite  and  acetate  of  copper ;  it  is  applied  as  an   oxidizing 
agent  in  the  manufacture  of  glass ;  it  is  consumed  in  considerable 
quantities  for  poisoning  vermin,  and  for  producing  the  arsenic  acid 
which  is  used  in  the  dyeing  and  printing  of  cloth  ;  it  is  used  in 
very  small  doses  as  a  remedy  for  asthma,  and  in  some  skin  dis- 
eases.    Although  the  acid  is  so  violent  a  poison,  it  seems  to  be 
possible,  by  beginning  with  small  doses  and  gradually  increasing 
them,  to  accustom  the  human  body  to  sustain,  without  injury, 
doses  of  2  to  3  decigrammes,  or  even  more ;  the  arsenic  thus 
taken  is  said  to  produce  a  plump  and  healthy  appearance  in  those 
who  use  it,  and  especially  to  increase  the  power  of  the  respira- 
tory  organs.     In   veterinary  practice,  it   has   been  found  that 
arsenious  acid  administered  to  animals  in  this  manner  improves 
the  appearance  of  the  skin. 

318.  Arsenic  Acid  (As2O5).  *  This  compound  is  produced  by 
oxidizing  arsenious  acid  with  nitric  acid,  aqua  regia,  hypochlorous 
acid,  or  other  oxidizing  agents. 


ARSENIC    ACID.  247 

Exp.  131.  —  Add  8  grms.  of  powdered  arsenious  acid,  little  by  little, 
to  6  grms.  of  concentrated  nitric  acid  (specific  gravity,  1.35) ;  the  mix- 
ture becomes  hot,  and,  after  24  hours,  yields  a  syrupy  liquid,  resemb- 
ling oil  of  vitriol,  which  consists  of  arsenic  acid,  contaminated,  perhaps, 
with  a  little  arsenious  acid  which  may  be  completely  oxidized  by  the 
addition  of  a  little  more  nitric  acid. 

319.  The  syrupy  solution,  thus  obtained,  deposits,  after  stand- 
ing  for   some    days,   at  the  ordinary  temperature,  transparent 
elongated  prisms  or  rhomboidal  laminae.     These  crystals,  heated 
to  100°,  first  melt  and  then  yield  the  terhydrate  of  arsenic  acid 
(3H2O,As2O5  —  2H3AsO4)   as  a  crystalline    precipitate.     The 
same  hydrated  acid  separates  in  large  prismatic  crystals,  when  a 
concentrated  aqueous   solution  is  cooled  to  a  low  temperature. 
There  are  two  other  hydrates  of  the  oxide  As205 ,  —  a  bihydrate, 
2H20,As2O5  =  H4As2Or,  and    a  monohydrate,   H20,A?2O5  = 
2HAsO3;  both  these  lower  hydrates  are  obtained  from  the  ter- 
hydrate by  subjecting  the  latter  to  the  prolonged  action  of  certain 
temperatures.     If  either  of  the  hydrates  be  heated  to  dull  red- 
ness, a  white  amorphous  mass  remains,  which  is  the  anhydrous 
acid,  As2O5 ;  this  substance  has  no  action  upon  litmus,  and  seems 
to  be  scarcely  soluble  in  water.     After  long  exposure  to  moist 
air,  it  slowly  deliquesces,  and  if  covered  with  water  and  soaked 
for  a  long  time,  it  at  last  dissolves,  being  probably  converted  into 
the  soluble  terhydrate.     At  a  full  red  heat  it  is  resolved  into 
arsenious  acid  and  oxygen. 

320.  In  spite  of  the  recognized  existence  of  three  solid  hy- 
drates of  arsenic  acid,  there  is  but  one  aqueous  solution  of  this 
acid,  inasmuch  as  the  monohydrate,  the  bihydrate,  and  the  anhy- 
dride, are  all  immediately  converted  into  the,  terhydrate  when 
dissolved  in  water.     The  solution  has  a  very  sour  taste  and  a 
strong  acid  reaction  on  vegetable  colors.     The  concentrated  liquid 
is  highly  corrosive  and  produces  blisters  on  the  skin.     Arsenic 
acid  and  its  salts  are  poisonous,  but  not  in  so  high  a  degree  as 
arsenious  acid  and  the  arsenites. 

321.  Arsenic  acid  is  a  strong  acid,  capable  of  expelling  all 
the  more  volatile  acids  from  their   salts  at  high  temperatures. 
Its  three  hydrates  are  strictly  comparable  with  the  three  hydrates 
of  phosphoric  acid. 


248  SALTS  OF  ARSENIC  ACID. 

Hydrates          ( HPO3  HAs03  )       Hydrates 

of  mP2Or  H4AsA[-'          of 

Phosphoric  Acid.  (  H3PO4  H3AsO4  )  Arsenic  Acid. 

Either  one,  two,  or  all  three  of  "the  hydrogen  atoms  in  common 
arsenic  acid,  H3As04 ,  may  be  replaced  by  a  metal,  so  that  three 
arseniates  of  any  one  metal  may  exist,  as  for  example, 

NaH2AsO4  Na2HAsO4  Na3AsO4 

Acid  Arseniale  "  Neutral  "  Arseniate  Basic  Arseniate 

of  Sodium.  of  Sodium.  of  Sodium. 

If  an  acid  arseniate  be  suitably  heated,  a  meta-arseniate  results, 
as,  for  example,  NaAsO8  =  NaH2As04  —  H2O ;  if  a  neutral 
arseniate  be  sufficiently  heated,  a  pyro-arseniate  results,  as,  for 
example,  Na4As2O7  =  2Na2HAsO4  —  H2O ,  but  such  meta-  and 
pyro-ar=eniates,  unlike  the  corresponding  meta-  and  pyro-phos- 
phates,  have  very  little  stability,  take  up  again  the  molecule  of 
water,  which  the  heat  .expelled,  the  moment  they  are  brought  in 
contact  with  water,  and  are  so  changed  back  again  into  salts  of 
ordinary  arsenic  acid.  The  salts  of  arsenic  acid  are  isomorphous 
throughout  with  the  corresponding  phosphates. 

322.  Arsenic  acid  is  readily  reduced  to  arsenious  acid,  and, 
consequently,  acts  in  some  cases   as  an  oxidizing  agent.     Thus 
sulphurous  acid  reduces  arsenic  acid,  and  is  itself  converted  into 
sulphuric  acid :  — 

2H3As04  +  2S02  =  2H2S04  =  As2O3  +  H2O . 

Sulphydric  acid  gas,  passed  through  a  not  too  concentrated  solu- 
tion of  arsenic  acid,  slowly  precipitates  the  yejlow  tersulphide  of 
arsenic,  the  action  being  assisted  by  heat  and  by  the  presence  of 
another  acid.  Charcoal  and  the  metals  at  a  red  heat  reduce 
arsenic  acid  to  the  metallic  state,  just  as  they  do  arsenious  acid. 

323.  Arsenic  acid  has  been  extensively  used  in  calico  print- 
ing, in  place  of  the  more  expensive  tartaric  acid,  for  developing 
white  patterns  on  a  colored  ground  in  the  chloride  of  lime  vat. 
It  is  also  an  excellent  preservative  of  animal  substances,  and  is 
accordingly  used  to  defend  the  specimens  and  preparations  of 
the  anatomist  and  naturalist  from  decay  and  from  the  attack  of 
insects. 

324.  detection  of  arsenic  in  cases  of  poisoning.     Nearly  all 
compounds  of  arsenic  are  poisonous,  but  arsenious  acid  is  best 


DETECTION    OF    ARSENIC.  249 

known  and  most  easily  procured,  and  is,  therefore,  most  likely  to 
be  met  with  in  cases  of  poisoning  by  arsenic,  whether  accidental 
or  intentional.  In  criminal  trials  the  solubility  of  arsenious  acid 
in  water  has  often  been  much  discussed,  but  this  is  practically  a 
point  of  little  importance,  for  the  tasteless  poison  is  generally 
administered  in  the  solid  state  mixed  with  soup,  gruel,  milk,  or 
even  with  solid  food.  It  thus  sometimes  happens  that  small  par- 
ticles of  the  poison  can  be  found  adhering  to  culinary  vessels, 
cups,  plates,  or  spoons,  or  even  to  the  coatings  of  the  stomach 
and  intestines  after  death.  If  the  arsenious  acid  is  too  finely 
divided  to  be  picked  out  in  lumps,  it  may  sometimes  be  sepa- 
rated by  stirring  up  the  mass,  under  examination,  with  water,  and 
leaving  the  heavier  particles  to  settle.  Any  solid  arsenious  acid 
that  may  be  present  will  be  sure  to  be  found  in  the  residue  ;  it 
may  be  washed  with  cold  water.  It  is  always  very  satisfactory 
thus  to  obtain  the  solid  poison  in  the  condition  in  which  it  was 
administered,  because  the  examination  is,  in  such  cases,  very 
direct  and  conclusive.  It  is  only  necessary  to  try,  with  the  white 
powder  thus  obtained,  the  experiments  already  given  to  illustrate 
the  properties  of  arsenious  acid  (Exps.  127-130),  together  with 
certain  other  discriminating  tests  shortly  to  be  described. 

325.  It  more  frequently  happens,  however,  that  the  arsenic 
has  been  dissolved  by  the  acid  secretion  of  the  stomach,  and  has 
become  intimately  mixed  with  the  food  or  excretions,  or  incor- 
porated into  the  substance  of  the  organs  themselves.  The  exam- 
ination then  becomes  more  difficult.  The  reduction  of  arsenious 
acid  by  copper  (Exp.  130)  is  an  available  test  in  such  cases. 
To  the  suspected  matter,  if  liquid,  about  one-sixth  of  its  bulk  of 
chlorhydric  acid  is  added,  and  the  mixture  is  gently  boiled. 
Solid  tissues  must  be  cut  into  small  pieces  and  boiled  for  some 
time  with  dilute  chlorhydric  acid  (1  part  acid  to  6  parts  water) 
until  the  whole  is  disintegrated ;  this  solution  is  finally  clarified 
by  filtration.  Strips  of  copper  gauze  or  foil  are  then  immersed 
in  the  boiling  liquid,  and  if  any  gray  deposit  is  produced,  fresh 
pieces  of  metals  are  added  so  long  as  the  color  of  the  copper  is 
perceptibly  changed.  They  are  then  removed,  washed  with 
water,  dried,  folded  up,  placed  in  a  dry  tube  of  hard  glass  and 
gently  heated.  Some  of  the  metallic  arsenic  in  the  gray  alloy 


250  DIALYSIS. 

will  be  converted  into  arsenious  acid,  which  collects  on  the  cold 
part  of  the  tube  in  the  form  of  a  crystalline  sublimate.  To  this 
sublimate  all  tests  for  the  identification  of  arsenious  acid  can  be 
applied.  This  mode  of  operation  is  known  as  Reinsch's  test. 
The  chlorhydric  acid  employed  must  be  proved  to  be  free  from 
arsenic. 

3*26.  Another  method  of  separating  arsenious  acid  from  the 
organic  matters  with  which  it  is  mixed  is  that  of  dialysis,  a  pro- 
cess which  depends  upon  the  very  different  rates  at  which  differ- 
ent substances  diffuse  through  water.  This  process  it  is  now 
necessary  to  explain. 

Exp.  132.  —  Select  two  straight-sided  bottles  of  clear  glass,  about  15 
c.  m.  deep  and  8  to  9  c.  m.  wide.  Fill  them  seven-eighths  full  of  distilled 
water.  Dissolve  10  grms.  of  bichromate  of  potassium  in  100  c.  c.  of 
water  ;  suck  as  much  of  this  solution  as  will  fill  the  remaining  eighth 
of  one  of  the  above-mentioned  bottles  into  a  pipette  (Appendix,  §  22), 
and  carefully  convey  the  colored  fluid  to  the  bottom  of  the  bottle  by 
bringing  the  fine  point  of  the  pipette  to  the  bottom  of  the  bottle  and 
then  allowing  the  liquid  to  flow  very  slowly  out  of  the  pipette.  If  time 
enough  (5  or  6  minutes)  be  taken  for  this  process,  no  sensible  inter- 
mixture of  the  two  liquids  will  take  place  during  the  delivery.  Dis- 
solve 10  grms.  of  caramel  (melted  and  partially  burnt  sugar)  in  100 
c.  c.  of  water,  and  convey  to  the  bottom  of  the  second  bottle,  in  the 
same  manner  as  before,  enough  of  the  dark-colored  solution  to  fill  the 
bottle. 

The  two  bottles  are  left  at  rest  for  several  days  in  a  room  where  the 
temperature  is  nearly  constant.  Spontaneous  diffusion  immediately 
begins,  and  the  very  different  rates  at  which  the  two  colored  substances 
diffuse  upwards  through  the  water  should  be  from  time  to  time  ob- 
served. 

327.  Substances  which  have  a  comparatively  high  diffusive 
power  have  generally,  though  not  invariably,  the  power  of  crys- 
tallizing ;  their  solutions  are  generally  free  from  viscosity  and 
always  have  taste.  Such  substances  are  designated  by  the  term 
crystalloids.  Among  crystallojdjjL-there  are  wide  differences  of 


diffusive  'power  ;  thus,  caustic  Taofieh  Diffuses  twice  as  fast  as  sul- 
phate of  potassium,  and  sulphate  of  potassium  twice  as  fast  as 
sulphate  of  magnesium. 

Substances  of  very  low  diffusive  power  have  little,  if  any, 


CRYSTALLOIDS    AND    COLLOIDS.  251 

tendency  to  crystallize,  and  affect  a  vitreous  structure.  Such 
substances  are  often  very  soluble  in  water,  but  their  solution 
have  always  a  certain  degree  of  viscosity,  when  concentrated, 
and  are  insipid  or  wholly  tasteless.  By  combining  with  water, 
these  substances  are  apt  to  form  jellies.  Gelatine  has  been 
taken  as  the  type  of  this  class,  and  they  have  hence  been 
called  colloids,  a  name  derived  from  Greek  words  signifying 
glue-like.  Among  the  colloids  rank  hydrated  silicic  acid,  alumina 
starch,  gums,  caramel,  albumen,  and  animal  and  vegetable  extrac- 
tive matters. 

As  we  can  separate,  by  means  of  distillation  or  evaporation,  two 
bodies  of  different  volatility,  so  by  the  aid  of  diffusion  we  can  separate 
one  substance  more  or  less  completely  from  another.  Jellies  and  col- 
loid membranes  are  permeable  to  crystalloids,  but  are  practically  im- 
permeable by  colloids  like  themselves.  By  means,  therefore,  of  a 
colloidal  diaphragm,  or  partition,  crystalloids  can  be  separated  from 
colloids  by  diffusion.  The  most  suitable  substance  for  the  dialytic 
diaphragm  is  parchment  paper,  prepared  by  soaking  unglazed  paper, 
for  a  few  seconds,  in  a  mixture  of  6  parts  of  strong  sulphuric  acid  and 
1  part  of  water,  and  immediately  washing  it,  first  in  water  and  then  in 
water  containing  ammonia.  The  paper  subjected  to  this  treatment 
becomes  semi-transparent  and  tough,  like  parchment. 

A  dialyser,  as  the  apparatus  for  effecting  separation  by  diffusion  is 
called,  consists  of  two  gutta-percha,  or  wooden,  hoops,  one  of  which 
should  be  5  c.  m.,  and  the  other,  2.5  c.  m.  deep.  The  deeper  hoop  is 
slightly  conical,  and  the  shallower  must  slip  over  the  small  end  of  the 
deeper.  The  hoops  maybe  from  15  to  25  c.  m.  in  diameter.  The 
parchment  paper,  which  is  to  form  the  bottom,  must  be  about  8  c.  m. 
wider  than  the  small  end  of  the  5  c.  m.  hoop.  To  prepare  the  dialyser 
for  use,  soak  the  parchment  paper  for  about  a  minute  in  distilled  water  ; 
stretch  it  evenly  over  the  small  end  of  the  5  c.  m.  hoop  and  strain  it 
on  tightly  by  pushing  over  it  the  2.5  c.  m.  hoop.  The  paper  must  be 
pressed  smoothly  up  round  the  outside  of  the  deeper  hoop,  and  the 
bottom  must  be  flat  and  even. 

There  must  be  no  small  holes  in  the  paper.  To  detect  such,  put 
distilled  water  into  the  dialyser  to  the  depth  of  5  m.  m.,  and  place  the 
dialyser  ou  some  white  blotting-paper.  If  any  wet  or  dark  spots  ap- 
pear, they  indicate  the  existence  of  small  holes.  To  close  such  holes, 
apply  to  the  under  surface  of  the  paper  about  the  holes  some  solution 
of  albumen,  put  on  a  small  patch  of  parchment  paper,  and  iron  the 
patch  with  a  hot  smoothing-iron.  The  albumen  will  coagulate,  fix  the 
patch,  and  close  the  hole. 


252  DIALYSIS    OF    ARSENIOUS    ACID. 

Exp.  133. —  Into  the  dialyser  so  prepared  pour  an  aqueous  solution 
containing  five  per  cent,  of  cane-sugar  and  five  per  cent,  of  gum 
arabic,  to  the  depth  of  about  1.25  c.  m.  Then  float  the  dialyser  on 
distilled  water  contained  in  a  flaf  basin.  The  volume  of  water  in  the 
basin  should  be  from  5  to  10  times  greater  than  the  volume  of  the  fluid 
in  the  dialyser.  The  wider  the  dialyser  and  the  greater  the  quantity 
of  distilled  water  in  the  outer  basin,  the  more  rapid  and  effective  the 
diffusion.  A  dialyser  1 5  c.  m.  in  diameter,  serves  to  operate  upon  200 
to  250  c.  c.  of  liquid  ;  one  of  20  c.  m.  upon  400  to  450  c.  c. ;  one  of  25 
c.  m.  upon  600  c.  c. 

After  the  lapse  of  twenty-four  hours,  the  water  in  the  basin  should 
be  poured  into  an  evaporating-dish  and  gently  evaporated  over  a  water- 
bath.  Pure  sugar  will  crystallize  from  the  solution.  The  sugar,  a 
crystalloid,  has  passed  through  the  diaphragm ;  the  gum,  a  colloid,  has 
remained  in  the  dialyser.  It  should  be  remarked  that  gum  is  wholly 
uncrystallizable,  and  that  the  mixed  solution  of  gum  and  sugar  will  not 
yield  crystals,  but  only  an  amorphous  mass,  when  evaporated. 

328.  By  means  of  this  dialysing  apparatus,  arsenious  acid, 
salts  of  the  metals,  strychnine,  and  other  crystallizable  poisons, 
mineral  and  organic,  can  be  readily  separated  from  organic 
fluids,  and  the  process  has  the  very  great  advantage  of  intro- 
ducing no  metal,  chemical  reagent,  or  other  foreign  substance 
into  the  fluids  under  examination.  After  twenty-four  hours,  the 
crystallizable  poison,  or  a  large  proportion  of  it,  will  have  been 
transferred  to  the  distilled  water  in  the  outer  basin,  and  in  this 
solution  it  may  be  sought  for  by  the  application  of  the  appro- 
priate tests. 

Exp.  134.  —  Dissolve  0.1  grin,  of  arsenious  acid  in  about  30  c.  c.  of 
hot  water,  and  stir  the  solution  into  about  200  c.  c.  of  milk,  ale,  soup, 
gruel,  or  other  thick  organic  fluid;  place  the  poisonous  fluid  in  a  15 
c.  m.  dialyser  and  float  the  dialyser  on  two  litres  of  distilled  water  in 
a  clean  basin.  Allow  the  apparatus  to  stand  at  rest  in  a  room  where 
the  temperature  is  tolerably  uniform  for  forty-eight  hours.  At  the  ex- 
piration of  this  time,  transfer  the  clear  solution  in  the  basin  to  an 
evaporating-dish,  without  losing  a  drop,  rinse  the  basin  carefully  with 
distilled  water,  and  add  the  rinsings  to  the  contents  of  the  dish ;  evap- 
orate the  solution  over  a  water-bath  (see  Appendix,  §  1 7)  to  the  bulk 
of  50  c.  c.  To  one-third  of  this  concentrated  solution,  add  a  few  drops 
of  pure  chlorhydric  acid,  and  apply  Reinsch's  test  for  arsenic  (§  325) 
with  due  regard  to  the  small  scale  on  which  the  operation  must  be  con- 
ducted. About  0.025  grm.  of  arsenious  acid  is  the  quantity  which  may 


EXAMINATION    FOR    ARSENIC.  253 

be  expected  to  respond  to  the  test  by  copper.  Three-quarters  of  the 
original  decigramme  should  be  transferred  by  diffusion  through  the 
dialyser  in  the  course  of  forty-eight  hours,  and  of  this  solution  of  this 
0.075  grm.  of  arsenious  acid  we  have  taken  one-third.  The  rest  of 
the  solution  is  to  be  reserved  for  tests  hereafter  to  be  described. 

329.  When  the  arsenious  acid  must  be  sought  in  large  organs 
of  the  body,  like  the  stomach,  liver,  or  intestines,  or  in  consider- 
able quantities  of  disgusting  semi-fluid  materials,  it  is  sometimes 
necessary  to  utterly  destroy  the  organic  matters  by  processes 
which  cannot  cause  the  loss  of  arsenic.  Several  methods  may 
be  employed  for  this  purpose.  1.  The  organic  matter  is  gently 
heated  in  a  tubulated  retort  with  strong  chlorhydric  acid,  and 
strong  nitric  acid  is  added  from  time  to  time.  The  organic  mat- 
ter is  thus  completely  destroyed,  with  the  exception  of  the  fat. 
A  cooled  receiver  is  connected  with  the  retort  to  condense  the 
distillate  from  the  hot  mass.  The  fat  is  separated  from  the  clear 
liquid  in  the  retort  by  decantation,  and  well  washed  with  water ; 
these  washings,  together  with  the  distillate  in  the  receiver,  are 
added  to  the  main  bulk  of  the  fluid.  2.  Chlorate  of  potassium 
may  be  added  instead  of  nitric  acid.  3.  The  organic  matter, 
after  being  made  as  fine  as  possible,  is  stirred  up  with  water, 
and  chlorine  gas  is  passed  through  the  liquid  until  the  organic 
substances  are  partly  destroyed  and  partly  deposited  in  brown 
flakes. 

All  these  processes,  and  there  are  others  based  on  like  prin- 
ciples, are  processes  of  combustion  ;  aqua  regia,  chlorate  of  po- 
tassium, and  chlorine  are  oxidizing  agents  of  great  power,  as 
we  have  already  seen ;  they  burn,  the  carbon  and  hydrogen  of 
the  organic  materials  as  literally  as  the  oxygen  of  the  air  burns 
the  coal  in  the  grate.  The  arsenic  also  is  oxidized  and  converted 
into  its  highest  oxide,  arsenic  acid.  Whenever  chlorhydric  acid 
is  used,  and  heat  is  applied,  there  is  danger  that  chloride  of 
arsenic  (AsCl3)  may  be  formed ;  this  chloride  is  a  volatile  body, 
against  who^e  loss  precautions  must  be  taken,  by  never  allowing 
the  temperature  of  the  fluids  to  rise  much  above  100°,  and  by 
collecting  any  distillate  which  may  be  formed  under  circumstances 
which  make  it  possible  for  this  chloride  to  be  evolved. 

330.    All  these  methods  of  destroying  the  organic  matters  in 


254  DETECTION    OF    ARSENIC. 

which  arsenious  acid  is  to  be  sought  for  are  liable  to  one  objec- 
tion. Considerable  quantities,  even  kilogrammes,  of  acids,  must 
be  used,  if  the  quantity  of  organic  substance  to  be  destroyed  is 
large  ;  chlorhydric  and  sulphuric  acids  very  commonly  themselves 
contain  arsenic,  and  since  the  liquids,  which  result  from  the  de- 
struction of  the  organic  tissues,  are  finally  evaporated  to  a  very 
small  bulk,  all  the  arsenic  in  several  kilogrammes  of  the  acids 
employed,  as  well  as  all  the  arsenic  which  may  have  been  con- 
tained in  the  bodily  organs  or  fluids  submitted  to  examination,  will 
be  concentrated  into  a  small  cupful  of  liquid.  It  is  obviously 
necessary  to  demonstrate  that  the  arsenic  reactions  cannot  be 
obtained  from  the  same  quantities  of  the  same  acids  actually 
employed,  subjected  to  the  same  series  of  operations.  The  best 
way  is  to  conduct  a  parallel  examination  of  normal  animal 
organs  or  fluids  ;  in  this  examination  the  identical  processes  and 
the  same  weights  of  the  same  chemical  materials  must  be  em- 
ployed as  in  the  examination  of  the  suspected  substances ;  if 
arsenic  is  found  in  the  latter  investigation,  but  not  in  the  parallel 
examination  of  the  normal  animal  substances,  it  will  be  quite 
certain  that  the  arsenic  was  not  derived  from  the  chemicals 
employed  in  the  research. 

331.  When,  by  any  of  the  processes  above  described,  a  clear 
arsenical  solution,  free  from  organic  matter,  has  been  obtained, 
the  identification  of  the  arsenic  may  be  accomplished  by  many 
methods,  of  which  the  two  following  will  serve  as  examples :  — 

1.  By  precipitation  as  sulphide  of  arsenic.  If  the  clear  solu- 
tion contains  arsenic  acid,  it  is  necessary  to  reduce  this  oxide  to 
arsenious  acid  before  the  precipitation  can  be  effected.  This  re- 
duction may  be  accomplished  by  passing  a  slow  stream  of  washed 
sulphydric  acid  gas  (§  202)  through  the  solution  for  several 
hours,  but  may  be  immediately  effected  by  saturating  the  solu- 
tion with  sulphurous  acid  gas,  the  superfluous  gas  being  finally 
expelled  by  gentle  heating.  After  the  reduction  has  been 
effected,  a  slow  stream  of  washed  sulphydric  acid  gas  precipi- 
tates the  yellow  sulphide  of  arsenic  from  the  liquid. 

Exp.  135. —  Acidulate  one-half  of  the  liquid  reserved  from  Exp. 
134,  with  pure  chlorhydric  acid,  place  it  in  a  small  beaker-glass  and 
pass  a  slow  stream  of  washed  sulphydric  acid  gas  through  the  solution. 


MARSH'S  TEST.  255 

The  delivery-tube  of  the  gas  should  be  small  and  the  current  slow ;  a 
piece  of  unglazed  paper  should  be  used  as  a  cover,  in  order  to  keep 
the  beaker  full  of  the  gas.  A  yellow  precipitate  (A's2S3)  will  appear, 
indicating  the  probable  presence  of  arsenic.  When  no  more  precipi- 
tate seems  to  form,  stop  the  current  of  gas,  and  let  the  beaker  stand  in 
a  warm  place  till  the  odor  of  the  gas  has  nearly  disappeared.  Collect 
the  precipitate  on  a  small  filter  (see  Appendix,  §  14),  wash  it  thor- 
oughly with  water,  and  dry  it. 

Exp.  136. —  Mix  intimately  the  dry  precipitate  obtained  in  the  last 
experiment  with  its  bulk  of  dry  carbonate  of  sodium  and  its  bulk  of 
dry  cyanide  of  potassium,  and  introduce  this  mixture  into  a  hard  glass 
tube  (No.  5),  the  end  of  which  has  been  closed  and  expanded  to  a  small 
bulb.  If  the  precipitate  stick  to  the  filter-paper,  it  must  be  scraped 
off*.  Warm  the  bulb  and  its  contents  over  the  lamp  to  expel  mois- 
ture, then  wipe  the  tube  out  with  a  tuft  of  cotton  on  the  end  of  a  wire, 
and  bring  the  bulb  to  a  red  heat.  A  ring  of  metallic  arsenic,  like  that 
of  Exp.  1 28,  will  be  deposited  in  the  tube.  Preserve  this  metallic  mirror 
for  subsequent  study. 

2.  *  By  conversion  into  arseniuretted  hydrogen.  When  an 
aqueous  or  acid  solution,  containing  arsenious  or  arsenic  acid,  is 
added  to  the  contents  of  a  flask  in  which  hydrogen  is  being  gen- 
erated, the  nascent  hydrogen  reduces  the  oxide  of  arsenic,  and 
there  is  formed  a  quantity  of  arseniuretted  hydrogen,  which  mixes 
with  the  uncombined  hydrogen  evolved.  (Compare  §  307.) 
This  arseniuretted  hydrogen  is  decomposed,  with  deposition  of 
metallic  arsenic,  by  being  passed  through  a  red-hot  tube ;  the 
undecomposed  gas  burns  with  a  whitish  flame,  and  if  a  cold  body 
be  held  in  the  flame,  a  spot  of  metallic  arsenic  will  be  deposited 
upon  it.  (See  §  308.)  Upon  these  properties  and  reactions  is . 
based  the  process  for  detecting  arsenic  known  as  Marsh's  test. 

FIG.  44. 


Exp.  137.  — To  a  bottle  prepared  for  generating  hydrogen  from 
pure  zinc  and  dilute  sulphuric  acid,  adapt  a  chloride  of  calcium  tube, 


256  MARSH'S  TEST. 

and  with  the  outer  end  of  this  drying-tube  connect  a  tube  of  hard  glass 
(No.  4)  which  has  been  twice  drawn  to  a  fine  bore  and  which  termi- 
nates in  a  fine  open  point.  (Fig.  44.)  {Support  this  long  tube  at  three 
or  four  points,  in  such  a  manner  that  the  softening  of  the  glass,  first  at 
the  point  a,  and  then  at  the  point  6,  shall  not  distort  the  tube.  By 
adding  acid  through  the  funnel-tube  of  the  flask,  evolve  hydrogen,  and 
when  the  whole  apparatus  is  full  of  hydrogen,  light  the  gas  at  the  tip 
of  the  hard  glass  tube.  By  means  of  an  efficient  gas-lamp,  heat  about 
2  c.  m.  of  this  tube  to  dull  redness  at  the  point  a;  just  beyond  the  hot 
part  of  the  tube,  place  a  small  sheet-iron  screen,  as  shown  in  the  figure, 
to  cut  off  the  heat  from  the  adjoining  narrow  part  of  the  tube.  Main- 
tain the  apparatus  in  this  condition  for  ten  minutes, — the  glass  tube 
red-hot  at  one  point  and  the  hydrogen  flowing  steadily  through  the 
tube  and  burning  with  a  colorless  flame  at  the  point.  If  no  deposit,  or 
only  a  scarcely  perceptible  deposit,  appears  in  the  fine  tube  adjoining 
the  heated  portion,  the  zinc  and  sulphuric  acid  are  pure  enough  for  the 
experiment,  but  if  a  black,  shining  deposit  appears  in  the  fine  tube,  the 
materials  themselves  contain  arsenic  and  are,  of  course,  unsuitable  for 
use  in  testing  for  this  substance.  • 

Jf  the  zmc  and  sulphuric  acid  prove  to  be  sufficiently  free  from 
arsenic,  add  to  the  contents  of  the  flask  a  few  drops  of  the  liquid  ob- 
tained by  dialysis,  (Exp.  134).  In  a  moment  a  mirror  of  arsenic 
will  be  deposited  in  the  fine  tube  adjoining  a  ;  when  this  mirror  has 
become  large  and  dense,  move  the  lamp  to  &,  transfer  the  screen,  and 
obtain  a  similar  mirror  in  the  second  attenuated  portion  of  the  tube  ; 
finally,  extinguish  the  lamp  and  allow  the  arseniuretted  hydrogen  to 
reach  the  burning  jet  of  gas  at  the  extreme  point  of  the  apparatus ; 
the  white  coloration  of  the  flame  will  now,  for  the  first  time,  be  seen ; 
introduce  into  the  jet  a  bit  of  cold  porcelain,  and  obtain  the  character- 
istic black  and  lustrous  spot  of  metallic  arsenic ;  this  experiment  may 
be  repeated  indefinitely  and  a  large  number  of  spots  obtained  for  sub- 
sequent use.  Preserve  the  two  mirrors  and  a  number  of  arsenic  spots 
for  future  study.  In  order  to  prevent  the  possibility  of  any  arseniu- 
retted hydrogen  escaping  into  the  air  of  the  room,  the  jet  of  gas  must 
be  kept  constantly  burning,  and  when  the  experiments  are  ended,  the 
flask  must  be  washed  out  promptly  and  thoroughly. 

332.  This  method  is  very  well  adapted  for  the  speedy  and 
certain  detection  of  arsenic  in  green  paints,  such  as  are  applied 
to  wall-papers,  artificial  flowers,  lamp-shades,  and  the  like,  for  in 
such  cases,  if  any  arsenic  is  present,  there  is  so  much  as  to  make 
any  traces  of  arsenic  which  may  contaminate  the  zinc  and  sul- 


DETECTION  OF  ARSENIC.  257 

phuric  acid  of  no  consequence  whatever.  It  is  only  necessary,  in 
such  examinations,  to  scrape  off  some  of  the  green  coloring  mat- 
ter, dissolve  it  in  dilute  chlorhydric  acid,  and  add  the  solution  to 
the  hydrogen  flask  of  the  apparatus  described  above.  Arsenic 
greens  instantly  give  enormous  mirrors  and  spots  under  these 
conditions. 

333.  In  medico-legal  investigations,  upon  whose  results  life 
often  depends,  it  must  always  be  remembered  that  arsenic  is 
very  widely  diffused  in   the  mineral  kingdom,  and  that  it  is  a 
matter  of  great  difficulty  to  procure  reagents  absolutely  free 
from  it.     The  substances  employed  as  reagents  in  Marsh's  test 
are  often  contaminated  with  it,  and  the  acids  used  in  destroying 
organic  matter  may  well  contain  arsenic  enough  to  become  visible 
after  the  great  concentration  of  this  impurity  which  inevitably 
occurs  in  the  evaporation  of  the  liquid  which  results  from  the 
burning  of  the  organic  matter.     The  use  of  zinc  is  avoided,  and 
other  advantages  gained,  by  obtaining  the  necessary  hydrogen 
by  the  electrolysis  of  acidulated  water.     (§  35.)     When  a  solu- 
tion of  arsenious  acid,  acidulated  with  chlorhydric  or  sulphuric 
acid,  is  decomposed  by  the  electric  current,  the  greater  part  of 
the  arsenic  eliminated  at  the  negative  pole  is  given   off  in  the 
form  of  arseniuretted  hydrogen,  which  may  be   examined  pre- 
cisely as  if  it  were  generated  in  Marsh's  apparatus.    'This  method 
is  very  delicate,  and  seems  to  possess  considerable  advantages 
over  Marsh's  process,  but  it  has  not  yet  (1866)  been  actually 
applied  in  judicial  investigations. 

334.  To  describe  the  methods  by  which  the  analytical  chemist 
purifies  his  reagents  and  proves  their  purity,  would  involve  de- 
scending into  technical   details    which  are   unsuitable  for  this 
manual.     Zinc  and  acids,  pure  enough  for  illustrative  experi- 
ments, can  be  bought  of  the  dealers  in  pure  chemicals.     None 
but  expert  analysts  should  ever  be  intrusted  with  the  chemical 
investigation  in  a  supposed  case   of  poisoning  by  arsenic.     A 
difficulty,  attending  such  examinations,  remains  to  be  discussed 
under  the  metal  antimony,  a  substance  which  combines  with 
hydrogen,  as  arsenic  does,  to  form  a  gas  which  is  decomposed  by 
heat,  as  arseniuretted  hydrogen  is,  with  deposition  of  a  metallic 
mirror  which  cannot  be  distinguished  by  mere  inspection  from 

17 


258  CHLORIDE    OP    ARSENIC. 

that  of  arsenic.  Since  preparations  of  antimony  are  much  em- 
ployed as  medicines,  and  particularly  since  tartar-emetic,  a  salt 
containing  antimony,  is  often  administered  in  cases  of  poisoning, 
it  is  essential  to  find  means  of  dfstinguishing  between  compounds 
of  arsenic  and  the  analogous  compounds  of  antimony. 

335.  Chloride  of  Arsenic.     Only  one  chloride  of  arsenic  is 
known,  the  terchloride  (AsCl3),  corresponding  to  the  terchloride 
of  phosphorus.     No  quinquichloride  corresponding  to  the  quin- 
quichloride  of  phosphorus  is  known.     The  chloride  of  arsenic  is 
formed  by  passing  dry  chlorine  gas  over  finely  divided  metallic 
arsenic  placed  in  a  retort.     The  combination  is  usually  attended 
with  combustion,  and  the  heat  developed  is  sufficient  to  distil  the 
chloride  over  into  the  receiver.     It  may  also  be  made  by  dis- 
tilling a  mixture  of  metallic  arsenic  and  the  mercury  compound 
called  corrosive  sublimate,  in  accordance  with  the  following  equa- 
tion, in  which  Hg  stands  for  mercury  (Hydrargyrum)  :  — 

>  6  HgCl2  +  2As  =  3Hg2Cl2  -$AsCl3. 
Corrosive  Sublimate.  Calomel. 

Terchloride  of  arsenic  may  also  be  procured  by  distilling  arsenious 
acid  with  common  salt  and  sulphuric  acid.  Small  lumps  of  fused 
salt  should  be  added  from  time  to  time  to  a  mixture  of  arsenious 
acid  with  a  large  excess  of  sulphuric  acid  :  — 

As2O3  +  6&aCl  -f  6H2SO4  =  3H2O  -f  2AsCl3  -f  6NaHSO4 . 

336.  Terchloride  of  arsenic  is  a  dense,  colorless,  oily  liquid, 
whose  specific  gravity  is  2.205.     It  boils  at  132°,  producing  a 
vapor  whose  density  is  90.91.     It  evaporates  freely  at  the  ordi- 
nary  temperature,   producing  fumes   of  arsenious   acid.     It  is 
highly  poisonous.     The  chloride  is  decomposed  by  an  excess  of 
water  into  chlorhydric  acid  and  arsenious  acid,  just  as  the  chlo- 
ride of  phosphorus  is  resolved  by  water  into  chlorhydric  and 
phosphorous  acids ;  this  reaction  is  the  basis  of  the  best  deter- 
mination of  the  atomic  weight  of  arsenic. 

2AsCl8  +  3H2O  =.  6HC1  +  As2O3. 

All  the  chlorine  in  a  known  weight  of  chloride  of  arsenic  is  con- 
verted by  this  reaction  into  chlorhydric  acid ;  the  weight  of  the 
chlorine  contained  in  this  chlorhydric  acid  can  be  accurately  de- 


SULPHIDES    OF    ARSENIC.  259 

termined,  and  the  weight  of  the  arsenic  with  which  this  quantity 
of  chlorine  was  originally  combined  is  obtained  by  simple  sub- 
traction. The  proportions  by  weight  in  which  arsenic  and 
chlorine  combine  are  thus  determined.  In  terchloride  of  arsenic, 
as  in  terchloride  of  phosphorus,  three  volumes  of  chlorine  unite 
with  only  half  a  volume  of  arsenic  vapor  to  produce  two  vol- 
umes of  the  terchloride  vapor.  Indeed,  all  the  volatile  compounds 
of  arsenic  illustrate  the  fact,  already  mentioned  (§§  305,  309), 
that  the  unit-volume  weight,  or  specific  gravity,  of  arsenic  vapor 
is  the  double  of  its  atomic  weight. 

337.  Bromide  and  Iodide  of  Arsenic.     It  is  enough  to  say  of 
these  two  compounds  that  they  are  crystallizable  solids,  obtain- 
able by  the  direct  action  of  the  elements  upon  each  other,  and 
answering  to  the  formulae  AsBr3  and  AsI3  respectively. 

338.  Sulphides   of  Arsenic.     There   are    three   well-defined 
sulphides  of  arsenic,  corresponding  to  the  formulae  As2S2 ,  As2S8  r 
and  As2S5 .     The  first  two  occur  as  natural  minerals,  realgar  and 
orpiment,  and  may  also  be  obtained  in  the  free  state  by  artificial 
processes  ;  the  third  is  known  only  in  combination. 

339.  Bisulphide   of  Arsenic   (As2S2).      The  native   mineral 
realgar  has  this  composition.     The  compound  is  obtained  arti- 
ficially by  melting  arsenic  with  sulphur,  or  arsenic  with  orpiment 
(see  the  next  section),  or  sulphur  with  arsenious » acid,  in  such 
proportions  in  either  case  as  will  bring  together  those  parts  by 
weight  of  the  two  elements  which  the  above  formula  requires. 
The  commercial  product  is  a  brownish-red,  opaque  substance  of 
variable  composition,  generally  containing  free  arsenious  acid. 
Realgar  is  one  oi  the  ingredients  of  white  Indian  fire,  a  mixture 
of  24  parts  of  nitre,  7  of  sulphur,  and   2  of  realgar,  sometimes 
used  as  a  signal  light. 

340.  Tersulphide  of  Arsenic  (As2S3).     This  sulphide  occurs 
native  in  translucent  rhombic  prisms  of  a  yellow  color.     It  is 
obtained  artificially  by  passing  sulphydric  acid  gas  through  a 
solution  of  arsenious  acid  or  an  arsenite,  acidulated  with  chlor- 
hydric  acid;  the   sulphide  falls  as  a  bright  yellow  amorphous 
powder,  insoluble  in  water  and  dilute  acids.     It  melts  easily,  find 
burns  in  the  air  with  a  pale  blue  flame  ;  in  closed  vessels  it  may 
be  sublimed  without  change. 


260  SULPHARSENITES. 

Under  the  name  of  orpiment,  this  sulphide  is  used  as  an 
orange  pigment ;  a  mixture  of  the  sulphide  with  arsenious  acid, 
called  king's  yellow,  was  formerly  employed  as  a  yellow  pig- 
ment. This  impure  tersulphide  was  made  by  subliming  7  parts 
of  arsenious  acid  with  1  part  of  sulphur,  a  proportion  of  sulphur 
not  sufficient  to  convert  all  the  acid  into  tersulphide.  If  a  pat- 
tern be  printed  upon  cotton  cloth  with  a  preparation  containing 
arsenious  acid,  and  the  cloth  be  then  passed  through  water  con- 
taining sulphydric  acid,  orpiment  will  be  deposited  in  the  fibre 
of  the  cloth  and  the  pattern  will  be  brought  out  in  orange-yellow. 
We  have  already  seen  (Exp.  136)  that  the  tersulphide  of  arsenic 
yields  a  mirror  of  metallic  arsenic  when  heated  in  a  closed  tube 
with  a  mixture  of  carbonate  of  sodium  and  cyanide  of  potassium. 
The  sulphide  is  readily  dissolved  by  a  cold  solution  of  potash, 
soda,  or  ammonia,  the  oxygen  of  the  alkali  converting  part  of 
the  arsenic  in  the  sulphide  into  arsenious  acid,  while  the  alkali- 
metal  combines  with  the  sulphur  liberated  ;  this  alkaline  sulphide 
then  unites  with  the  undecomposed  portion  of  sulphide  of  arsenic 
to  form  a  sulphur-salt,  whose  composition  is  that  of  an  arsenite 
in  which  the  oxygen  has  been  replaced  by  sulphur. 

4As2S3  +  5K20  =  3(K2S,As2S8)  +  2K2O,As2O3.' 

If  an  acid  be  added  to  this  solution,  no  sulphuretted  hydrogen  is 
evolved,  as  is  generally  the  case  when  an  acid  is  brought  in  con- 
tact with  an  alkaline  sulphide,  but  the  sulphur  and  arsenic  re- 
combine  and  are  separated  as  tersulphide  of  arsenic. 

.3(K2S,As.,S3)  -j-  2K2O,As2O3  +  10HC1=  10KC1  +  5H2O  -f  4As2S3. 

341.  Sulpharsenites.  The  tersulphide  of  arsenic  unites  with 
basic  metallic  sulphides  in  three  different  proportions,  forming 
with  potassium,  for  example,  the  three  salts  3K2S,As2S3,  2K2S, 
As2S3,  and  K2S,As2S3 .  One  mode  of  preparing  a  sulpharsenite 
Iras  been  mentioned  in  the  last  section ;  another  method  is  to  dis- 
solve arsenious  acid  in  an  alkaline  sulphydrate,  in  which  case 
one-half  of  the  alkali  is  converted  into  arsenite :  — 
Dualistic :  2As,O3  -f  4KHS  =  E^As^  +  K2O,As2O3  +  2H2O. 
Empirical:  As.Of  -f  2KHS  =  KAsS2  -f  KAsO2  +  H2O . 

The  sulpharsenites  of  the  other  metals  are  mostly  obtained  from 


.  SULPHARSEN[ATES.  261 

the  sulpharsenites  of  sodium  and  potassium  by  the  method  of 
double  decomposition.  The  sulpharsenites  are  either  yellow  or 
red;  they  are  obscure  bodies,  of  no  practical  importance  at 
present.  They  illustrate,  however,  two  points  of  theoretical 
interest,  namely,  the  existence  of  sulphur-salts  which  bear  to 
sulphides  the  same  relation  which  oxygen  salts  bear  to  oxides, 
and  the  parallelism  of  composition  between  these  two  classes  of 
salts.  We  place  beside  each  other  the  empirical  formulas  of  the 
sulphur-salts  of  potassium  and  arsenic,  and  the  corresponding 
oxygen-salts  :  — 

Sulphur-salts.  Oxygen-salts. 

K3AsS3  K3As03 

K4As2S5  K4As205 

KAsS2  KAsO2 

342.  Quinquisulphide  of  Arsenic  (As2S5).  A  sulphide  of 
arsenic  corresponding  to  anhydrous  arsenic  acid  is  not  known  in 
the  free  state.  The  quinquisulphide  is  known  only  in  combina- 
tion with  sulphides  of  the  metals  in  sulphur-salts  called  sulph- 
arseniates.  When  a  solution  of  sulphide  of  sodium  is  digested 
with  some  tersulphide  of  arsenic  and  sulphur  enough  to  permit 
the  formation  of  the  quinquisulphide,  and  the  solution,  after  long 
standing,  is  concentrated  by  evaporation  and  then  cooled,  large 
colorless  crystals  of  snlpharseniate  of  sodium  are  obtained,  which 
are  not  changed  by  exposure  to  the  air.  The  crystals  have  the 
composition  indicated  by  the  formula  3Na2S,As2S5  -|-  15H2O. 
The  sulpharseniate,  2Na2S,As2S5 ,  may  be  prepared  by  saturating 
the  aqueous  solution  of  the  corresponding  oxygen-salt  2Na20, 
As2O5  with  sulphydric  acid  gas.  The  sulpharseniates  of  the 
alkali-metals,  and  a  few  others,  are  soluble  in  water,  but  the 
greater  number  of  sulpharseniates  are  'insoluble  ;  these  insoluble 
salts  are  prepared  by  mixing  a  solution  of  an  alkaline  sulph- 
arseniate with  a  solution  of  some  salt  of  the  metal  whose 
sulpharseniate  is  desired.  The  same  parallelism  is  observable 
between  sulpharseniates  and  arseniates  as  between  sulpharsenites 
and  arsenites. 


262  ANTIMONY. 

CHAPTER    XVIII. 

ANTIMONY. 

343.  Antimony  is  found  native,  both  alone  and  alloyed  with 
other  metals,  especially  with  arsenic,  nickel,  and  silver.     There 
exist  also  a  considerable  number  of  minerals  which  consist  of, 
or  contain  large  proportions  of  the  compounds  of  antimony  with 
oxygen  and  sulphur. 

344.  All  the   antimony  of  commerce  is  obtained  from    the 
mineral  tersulphide,  Sb2S8.     The  symbol  for  antimony  is  Sb, 
from  the  Latin  name  of  the  substance,  Stibium.     This  sulphide 
is  very  fusible,  melting  readily  in  the  flame  of  a  candle  ;  it  may, 
therefore,  be  separated  from  the  earthy  or  rocky  gangue,  in  which 
it  occurs,  by  simple  fusion  at  a  low  temperature.     The  metal  is 
obtained  from  the  sulphide  by  several  different  processes  :  1.  By 
adding  to  the  melted  sulphide,  iron  nails,  filings,  or  scraps ;  the 
iron  and  the  antimony  change  places. 

Sb2S3  +  3Fe  =  3FeS  +  2Sb . 

2.  By  roasting  the  sulphide  of  antimony,  reduced  to  a  coarse 
powder,  until  the  greater  part  of  the  sulphur  has  been  burnt  off 
and  the  antimony  converted  into  the  oxide ;  this  residue  is  then 
mixed  into  a  paste  with  water,  charcoal  powder,  and  carbonate 
of  sodium,  or  some  equivalent  reducing  flux,  and  heated  in  cov- 
ered crucibles  to  full  redness ;  the  metal  sinks  to  the  bottom  of 
the  crucible.  3.  By  fusing  together  a  mixture  of  sulphide  of 
antimony,  the  scales  which  fall  from  hot  iron  when  it  is  ham- 
mered (an  oxide  of  iron),  carbonate  of  sodium,  and  charcoal ; 
this  process  is  a  sort  of  combination  in  a  single  operation  of  the 
two  preceding  methods.  Since  the  sulphides  and  oxides  of  anti- 
mony and  the  metal  itself  are  somewhat  volatile  at  moderate 
temperatures,  it  has  thus  far  been  found  impossible  to  avoid  a 
considerable-  loss  of  metal  during  the  melting,  roasting,  and  re- 
ducing of  the  ore.  From  one-fifth  to  one-half  of  the  metal  is 
lost,  according  to  the  skill  and  care  of  the  workmen. 

345.  The  commonest  impurities  in  commercial  antimony  are 


PROPERTIES    OF   ANTIMONY.  263 

sulphur,  sodium,  arsenic,  lead,  iron,  and  copper.  These  impuri- 
ties injure  the  antimony  for  many  of  its  applications  in  the  arts, 
and  the  extensive  use  of  aritimonial  preparations  in  medicine, 
renders  the  removal  of  the  arsenic  a  point  of  particular  impor- 
tance. The  purification  may  be  effected  by  fusing  the  powdered 
metal,  first,  with  a  mixture  of  sulphide  of  antimony  and  carbon- 
ate of  sodium,  and,  secondly,  with  a  mixture  of  carbonate  of 
sodium  and  nitre.  These  fusions  may  be  several  times  repeated ; 
the  impurities  are  either  oxidized,  or  converted  into  sulphides, 
and  enter  the  slag.  Lead,  however,  cannot  be  got  rid  of  by 
these  processes ;  this  impurity  is  removed  by  fusing  the  anti- 
mony with  oxide  of  antimony  ;  the  lead  changes  places  with  the 
antimony  in  the  oxide  of  antimony,  and  is  converted  into  litharge. 
346.  Antimony  is  a  brittle  metal,  having  a  bluish- white  color, 
a  brilliant  lustre,  and  a  highly  crystalline  structure.  The  cakes 
of  the  commercial  metal  usually  present  upon  their  upper  sur- 
faces beautiful  stellate  or  fern-like  markings.  Like  phosphorus 
and  arsenic,  it  is  dimorphous,  crystallizing  both  in  rhombohedrons 
and  octohedrons.  The  specific  gravity  of  the  metal  is  from  6.60 
to  6.85  ;  its  atomic  weight  is  122.  For  a  metal,  it  is  a  poor  con- 
ductor of  heat  and  electricity.  At  450°  it  melts,  gives  off  vapors 
at  a  low  red  heat,  and  takes  fire  at  full  redness,  burning  brilliantly 
with  evolution  of  white  fumes  of  the  teroxide  (Sb2O3).  If  the 
antimony  is  contaminated  with  arsenic,  as  is  often  the  case,  a 
garlic  odor,  due  to  the  presence  of  this  impurity,  may  be  im- 
parted to  the  vapors. 

Exp.  138. — Melt  about  0.5  grm.  of  antimony  by -heating  it  on  a 
piece  of  charcoal  before  the  blow-pipe.  (See  Chapter  XX.)  Throw 
the  white,  glowing,  globule  into  the  middle  of  a  large  tray  made  of 
coarse  paper  ;  the  globule  bursts  into  a  multitude  of  small  beads  which 
fly  over  the  paper,  leaving  in  their  trail  a  white,  powdery  oxide. 

Exp.  139. —  Melt  a  second  small  fragment  of  antimony  upon  char- 
coal as  before,  but,  instead  of  throwing  it  from  the  coal,  allow  it  to  cool 
there  slowly.  The  globule  will,  in  this  case,  become  covered  with  an 
efflorescence  of  crystals  of  the  oxide. 

The  metal  is  not  oxidized  by  exposure  to  dry  or  moist  air  at 
ordinary  temperatures.  Nitric  acid  oxidizes  it  easily,  but  does 
not  dissolve  it;  the  insoluble  quinqui-oxide,  or  some  mixed 


264  ALLOYS    OF    ANTIMONY. 

oxide,  is  formed,  according  to  the  strength  of  the  acid  employed. 
Powdered  antimony  takes  fire  when  thrown  into  chlorine  gas, 
and  combines  very  energetically  with  bromine  and  iodine. 
When  finely  powdered,  it  is  dissolved  by  boiling  chlorhydric  acid, 
with  evolution  of  hydrogen  ;  if  a  little  nitric  acid  be  added  to  the 
chlorhydric,  the  metal  dissolves  easily,  to  form  a  solution  of 
terchloride  of  antimony  (SbCl3).  The  metal,  when  in  fine  pow- 
der, is  also  dissolved  readily  by  solutions  of  the  higher  sulphides 
of  sodium  and  potassium,  with  formation  of  sulphantimonites  and 
sulphantimoniates. 

347.  In  spite  of  the  strong  tendency  of  this  metal  to  crystal- 
lize, it  can  be  obtained  in  an  amorphous  form  by  the  electrolysis 
of  concentrated  antimonial  solutions.     This  amorphous  antimony 
always  contains,  however,  5  or  6  per  cent,  of  terchloride  of  an- 
timony and  a  trace  of  chlorhydric  acid;  whether  these  foreign 
substances  are  retained  mechanically,  or  not,  within  the  mass,  is 
not  clear.  /The  amorphous  metal  has  a  dark  steel  color,  a  smooth 
surface,  a  comparatively  soft  texture,  a  lustrous  amorphous  frac- 
ture and  a  specific  gravity  varying  from   5.74   to  5.83.     When 
gently  heated  or  sharply  struck,  the  amorphous  antimony  sud- 
denly manifests  a  great  heat,  the  temperature  rising  from  15°  to 
230°  and  upwards,  and  fumes  of  terchloride  of  antimony  are 
evolved.     After  undergoing  this  peculiar  change,  the  metal  ap- 
proximates to  the  crystalline  variety  in  structure,  density,  and 
color. 

348.  Antimony  enters  into  the  composition   of  several  very 
valuable  alloys.      Type  metal  is  an  alloy  of  lead  and  antimony, 
containing  abdut   20  per   cent,    of  antimony.     For   stereotype 
plates  ^  to  §L  of  tin  is  usually  added  to  this  alloy.     The  com- 
mon white  metallic  alloys  used  for  cheap  teapots,  spoons,  forks, 
and  like  utensils,  are  variously  compounded  of  brass,  tin,  lead, 
bismuth,  and  antimony ;  for  example,  a  superior  kind  of  pewter 
is  made  of  12  parts  tin,  1  part  antimony,  and  a  small  proportion 
of  copper ;  Britannia  metal  is  sometimes  compounded  of  equal 
parts  ®f  brass,  antimony,  tin,  bismuth,  and  lead.     The  value  of 
antimony  in  these  alloys  depends  upon  the  hardness  which  it 
communicates  to  the  compounds,  without  rendering  them  incon- 
veniently brittle. 


ANTIMONY    AND    HYDROGEN.  265 

"With  zinc,  antimony  forms  two  alloys  having  a  definite  crys- 
talline character.  The  alloy  containing  43  per  cent,  of  zinc 
crystallizes  in  silver-white  needle-like  prisms  ;  it  answers  to  the 
formula  Sb2Zn3.  The  alloy  containing  33  per  cent,  of  zinc 
crystallizes  in  broad  plates  presenting  no  similarity  to  the  form 
of  the  other  alloy ;  it  answers  to  the  formula  SbZn  .  These 
alloys,  especially  Sb2Zn3 ,  decompose  boiling  water  with  evolu- 
tion of  hydrogen.  The  crystals  of  these  two  alloys  are  obtained 
by  the  method  of  fusion  (§  194).  In  each  of  these  crystallized 
alloys,  the  crystalline  form  may  be  preserved,  although  the  pro- 
portions of  the  ingredients  may  vary  considerably  from  the  exact 
atomic  proportions  indicated  by  their  formulae  ;  thus,  needles  may 
be  obtained  in  which  the  actual  proportion  of  antimony  present 
varies  from  35.77  per  cent,  to  57.24  per  cent.,  the  exact  atomic 
proportion  being  55.7  per  cent.,  and  the  percentage  of  antimony 
in  the  plates  may  fall  as  low  as  64.57,  or  may  rise  as  high  as 
79.42,  although  65.07  per  cent,  is  the  true  atomic  proportion. 
Tljese  interesting  crystalline  alloys  strikingly  illustrate,  therefore, 
a  principle  of  wide  applicability,  namely,  that  a  definite  crystal- 
line form  is  not  necessarily  a  guaranty  of  an  unvarying  chemical 
composition. 

349.  Antimony  and  Hydrogen.  The  composition  of  the  gas- 
eous compound  of  these  two  elements  is  not  certainly  known, 
inasmuch  as  it  has  never  yet  been  prepared  free  from  admixed 
hydrogen.  When  a  solution  of  any  salt  of  antimony  is  poured 
into  a  mixture  of  zinc  and  dilute  acid  which  is  disengaging  hy- 
drogen, the  antimony  compound  is  decomposed ;  o^ne  portion  of 
the  antimony,  and  sometimes  even  the  whole  of  it,  is  deposited 
upon  the  zinc,  while  another  portion  usually  combines  with  the 
hydrogen,  and  assumes  the  gaseous  state.  When  this  compound 
gas  is  passed  through  a  solution  of  nitrate  of  silver,  a  precipitate 
is  produced  which  has  been  found  to  consist  of  antimonide  of 
silver,  SbAg3 .  Since  silver  is  a  metal  which  replaces  hydrogen, 
atom  for  atom,  it  is  a  natural  inference  that  the  gas  which  has 
produced  this  precipitate  must  have  the  composition  represented 
by  the  formula  SbH8 .  This  supposition  derives  strength  from 
the  analogous  formulae  of  the  well-known  gases  ammonia,  NH3 , 
phoaphuretted  hydrogen, PH3,  and  arseniuretted  hydrogen,  AsH3. 


266 


ANTIMONIURETTED    HYDROGEN. 


Antimoniuretted  hydrogen  is  a  colorless  gas,  inodorous  when 
free  from  arseniuretted  hydrogen,"  and  insoluble  in  water  and 
alkaline  liquids.  The  gas  is  decomposed  at  a  red  heat  into  anti- 
mony and  hydrogen ;  it  burns  in  the  air  with  a  whitish  flame, 
and  gives  off  a  white  smoke  of  teroxide  of  antimony ;  when  a 
bit  of  cold  porcelain  is  held  against  a  burning  jet  of  the  gas,  a 
sooty  spot  of  metallic  antimony  is  deposited  on  the  porcelain. 
These  reactions  resemble  those  of  arseniuretted  hydrogen  (§  308). 

Exp.  140. — Dissolve  0.5  grm.  of  tartar-emetic  (tartrate  of  antimony 
and  potassium)  in  about  30  c.  c.  of  water.  Add  a  few  centimetres  of 
the  solution  thus  obtained  to  the  bottle  of  the  apparatus  represented  in 
Figure  45,  in  which  hydrogen  is  already  being  generated  from  zinc 

FIG.  45. 


and  dilute  sulphuric  acid.  Antimoniuretted  hydrogen  will  be  produced, 
and  should  be  submitted  to  precisely  the  same  series  of  operations  by 
which  arseniuretted  hydrogen  was  examined.  (Exp.  137.)  By  heat- 
ing the  hard  glass  tube  at  a  and  b  successively,  two  mirrors  of  antimony 
will  be  obtained ;  when  the  gas  reaches  the  jet  without  decomposition, 
the  white  color  of  the  flame  will  be  observable ;  when  a  cold  piece  of 
porcelain  is  pressed  against  the  burning  jet,  spots  of  antimony  will  be 
deposited  thereon.  Preserve  these  mirrors  and  spots. 

Exp.  141.  —  Compare  together  the  spots  obtained  on  porcelain  from 
arseniuretted  hydrogen  (Exp.  137)  and  from  an timoniuretted  hydrogen 
(Exp.  140).  1.  The  arsenical  spot  has  a  metallic  lustre,  and  a  brown 
color,  when  thin;  the  stain  of  antimony  has  a  feeble  lustre,  and  is 
smoky-black.  2.  The  arsenical  stain  disappears  readily  on  the  appli- 
cation of  a  heat  below  redness ;  the  stain  of  antimony  is  volatile  only 
at  a  red  heat.  On  account  of  the  comparative  want  of  volatility,  which 
characterizes  •  the  antimony  deposit,  the  mirrors  of  antimony  obtained 
in  the  glass-tube  (Exp.  140)  are  always  deposited  nearer  the  heated 
portion,  of  the  tube  than  the  arsenic  mirrors  are.  3.  The  arsenical 
stains  may  be  distinguished,  moreover,  from  the  antimonial  stains  by 


TESTING    FOR    ANTIMONY.  267 

means  of  a  solution  of  "  chloride  of  soda  "  (a  mixture  of  hypoehlorite  of 
sodium  with  chloride  of  sodium,  prepared  by  mixing  a  solution  of 
chloride  of  lime  with  carbonate  of  sodium  in  excess,  and  filtering)  ; 
this  solution,  which  is  analogous  to,  and  indeed  may  be  replaced  by, 
a  solution  of  chloride  of  lime  (§  120),  immediately  dissolves  arsenical 
spots,  but  leaves  antimonial  spots  unaffected  for  a  long  time.  For  the 
application  of  this  test  it  is  convenient  to  produce  some  spots  on  the 
interior  of  a  concave  bit  of  porcelain.  4.  On  warming  an  arsenic 
spot  with  a  drop  or  two  of  aqua  regia,  and  evaporating  to  dryness,  a 
slight  residue  of  arsenic  acid  is  left,  recognizable  by  its  ready  solubility 
in  a  drop  of  water ;  if  to  this  drop  of  arsenic  acid  solution  a  drop  of 
ammonio-nitrate  of  silver  be  added,  a  brick-red  turbidity,  due  to  the 
formation  of  arseniate  of  silver,  will  be  produced.  This  ammonio- 
nitrate  of  silver  is  prepared  by  adding  exactly  ammonia  enough  to  a 
solution  of  nitrate  of  silver  to  redissolve  the  precipitate  which  forms  at 
first.  The  antimony  spot  treated  in  the  same  way  yields  no  such  red 
precipitate.  5.  An  antimony  stain  will  dissolve  readily  in  a  few  drops 
of  a  solution  of  sulphydrate  of  ammonium  which  has  become  yellow 
by  keeping ;  when  such  a  solution  is  evaporated  to  dryness,  a  bright 
orange  stain  remains.  The  arsenical  stain,  on  the  contrary,  is  not  per- 
ceptibly affected  by  the  yellow  sulphydrate  of  ammonium  solution, 
unless  heat  is  applied. 

Exp.  142. —  Connect  the  tube  of  hard  glass  in  which  two  arsenic 
mirrors  were  formed,  in  Exp.  137,  with  a  sulphuretted  hydrogen  gen- 
erator (Appendix,  §  19),  interposing  between  the  tube  and  the  gener- 
ator a  suitable  drying-tube  or  bottle  filled  with  chloride  of  calcium ; 
then  transmit  through  the  tube  a  very  slow  stream  of  sulphydric  acid 
gas,  and  heat  the  mirrors  with  a  small  gas-flame,  proceeding  from  the 
outer  to  the  inner  border  of  the  mirrors  in  the  direction  opposite  to  that 
of  the  gas  current. 

Repeat  the  same  process  with  the  tube  containing  the  antimony 
mirrors  obtained  in  Exp.  140. 

Yellow  tersulphide  of  arsenic  is  formed  in  one  case,  and  orange-red 
or  black  tersulphide  of  antimony  in  the 'other.  When  both  metals  are 
present  in  one  mirror,  the  two  sulphides  appear  side  by  side,  the  sul- 
phide of  arsenic  as  the  more  volatile  lying  invariably  beyond  the 
sulphide  of  antimony. 

Exp.  143.  —  Transmit  through  the  tube  which  contains  the  sulphide 
of  arsenic  a  stream  of  dry  chlorhydric  acid  gas  (§  95),  without  apply- 
ing heat ;  no  alteration  will  take  place  in  the  yellow  sulphide. 

Transmit  the  same  gas  through  the  tube  containing  the  sulphide  of 
antimony ;  the  sulphide  of  antimony  will  immediately  disappear.  If 


268  TEROXIDE    OF    ANTIMONY. 

the  gaseous  current  be  then  passed  through  some  water,  the  presence 
of  antimony  in  the  water  can  be  demonstrated  by  means  of  sulphydric 
acid  (§  210). 

When  both  sulphides  are  present  at  once,  the  chlorhydric  acid  at- 
tacks and  removes  the  sulphide  of  antimony,  while  the  sulphide  of 
arsenic  remains  behind.  A  drop  or  two  of  ammonia-water,  drawn  into 
the  tube,  will  then  dissolve  the  sulphide  of  arsenic.  This  solubility  in 
ammonia  distinguishes  the  yellow  sulphide  from  sulphur  itself,  with  which 
it  might  otherwise  be  sometimes  confounded. 

Antimony  and  Oxygen.  Antimony  forms  two  well-marked 
oxides  analogous  to  the  oxides  of  arsenic,  the  teroxide  or  anti- 
monious  acid,  Sb2O8  ,  and  the  quinqui-oxide  or  antimonic  acid, 
Sb2O5  ;  a  compound  of  these  two  oxides  Sb2O3,  Sb2O5  =  2Sb2O4  , 
is  sometimes  recognized  as  a  distinct  oxide  under  the  name  of 
the  quadroxide. 

350.  Teroxide  of  Antimony.  This  oxide  occurs  as  a  natural 
mineral,  called  White  Antimony  or  Antimony  Bloom.  Like 
arsenious  acid,  it  is  dimorphous,  crystallizing  in  rhombic  prisms 
belonging  to  the  trimetric  system  and  also  in  regular  octohedrons. 
The  artificial,  as  well  as  the  native  teroxide.  is  dimorphous. 
Antimonious  oxide  is  produced  when  antimony  is  burnt  in  the 
air,  or  heated  to  full  redness  in  imperfectly  covered  crucibles. 
The  easiest  mode  of  getting  it  is  to  heat  the  tersulphidc  (Sb2S3) 
with  strong  chlorhydric  acid  as  long  as  sulphydric  acid  continues 
to  escape,  and  pour  the  resulting  solution  of  the  terchloride 
(SbCl3)  into  a  boiling  solution  of  carbonate  of  sodium  :  — 

2SbCl3  +  SNajCOg  =  Sb2O3  +  GNaCl  +  3CO2. 


If  the  solution  of  carbonate  of  sodium  be  cold  or  only  warm, 
instead  of  boiling,  a  hydrate  of  the  teroxide  is  precipitated  : 
Sb2O8,H2O  =  2SbH02. 

Antimonious  oxide  is  white  or  grayish-white  at  ordinary  tem- 
peratures, but  turns  yellow  when  heated.  It  melts  below  a  red 
heat,  and  sublimes  when  raised  to  a  higher  temperature  in  a 
closed  vessel.  When  heated  in  the  air  it  is  partly  converted  into 
antimonic  acid.  It  is  readily  reduced,  to  the  metallic  state  by 
ignition  with  hydrogen,  charcoal,  or  potassium.  Teroxide  of 
antimony  dissolves  sparingly  in  water,  but  freely  in  strong  chlor- 
hydric acid  ;  it  also  dissolves  in  a  hot  solution  of  tartaric  acid,  or 


ANTIMONIATE    OF    ANTIMONY.  269 

of  acid  tartrate  of  potassium  (cream  of  tartar).  The  solution 
obtained  in  the  latter  case  contains  the  tartrate  of  antimony  and 
potassium  (C4H4KSbOr),  commonly  called  tartar-emetic.  Ordi- 
nary nitric  acid  does  not  dissolve  the  teroxide,  but  fumino-  nitric 
acid  and  fuming  sulphuric  acid  both  dissolve  it,  forming  solutions 
which  ultimately  deposit  shining  scales  of  a  nitrate  in  the  one 
case  and  a  sulphate  in  the  other. 

It  is  obvious,  from  these  facts,  that  this  oxide  of  antimony  dif- 
fers from  all  the  oxides  which  we  have  heretofore  studied,  in  that 
it  is  capable  of  reacting  upon  strong  acids  in  such  wise  as  to  form 
salts,  wherein  the  antimony  plays  very  much  the  same  part  which 
lead  plays  in  nitrate  of  lead  PbN2Oc  (Exp.  42),  or  calcium  in 
CaS04  (p.  74).  This  truth  is  expressed  in  technical  language 
when  we  say,  that  the  teroxide  of  antimony  is  capable  of  acting 
as  a  base  ;  the  oxides  heretofore  studied  have  either  been  acids, 
like  the  oxygen  acids  of  the  chlorine  and  sulphur  groups,  of 
nitrogen,  phosphorus,  and  arsenic,  or  they  have  been  indifferent 
bodies  not  inclined  to  form  definite,  stable  compounds  by  union 
with  other  substances. 

But  if,  on  the  one  hand,  teroxide  of  antimony  is  thus  some- 
times a  base,  on  the  other  it  also  acts  as  a  feeble  acid.  The  arti- 
ficial teroxide  dissolves  readily  in  solutions  of  caustic  potash  and 
soda,  forming  very  unstable  antimonites  which  are  decomposed 
by  boiling,  or  mere  evaporation.  These  antimonites  are  analo- 
gous to  the  arsenites,  but  it  is  to  be  observed  that  arsenious  acid 
is  not  only  a  stronger  acid  than  antimonious,  but  that,  unlike 
antimonious  oxide,  it  never  plays  the  part  of  a  base. 

351.  Antirnoniate  of  Antimony  or  Quadroxide  of  Antimony 
(Sb204).  This  oxide  occurs  as  a  native  mineral.  It  may  be 
prepared  artificially  by  heating  strongly  the  quinqui-oxide  (Sb2O5), 
or  by  roasting  the  teroxide  or  the  tersulphide,  or  by  treating  pow- 
dered antimony  with  an  excess  of  nitric  acid.  As  thus  prepared, 
it  is  white,  infusible,  and  unalterable  by  heat,  slightly  soluble 
in  water,  more  soluble  in  chlorhydric  acid,  and  easily  resolvable 
by  boiling  with  a  solution  of  cream  of  tartar  into  antimonious 
oxide  and  antimonic  acid.  2Sb2O4  =  Sb2O3,Sb2O6 .  The  oxide 
may,  therefore,  be  regarded  as  a  compound  of  the  two  other 
oxides  of  antimony,  but  it  is  sometimes  considered  a  distinct 


270  ANTIMONIC    ACID. 

oxide  on  the  ground  that  it  yields  by  fusion  with  caustic  potash, 
or  carbonate  of  potassium,  an  amorphous,  saline  mass  whose 
composition  answers  to  the  formula  K2O,Sb204  .  This  salt  itself, 
however,  if  such  it  be,  can  be  regarded  as  a  mixture  of  an  anti- 
monite  and  an  antimoniate  :  — 

2(K2O,Sb204)  =  K20,Sb203  +  K2O,Sb2O5. 


352.  Quinqui-oxide  of  Antimony  or  Antimonic 
This  compound  is  obtained  as  a  hydrate  :  1.  By  treating  anti- 
mony with  nitric  acid,  or  aqua  regia  containing  an  excess  of 
nitric  acid.  2.  By  decomposing  the  quinquichloride  of  anti- 
mony, SbCl5  (§  354),  with  water  :  — 

2SbCl5  +  5H20  =  Sb205  +  10HC1. 

3.  By  precipitating  a  solution  of  antimoniate  of  potassium 
(K2O,Sb2O5  -\-  5H2O)  with  a  strong  acid.  This  antimoniate  of 
potassium  is  obtained  by  fusing  one  part  of  antimony  with  four 
parts  of  nitre,  digesting  the  fused  mass  with  tepid  water  to  re- 
move nitrate  and  nitrite  of  potassium,  and  boiling  the  residue  for 
an  hour  or  two  with  water  ;  the  white  insoluble  mass  of  anhy- 
drous antimoniate  is  thereby  transformed  into  a  soluble  hydrate, 
and  the  solution,  treated  with  a  strong  acid,  yields  a  precipitate 
of  hydrated  antimonic  acid.  The  hydrated  antimoniate  of  po- 
tassium itself  is  a  gummy,  uncrystallizable  salt. 

The  hydrated  oxide,  obtained  by  either  of  these  methods,  gives 
off  its  water  at  a  heat  below  redness,  and  yields  anhydrous  anti- 
monic acid  as  a  yellowish,  tasteless  powder,  insoluble  in  water 
and  acids.  At  a  red  heat  it  gives  off  one-fifth  of  its  oxygen,  and 
is  converted  into  the  quadroxide.  A  boiling  solution  of  caustic 
potash  dissolves  the  oxide. 

The  hydrated  oxide  obtained  by  the  first  and  third  of  the 
above  methods  is  not  identical  with  that  which  results  from  the 
second  process.  The  product  of  the  first  and  third  methods  is 
called  antimonic  acid  ;  the  product  of  the  second  is  called  met- 
andmonic  acid,  a  term  derived  from  a  Greek  adverb  which  was 
used  in  composition  to  denote  a  change  of  place,  condition,  or 
quality.  Antimonic  acid  forms  normal  salts  of  the  composition 
M20,Sb2O5  and  acid  salts  containing  M20,2Sb2O5,  while  met- 


TERCHLORIDE    OF    ANTIMONY.  271 

antimonic  acid  forms  normal  salts  containing  2M2O,Sb205  and 
acid  salts  answering  to  the  formula  2M20,2Sb2O5;  the  acid 
metantimoniates  are  isomeric  (§  312)  with  the  normal  anti- 
moniates. 

The  metantimoniates  of  sodium,  potassium,  and  ammonium  are 
crystalline  ;  the  antimoniates  of  the  same  bases  are  gelatinous 
and  uncrystallizable.  The  antimoniates  and  metantimoniates 
of  sodium,  potassium,  and  ammonium,  are  the  only  ones  which 
are  readily  soluble  in  water;  all  other  antimoniates  and  met- 
antimoniates are  insoluble  or  sparingly  soluble.  Normal  anti- 
moniates correspond  with  normal  nitrates  :  — 

M2O,Sb2O5  =  M2Sb2O6  =  2MSbO3. 
M2O,N2O5    =  M2N206    =  2MNO3  . 

Normal  metantimoniates  are  analogous  to  pyrophosphates  :  — 

2M2O,Sb205  =  M4Sb2O7. 
2MAFA    =  M4P207. 

Antimony  and  Chlorine.  Antimony  forms  two  chlorides,  a 
terchloride,  SbCls,  and  a  quinquichloride  SbCl5,  both  of  which 
have  their  analogues  in  the  chlorides  of  phosphorus  already 
studied  ;  the  terchloride  is  also  comparable  with  the  chloride  of 
arsenic.  The  metal  unites  directly  with  chlorine  on  contact  (Exp. 
57),  and  the  two  chlorides  are  bodies  of  considerable  stability. 

353.  Terchloride  of  Antimony  (SbCl3).  This  chloride  is 
formed  when  chlorine  gas  is  passed  slowly  through  a  tube  con- 
taining antimony  in  large  excess.  It  may  also  be  prepared  by 
distilling  3  parts  of  antimony  with  8  parts  of  corrosive  sublimate 
(chloride  of  mercury),  or  2  parts  of  the  tersulphide  of  antimony 
with  4.6  parts  of  corrosive  sublimate:  — 


2Sb  -f  3HgCl2  =  2SbCl3  -j-  3Hg  :  Sb2S8  +  3HgCl2=  2SbCl3+  3HgS  . 

The  easiest  method  of  preparing  this  chloride  is  to  dissolve  the 
tersulphide  of  antimony  in  strong,  hot  chlorhydric  acid,  or  me- 
tallic antimony  in  the  same  acid  to  which  a  little  nitric  acid  has 
been  added  ;  the  resulting  liquid,  in  either  case,  after  evapora- 
tion to  an  oily  consistency,  should  be  distilled. 

At  the   ordinary  temperature,  terchloride   of  antimony  is  a 


272  QUINQUICHLORIDE    OF    ANTIMONY. 

translucent  yellowish  substance  of  fatty  consistency,  whence  its 
popular  name,  butter  of  antimony.  It  melts  at  72°  and  boils  at 
about  200°,  fumes  slightly  in  the  air,  is  deliquescent  and  highly 
corrosive.  When  thrown  into  water,  it  is  decomposed  into  chlor- 
hydric  acid  and  teroxide  of  antimony,  which,  however,  remains 
united  with  a  portion  of  the  chloride,  forming  a  white  powder 
which  contains  antimony,  chlorine,  and  oxygen,  but  is  somewhat 
variable  in  composition.  This  white  precipitate  is  redissolved 
by  excess  of  chlorhydric  acid,  and  the  solution  thus  obtained  is 
the  most  convenient  that  can  be  used  for  exhibiting  the  reactions 
of  antimony.  The  addition  of  tartaric  acid  to  this  solution  pre- 
vents its  decomposition  by  water. 

Exp.  144. —  In  a  flask  of  about  200  c.  c.  capacity,  heat  gently  0.5 
grm.  of  finely  powdered  antimony  with  30  c.  c.  of  strong  chlorhydric 
acid,  to  which  10  drops  of  nitric  acid  have  been  added.  When  com- 
plete solution  has  been  effected,  pour  a  little  of  the  chloride  into  water, 
to  demonstrate  the  decomposition  just  referred  to.  Evaporate  the  rest 
of  the  solution  to  the  consistency  of  a  thick  syrup ;  it  is  the  butter  of 
antimony. 

The  anhydrous  terchloride  combines  with  the  chlorides  of 
sodium,  potassium,  and  ammonium,  and  certain  other  chlorides, 
to  produce  crystalline  saline  compounds,  analogous  in  composi- 
tion to  those  oxygen  and  sulphur  compounds  to  which  the  term 
salt  is  commonly  applied. 

354.  Quinqui-chloride  of  Antimony  (SbCl5).  This  compound 
is  formed,  with  brilliant  combustion,  when  finely  powdered  anti- 
mony is  thrown  into  chlorine  gas  (Exp.  57).  It  may  also  be 
prepared  by  passing  dry  chlorine  over  warm,  powdered  antimony, 
or  over  the  terchloride. 

Exp.  145.  —  Fill  a  hard  glass-tube,  No.  2,  150  c.  m.  long  with 
coarsely  powdered  antimony,  and  fit  one  end  of  the  tube  so  charged 
into  a  tubulature  of  a  two-necked  glass  receiver,  the  other  neck  of 
which  is  connected  with  a  source  of  dry  chlorine.  Support  the  long 
tube  at  an  angle  of  10°  or  15°  with  the  table,  so  that  its  open  end  shall 
be  some  20  c.  m.  higher  than  the  end  which  enters  the  receiver. 
Keeping  the  tube  warm  throughout  its  whole  extent,  pass  chlorine 
slowly  and  continuously  into  the  receiver.  Combination  takes  place 
in  the  tube  and  the  product  flows  back  into  the  receiver,  where  it  re- 
mains in  contact  with  chlorine  ;  the  long  layer  of  antimony-  prevent:; 


TERSULPHIDE  OF  ANTIMONY.  273 

the  escape  of  any  free  chlorine.     Preserve  the  product  in  a  glass- 
stoppered  bottle. 

The  quinquichloride  is  a  colorless,  or  yellowish  liquid,  which 
is  very  volatile  and  emits  suffocating  fumes.  Water  in  small 
proportion  forms  with  it  white  deliquescent  crystals,  but  in  large 
quantity  water  decomposes  the  chloride  into  chlorhydric  and 
antimonic  acids. 

355.  We  are  familiar  with  nitric  acid  (N2O5)  as  an  oxidizing 
agent,  as  a  substance  which  readily  yields  some  of  its  oxygen  to 
other  bodies  with  which  it  is  brought  in  contact ;  in  a  perfectly 
analogous   sense,  the  quinquichloride  of  antimony  and  its  ana- 
logue the   quinquichloride   of  phosphorus,  may  be   said  to  be 
chloridizing  agents  of  great  power,  for  they  readily  impart  chlo- 
rine to  other  substances.     These  two  chlorides  are  much  used  in 
organic  chemistry  for  preparing   chlorine   compounds ;  thus,  for 
example,  the  compound  of  carbon  and  hydrogen  called  eihylene 
or  defiant  gas,  C2H4,  is   converted  by  passing  through  boiling 
quinquichloride   of  antimony  into  an  oily  bichloride,  C2H4Cl2r 
known  as  Dutch  liquid.     The  quinquichloride  acts  as  a  carrier 
of  free  chlorine,  being  itself  reduced  to  the  terchloride. 

Terbromide  and  Teriodide  of  Antimony  (SbBr3  and  SbI3). 
It  is  enough  to  mention  the  existence  of  these  compounds, 
formed  by  the  direct  union  of  the  elements. 

Antimony  and  Sulphur.  Antimony  forms  two  sulphides, 
Sb2S3  and  SbL,S5 ,  corresponding  to  antimonious  oxide  and  anti- 
monic acid,  and  possibly  an  intermediate  sulphide  corresponding 
to  the  quadroxide. 

356.  Ter sulphide  of  Antimony  (Sb2S3).     This  compound  ex- 
ists in  the  crystalline  and  in  the  amorphous  state.     Crystallized 
tersulphide  of  antimony  is  a  natural  mineral  called  gray  antimony 
or  antimony-glance.     It  is  the  source  of  all  the  antimony  and 
antimony  compounds   of  commerce.     The  mineral   has  a  lead- 
gray  color  and  a  metallic  lustre ;  it  is  friable  and  very  fusible, 
melting  even  in  the  flame  of  a  candle.     At  a  white  heat  it  may 
be  distilled  unchanged  in   closed  vessels,  but  by  roasting  in  the 
open  air  it  is  converted  into  a  fusible  mixture  of  teroxide  and 
tersulphide   of   antimony.     This   oxysulphide,  after  it  has  been 
fused,  constitutes  the  commercial  glass  of  antimony  which  con- 

18 


274  TERSULPHIDE  OF  ANTIMONY. 

tains  about  8  parts  of  the  teroxide  to  1  part  of  the  tersulphide  ; 
the  greater  the  proportion  of  sulphide,  the  darker  the  tint  of  the 
glass. 

The  native  tersulphide  is  seldom  pure,  being  generally  con- 
taminated with  lead,  copper,  iron,  and  arsenic.  To  obtain  pure 
crystallized  tersulphide  of  antimony,  it  is  best  to  prepare  it  arti- 
ficially by  fusing  pure  metallic  antimony  with  sulphur  in  the 
required  proportions  by  weight.  The  materials  must  be  finely 
powdered  and  intimately  mixed,  and  the  mixture  is  thrown  by 
small  portions  into  a  heated  crucible.  The  reactions  of  crys- 
tallized sulphide  of  antimony  are  the  same  as  those  of  the 
amorphous  sulphide,  to  be  presently  described,  but  they  take 
place  less  quickly  on  account  of  the  greater  cohesion  of  the 
mass. 

Amorphous  tersulphide  of  antimony  can  be  procured  by  sev- 
eral processes,  from  which  we  may  select  the  two  simplest :  1.  The 
native  gray  tersulphide  is  changed  into  the  amorphous  variety 
by  keeping  it  in  the  fused  state  for  a  considerable  time,  and  then 
cooling  it  very  suddenly  by  throwing  the  vessel  in  which  it  has 
been  melted  into  a  large  quantity  of  cold  water.  The  product 
is  an  amorphous  mass,  having  a  conchoidal  fracture,  and  a  less 
specific  gravity,  but  a  greater  hardness  than  that  of  the  crystal- 
line variety.  Its  color,  in  thin  pieces,  is  hyacinth-red  ;  in  the  state 
of  powder,  orange-brown.  2.  When  sulphydric  acid  gas  is  passed 
into  an  acidulated  solution  of  an  antimony-salt,  that  of  tartar- 
emetic,  for  example,  a  bright  orange-red  precipitate  of  a  hy- 
drated  tersulphide  of  antimony  is  formed,  which  may  be  ren- 
dered anhydrous  at  a  moderate  heat  without  losing  its  red  color. 

Exp.  146.  —  Dissolve  2  grms.  of  tartar  emetic  in  50  c.  c.  of  water 
and  add  to  the  solution  a  few  drops  of  acetic  acid ;  pass  a  slow  current 
of  sulphuretted  hydrogen,  from  a  self-regulating  generator  (Appendix, 
§  19),  through  this  solution  for  ten  minutes.  The  precipitate  is  the 
hydrated  tersulphide  of  antimony.  Collect  this  precipitate  upon  a 
filter  and  wash  it. 

Exp.  147.  —  Pour  a  dilute,  cold  solution  of  caustic  soda  upon  th( 
washed  precipitate  of  the  last  experiment  as  it  lies  upon  the  filter,  ai 
collect  the  filtrate  in  a  test-tube ;  if  the  whole  precipitate  does  m 
shortly  redissolve,  pour  the  filtrate  a  second  time  upon  the  undissolvt 
precipitate  in  the  filter,  or  use  an  additional  quantity  of  soda-lye,  if 


RED    SULPHIDE    OF    ANTIMONY.  275 

necessary.     There  is  produced  a  mixture  of  sulphantimonite  of  sodium 
and  teroxide  of  antimony,  which  is  soluble  in  the  excess  of  soda-lye. 
2Sb2S3  +  6NaHO  —  3Na2S,Sb2S3  -j-  Sb2O3  -\-  3H2O . 

Exp.  148. —  Pour  the  clear  alkaline  solution,  ob lain ed  in  the  last 
experiment,  into  two  or  three  times  its  bulk  of  dilute  chlorhydric  acid. 
The  whole  of  the  antimony  will  be  thrown  down  again  as  tersulphide, 
without  any  evolution  of  sulphuretted  hydrogen,  because  the  gas 
evolved  from  the  sulphantimouite  is  exactly  absorbed  by  the  dissolved 
teroxide :  — 

SNa^SboSs  +  6HC1  =  GNaCl  +  Sb2S3  +  3H2S. 
Sb203  +  3H2S  =  Sb2S3    +  3H20. 

When  hydrated  amorphous  tersulphide  of  antimony  is  boiled 
with  a  solution  of  carbonate  of  sodium,  it  is  dissolved  ;  the  filtered 
liquid,  on  cooling,  deposits  a  reddish-brown  substance,  formerly 
much  used  in  medicine,  and  known  as  kermes  mineral.  This  sub- 
stance is  not  a  definite  compound,  but  is  a  variable  mixture  of 
tersulphide  and  teroxide  of  antimony,  the  latter  being  combined 
with  a  small  portion  of  the  alkali.  Minute  crystals  of  teroxide 
of  antimony  have  been  recognized  in  this  mixture  by  micro- 
scopic examination.  A  solution  of  cream  of  tartar  will  dissolve 
out  the  teroxide,  leaving  the  tersulphide.  On  acidulating  the 
cold  filtered  liquid,  after  the  deposition  of  the  kermes,  with  chlor- 
hydric acid,  a  particularly  bright  orange  precipitate  of  sulphide 
of  antimony,  known  as  the  golden  sulphide,  is  precipitated.  Ar- 
tificial sulphide  of  antimony  can,  indeed,  be  precipitated  of 
almost  any  color  between  a  light  orange  and  a  blackish  brown. 
A  vermilion-red  sulphide  has  found  some  applications  as  a  paint. 

Exp.  149. —  Place  in  a  porcelain  dish  10  grms.  of  a  solution  of 
chloride  of  antimony  of  about  1.35  specific  gravity ;  add  to  this  chloride 
a  cold  solution  of  hyposulphite  of  sodium  made  by  dissolving  15  grms. 
of  the  salt  in  30  c.  c.  of  water;  heat  the  dish  very  slowly,  and  stir  its 
contents  continually,  so  long  as  any  precipitate  separates  from  the 
liquid.  The  sulphide  of  antimony  is  thrown  down  of  a  brilliant  red 
color.  The  color  of  the  precipitate  is  darker  in  proportion  as  the  tem- 
perature of  the  mixture  is  higher ;  when,  therefore,  a  fine  red  is  pro- 
duced, the  lamp  may  be  withdrawn,  in  order  to  prevent  the  color  from 
growing  brown.  The  precipitate  is  collected  on  a  filter,  drained  thor- 
oughly, and  then  washed,  first  with  dilute  acetic  acid  and  subsequently 
with  water. 


276  QUINQUISULPHIDE    OF    ANTIMONY. 

Sulphantimonite  solutions,  similar  to  those  prepared  in  the 
wet  way,  may  be  obtained  by  fusing  tersulphide  of  antimony 
with  dry  caustic  soda  or  potash,  or  with  the  carbonates  of  sodium 
or  potassium,  and  boiling  the  residue  with  water.  During  the 
exposure  to  air  of  hot  sulphantimonite  solutions,  a  process  of 
oxidation  takes  place,  whereby  the  sulphur  set  free  from  one  por- 
tion of  the  salt  converts  another  portion  into  the  state  of  sulph- 
antimoniate,  so  that  on  acidulation  some  quinquisulphide  of  an- 
timony is  precipitated  along  with  the  tersulphide. 

Like  the  tersulphide  of  arsenic,  the  tersulphide  of  antimony 
is  a  sulphur-acid  which  unites  with  basic  metallic  sulphides  to 
form  sulphur-salts.  The  artificial  sulphantimonites  of  the  alka- 
lies have  been  alluded  to  above ;  there  are  many  natural  minerals 
of  analogous  composition ;  among  such  may  be  mentioned  Miar- 
gyrite,  Ag2S,Sb2S3,  Bournonite,  2Pb2S,Cu2S,Sb2S3 ,  and  Ber- 
thierite,  3FeS,2Sb2S3. 

357.  Quinquisulphide  of  Antimony  (Sb2S5).  This  compound, 
which  is  not  native,  is  made  by  passing  sulphuretted  hydrogen 
through  quinquichloride  of  antimony  dissolved  in  tartaric  acid. 
It  may  also  be  prepared  by  acidulating  the  solution  of  the  sulph- 
antimoniate  of  sodium,  3Na2S,Sb2S5 :  — 

3Na2S,Sb2S5  +  6HC1  =  GNaCl  +  Sb2S5  +  3H2S . 

The  quinquisulphide  is  an  orange-yellow,  anhydrous  amorphous 
powder,  and  is  chiefly  remarkable  for  the  facility  with  which  it 
unites  with  the  sulphides  of  the  metals  to  form  sulphantimo- 
niates  ;  on  this  account  this  sulphide  is  often  called  sulphantimonic 
acid.  It  is  readily  soluble  in  the  sulphides,  sulphydrates  and 
hydrated  oxides  of  sodium,  potassium,  and  ammonium. 

The  sulphantimoniates  have  generally  the  composition  repre- 
sented by  the  general  formula  3M2S,Sb2S5  =  2M3SbS4 ,  analo- 
gous to  that  of  the  tribasic  phosphates  3M2O,P2O5  =  2M3PO4 . 
The  sulphantimoniates  of  sodium^  potassium,  and  ammonium  are 
very  soluble  in  water  and  crystallize  with  facility ;  those  of  the 
heavy  metals  are  insoluble.  The  latter  are  precipitated  by  adding 
solutions  of  metallic  salts  to  a  solution  of  the  sulphantimoniate 
of  sodium,  keeping  the  latter  in  excess. 

Exp.  150. —  In  a  wide-mouthed  bottle,  or  other  vessel  which  can_be 


BISMUTH.  277 

closed,  mix  thoroughly  22  grms.  of  elutriated  tersulphide  of  antimony, 
26  grnis.  of  crystallized  carbonate  of  sodium,  2  grms.  of  flowers  of  sul- 
phur, 10  grms.  of  quick-lime,  slaked  after  weighing,  and  40  c.  c.  of 
water.  Let  the  mixture  digest  at  the  ordinary  temperature  for  24 
hours,  with  frequent  stirring;  then  filter  the  liquid,  wash  the  residue 
several  times  with  water,  and  evaporate  the  nitrate  and  the  wash-water 
in  a  porcelain  dish  or  clean  iron  pan,  until  a  sample  yields  crystals  on 
cooling.  The  formation  of  the  salt  is  accelerated  by  boiling.  The 
whole  is  then  left  to  cool ;  the  deposited  crystals  are  washed  two  or 
three  times  with  cold  water  and  dried  under  a  bell-jar  over  a  dish  of 
an  absorbent  like  quick-lime  or  oil  of  vitriol.  The  salt  is  sulphanti- 
moniate  of  sodium  Na3SbS4  -{-  9H2O ;  it  forms  transparent,  colorless, 
or  pale  yellow,  regular  tetrahedrons. 


CHAPTER    XIX. 

BISMUTH THE  NITROGEN  GROUP. 

358.  The  metal  bismuth  is  found  chiefly  in  the  metallic  state, 
but  also  occurs  in  combination  with  sulphur,  oxygen,  and  tel- 
lurium. It  is  prepared  for  the  arts  almost  exclusively  from 
native  bismuth.  The  process  of  extracting  the  metal  from  the 
gneiss  and  clay-slate  in  which  it  generally  occurs  is  very  simple, 
the  mineral  being  merely  heated  in  closed  iron  tubes,  inclined 
in  such  a  manner  that  the  melted  bismuth  runs  down  into  earthen 
pots',  which  are  heated  sufficiently  to  keep  the  metal  in  a  state  of 
fusion.  It  is  then  ladled  out  and  run  into  moulds.  The  impure 
metal,  which  often  contains  sulphur,  arsenic,  copper,  nickel,  and 
iron,  may  be  purified  by  melting,  it  two  or  three  times  with  about 
T\j  its  weight  of  nitre. 

Bismuth  is  a  tolerably  hard,  brittle  metal,  of  a  grayish-white 
color  with  a  reddish  tinge.  When  pure,  it  crystallizes  more 
readily  than  any  other  metal;  by  the  method  of  fusion  (§  188) 
it  may  be  obtained  in  most  beautiful  crystals,  made  highly  irri- 
descent  by  the  thin  film  of  oxide  which  forms  on  their  surfaces 
while  they  are  still  hot ;  these  crystals  look  like  cubes,  but  are 


278  TEROXIDE    OF   BISMUTH. 

really  rhombohedrons.  Bismuth,  like  phosphorus,  arsenic,  and 
antimony,  is  dimorphous,  presenting  forms  both  of  the  mono- 
metric  and  hexagonal  systems.  The  metal  melts  at  264°  and 
expands  about  -^  in  solidifying ;  Jience  its  specific  gravity  is 
greater  in  the  liquid  than  in  the  solid  state.  When  the  metal  is 
subjected  to  strong  pressure,  its  specific  gravity,  normally  9.83, 
has  been  said  to  become  less.  At  a  high  temperature  bismuth 
may  be  distilled.  Of  all  metals  it  exhibits  in  the  highest  degree 
the  phenomena  of  diamagnetism.  Its  atomic  weight  is  210. 

Exposed  to  dry  or  moist  air  the  metal  does  not  alter,  but  when 
exposed  to  the  combined  action  of  air  and  water,  it  is  super- 
ficially oxidized.  "When  heated  in  the  air,  it  burns  with  a  bluish 
flame,  forming  yellow  fumes.  Strong  chlorhydric  acid  acts  on  it 
with  difficulty ;  sulphuric  acid  attacks  it  only  when  hot  and  con- 
centrated ;  nitric  acid  attacks  it  immediately,  and  effects  complete 
solution,  with  formation  of  nitrate  of  bismuth  and  evolution  of 
nitric  oxide. 

359.  Bismuth  promotes  the  fusibility  of  metals  with  which  it 
is  alloyed  to  an  extraordinary  extent.     The  most  remarkable 
alloy  of  bismuth  is  that  known  as  "  fusible  metal."     When  com- 
posed of  1  part  of  lead,  1  part  of  tin,  and  2  parts  of  bismuth,  this 
alloy  melts  at  93°.75.     Solid  fusible  metal,  like  liquid  water, 
undergoes  an  anomalous  expansion  by  heat.     It  expands  regu- 
larly from  0°  to  35°,  then  contracts  gradually  as  the  temperature 
rises  to  55°,  at  which  point  it  is  less  bulky  than  at  0°.  again  ex- 
pands rapidly  to  80°,  and  beyond  that  temperature  continues 
expanding  regularly  up  to  its  melting  point.     On  account  of  this 
property  of  expanding  as  it  cools  while  still  in  the  soft  state,  the 
alloy  is  much  used  for  taking  impressions  from  dies ;  the  finest 
and  faintest  lines  are  reproduced  with  great  accuracy.     It  is  ob- 
vious that  an  alloy  possessing  such  properties  must  be  something 
more  than  a  mere  mixture  of  the  constituent  metals. 

No  compound  of  bismuth  and  hydrogen  is  as  yet  known. 

Bismuth  and  Oxygen.  Bismuth  forms  two  principal  oxides, 
a  teroxide  (Bi2O3)  and  an  acid  oxide  (Bi2O5)  ;  besides  these  there 
is  an  intermediate  oxide  (Bi204)  which  may  be  represented  as  a 
compound  of  the  other  two  Bi2O3 ,  Bi2O5  =  2Bi2O4 . 

360.  Teroxide  of  Bismuth   (Bi2O3).     This  oxide  is  formed 


BISMUTHIC    ACID.  279 

when  the  metal  is  roasted  in  the  air,  but  is  best  obtained  by 
gently  igniting  the  nitrate  or  subnitrate.  It  is  a  pale-yellow, 
insoluble  powder,  which  melts  at  a  red  heat,  and  is  easily  reduced 
to  the  metallic  state  by  heating  it  with  charcoal.  A  white  hy- 
drate of  this  oxide,  Bi2O8,H2O  =  2BiH02,  may  be  precipitated 
from  a  salt  of  bismuth  by  an  excess  of  ammonia.  Teroxide  of 
bismuth  combines  with  acids  to  form  the  bismuth  salts ;  in  the 
normal  salts  one  atom  of  bismuth  replaces  three  atoms  of 
hydrogen,  thus :  — 

[Bi2O3  +  3(H2N2O6)  =  3H2O  +  Bi23(N2O6) . 

Basic  salts  of  bismuth  are  also  known,  in  which  a  larger  propor- 
tion of  bismuth  is  present.  Some  of  the  normal  salts  crystallize 
well  from  acid  solutions,  but  they  cannot  exist  in  solution  unless 
an  excess  of  acid  is  present.  On  diluting  solutions  of  the  nor- 
mal salts  with  water,  insoluble  basic  salts  are  precipitated ;  this 
reaction  recalls  the  behavior  of  antimony  solutions.  The  nitrate 
of  bismuth,  Bi23(N2Oc)  -f-  9H2O,  is  the  commonest  soluble  salt 
of  bismuth ;  it  is  procured  by  dissolving  the  metal  in  nitric  acid. 
To  the  basic  nitrate,  which  is  precipitated  when  water  is  added 
to  the  solution  of  the  normal  nitrate,  the  formula  5Bi203,4N2O,5 
-f-  9H2O  has  been  assigned.  Bismuth  salts  are  heavy  com- 
pounds, which  are  colorless  unless  the  acid  be  colored ;  they  are 
poisonous  in  large  doses. 

361.  Quinqui-oxide  of  Bismuth,  or  Bismuthic  Acid  (Bi2O5). 
When  chlorine  gas  is  passed  through  a  concentrated  solution  of 
potash  holding  teroxide  of  bismuth  in  suspension,  a  blood-red 
solution  is  obtained,  from  which  there  soon  separates  a  red  pre- 
cipitate ;  this  substance  is  a  mixture  of  hydrated  bismuthic  acid 
and  teroxide  of  bismuth.  Cold  dilute  nitric  acid  dissolves  the 
oxide,  but  does  not  attack  the  .acid.  The  hydrated  acid  gives  up 
its  water  at  a  temperature  of  130°,  and  the  anhydrous  quinqui- 
oxide  remains  as  a  brown  powder,  which,  in  contact  with  acids, 
parts  very  readily  with  a  portion  of  its  oxygen  and  falls  back  to 
the  state  of  teroxide.  The  anhydrous  quinqui-oxide  may  be  also 
converted  by  a  gentle  heat  into  the  intermediate  oxide  Bi2O4 . 
Bismutliic  acid  combines  with  caustic  soda  and  potash,  but  the 
compounds  are  decomposed  by  mere  washing.  The  bismuthates 


280  CHLORIDE    OF    BISMUTH. 

are  little  known  and  are  of  interest  only  in  so  far  as  they  go  to 
show  the  feeble  acid  character  of  the  quinqui-oxide. 

362.  Ter chloride  of  Bismuth  (BiCl3).     This  compound  may 
be  obtained  by  heating  bismuth  in  chlorine,  or  by  mixing  the 
metal  in  fine  powder  with  twice  its  weight  of  corrosive  sublimate 
(chloride  of  mercury)  and  distilling.     The  same  substance  is 
produced  when  bismuth  is    dissolved   in   aqua  regia,  and  the 
excess  of  acid  evaporated.     It  is  a  very  fusible,  volatile,  deli- 
quescent body,  which  was  called  butter  of  bismuth,  from  its  re- 
semblance to  the  butter  of  antimony,  long  before  the  relationship 
now  established  between  bismuth  and  antimony  had  been  recog- 
nized.    The  terchloride  is  decomposed  by  water  into  chlorhydric 
acid,  which  dissolves  a  portion  of  the  chloride,  and  a  precipitate 
containing  bismuth,  chlorine,  and  oxygen,  and  called  oxy chloride 
of  bismuth. 

3BiCl3  +  4H20  =  6HC1  +  Bi3Cl3O3  +  H2O . 

The  same  oxychloride  is  precipitated  when  a  solution  of  nitrate 
of  bismuth  is  poured  into  a  solution  of  common  salt.  It  is  used 
as  a  pigment,  and  is  known  as  "  pearl-white."  It  may  be  distin- 
guished from  the  analogous  oxychloride  of  antimony  by  the  fact 
that  it  is  insoluble  in  tartaric  acid  and  in  potash,  both  of  which 
dissolve  the  antimony  compound.  Terchloride  of  bismuth  forms 
crystallizable  double  salts  with  the  chlorides  of  sodium,  potas- 
sium, and  ammonium.  These  chlorine  salts  are  analogous  in 
composition  to,  and  isomorphous  with,  the  corresponding  double 
chlorides  of  antimony. 

363.  Ter -sulphide  of  Bismuth   (Bi2S3).     Bismuth   glance,   a 
somewhat  rare  mineral  which  occurs  in  acicular  prisms  isomor- 
phous with  the  native  tersulphide  of  antimony,  is  a  tersulphide 
of  bismuth.     The  same  compound  may  be  formed  artificially  by 
fusing  the  pulverized  metal  with  one-third  its  weight  of  sulphur. 
Heated  in  close  vessels,  the  sulphide  evolves  sulphur ;  heated 
with  access  of  air,  it  forms  teroxide  of  bismuth  and  sulphurous 
acid.     The  tersulphide  is  also  obtained  as  a  brown-black  precipi- 
tate when  sulphuretted  hydrogen  is  passed  through  a  solution  of 
a  bismuth  salt. 

Exp.  151. —  Dissolve  2  grms.  of  powdered  bismuth  in  aqua  regia  ; 


THE    NITROGEN    GROUP.  281 

the  process  is  best  performed  in  a  small  flask,  —  the  aqua  regia  should 
be  added  by  small  portions  at  a  time,  so  as  to  avoid  an  unnecessary 
excess  of  acid,  and  a  gentle  heat  should  be  applied.  When  complete 
solution  has  been  effected,  pour  the  acid  solution,  drop  by  drop,  into  a 
comparatively  considerable  bulk  of  water,  and  observe  the  precipitation 
of  the  white  oxychloride  of  bismuth.  Filter  off  the  precipitate,  wash 
it  upon  the  filter  with  water,  and  dry  it ;  it  is  pearl-white.  Through 
the  filtrate  from  this  precipitate  pass  a  slow  stream  of  sulphuretted 
hydrogen  for  ten  minutes ;  the  brownish-black  precipitate  is  the  ter- 
sulphide  of  bismuth.  Filter  off  the  sulphide^  and  wash  it  with  water 
upon  the  filter ;  with  successive  portions  of  the  washed  sulphide  per- 
form the  following  experiments  in  a  test-tube  at  a  gentle  heat :  1.  Treat 
it  with  chlorhydric  acid,  —  sulphuretted  hydrogen  will  be  evolved  ;  2. 
With  nitric  acid,  —  sulphur  will  be  separated ;  3.  With  sulphuric  acid,  — 
sulphurous  acid  will  be  evolved  ;  4.  With  a  solution  of  caustic  soda,  — 
it  will  not  dissolve  ;  5.  With  a  solution  of  sulphydrate  of  ammonium,  —  it 
will  not  dissolve.  The  last  two  reactions  establish  distinct  differences 
between  the  sulphides  of  bismuth  and  antimony,  in  addition  to  their 
difference  of  color.  Lastly,  heat  moderately  a  little  of  the  washed 
sulphide  on  platinum  foil  over  the  gas-lamp ;  sulphurous  acid  will  be 
given  off,  and  the  oxide  of  bismuth  remains  ;  this  oxide  readily  melts 
to  dark-yellow  globules. 

364.  The  Nitrogen  Group  of  Elements.  The  five  elements, 
nitrogen,  phosphorus,  arsenic,  antimony,  and  bismuth,  form  a 
well-marked  natural  group  of  elements.  In  the  first  place,  the 
elements  themselves  exhibit  a  definite  gradation  of  properties, 
and,  secondly,  the  analogy  in  composition  and  properties,  mani- 
fested by  the  similar  compounds  of  the  five  elements  is  most 
striking  and  complete. 

Nitrogen  is  a  gas,  phosphorus  a  solid  whose  specific  gravity 
varies  from  1.8  to  2.2,  arsenic  has  the  specific  gravity  of  5.6, 
antimony  of  6.7,  while  that  of  bismuth  rises  to  9.8.  The  me- 
tallic character  is  most  decided  in  bismuth,  is  somewhat  less 
marked  in  antimony,  is  doubtful  in  arsenic,  and  almost  vanishes 
in  phosphorus.  All  four  of  these  elements  are  dimorphous,  pre- 
senting forms  both  of  the  monometric  and  hexagonal  systems. 
The  series  of  corresponding  hydrides,  oxides,  chlorides,  and  sul- 
phides, which  the  elements  of  this  group  form,  are  very  perfect ; 
they  prove  the  general  chemical  likeness  of  the  five  elements  :  — 


282  THE    NITROGEN    GROUP. 

Hydrides.       Oxides.  Oxides.  Oxides.  Chlorides.  Sulphides. 

NH3          N2O3  N2O4  N2O5          NC18(?)          P2S3 

PH3          P2O3          Sb2O4  PA          PC13  As,S3 

AsH3         As2O3         BiA  As205        AsCl3  Sb2S3 

SbH3         Sb2O3  SbA        SbClg  Bi2S3 

Bi2O3  BiA         BiCl3 

PC15  As2S5 

SbCl5  Sb2S5 

The  first  four  members  of  the  group  form  gaseous  terhydrides, 
in  which  three  volumes  of  hydrogen  and  one  atom  of  a  nitrogen- 
group  element  are  combined  to  form  two  volumes  of  the  com- 
pound gas.  We  have  already  spoken  of  the  similarity  of  chemi- 
cal composition,  and  the  gradation  of  properties  manifested  by 
these  four  hydrides.  Ammonia  is  a  powerful  base,  and  requires 
a  high  temperature  for  its  decomposition  ;  phosphuretted  hydro- 
gen is  a  very  feeble  base,  while  the  basic  character  is  not  per- 
ceptible in  arseniuretted  and  antimoniuretted  hydrogen.  Each 
of  the  last  three  hydrides  is  decomposed  by  simple  exposure  to 
heat,  phosphuretted  hydrogen  requiring  the  highest  temperature, 
arseniuretted  hydrogen  decomposing  at  a  lower,  and  antimoui- 
uretted  hydrogen  at  a  still  lower  degree  of  heat.  The  affinity  of 
bismuth  for  hydrogen  is  so  feeble  that  it  does  not  appear  to  form 
a  hydride. 

The  teroxides  also  show  a  gradation  of  physical  and  chemical 
qualities.  Teroxide  of  nitrogen  (nitrous  acid)  is  a  highly  vola- 
tile liquid,  that  of  phosphorus  (phosphorous  acid)  a  very  volatile 
solid,  that  of  arsenic  a  less  volatile  solid,  that  of  antimony  a 
solid  volatile  only  at  a  full  red  heat,  and  that  of  bismuth  a  solid 
which  requires  an  extremely  high  temperature  for  its  volatiliza- 
tion. The  teroxides  of  nitrogen  and  phosphorus  form  with  water 
strongly  acid  liquids ;  teroxide  of  arsenic  is  but  a  feeble  acid  ; 
teroxide  of  antimony  is  sometimes  an  acid  and  sometimes  a  base, 
while  teroxide  of  bismuth  is  a  decided  base.  Arsenious  and 
antimonious  acids  are  isodimorphous.  The  series  of  quinqui- 
oxides  also  shows  a  very  marked  gradation  of  chemical  energy, 
especially  when  the  compounds  which  they  form  with  the  ele- 


THE    NITROGEN    GROUP.  283 

ments  of  water  are  considered.  Nitric  acid  is  a  powerful  acid 
of  intense  energy  ;  phosphoric  acid  is  still  a  strong  acid,  but 
much  less  incisive  than  nitric  acid ;  arsenic  acid  has  the  corrosive 
properties  generally  attributed  to  acids,  but  it  is  chemically  a 
rather  less  vigorous  compound  than  phosphoric  acid.  The  phos- 
phates and  arseniates  are,  however,  isomorphous,  and  the  two 
acids  are  very  much  alike.  In  the  quinqui-oxide  of  antimony 
the  acid  character  becomes  comparatively  indistinct,  and  in  the 
so-called  bismuthic  acid  little  remains  but  the  name. 

The  terchloride  of  nitrogen  has  hardly  been  examined,  on 
account  of  its  extreme  instability.  The  other  four  terchlorides 
are  all  volatile  substances  of  analogous  composition,  since  three 
volumes  of  chlorine  and  one  atom  of  the  nitrogen-group  element 
unite  to  form  two  volumes  of  the  compound  vapor.  The  boiling 
points  of  the  terchlorides  of  phosphorus,  arsenic,  and  antimony 
are  74°,  132°,  and  223°  respectively,  while  that  of  the  terchlo- 
ride of  bismuth  is  considerably  higher  still.  All  four  terchlorides 
are.  decomposed  by  an  excess  of  water. 

The  tersulphides  of  antimony  and  bismuth  are  isomorphous. 

The  elements  of  this  group  do  not  form  many  combinations 
among  themselves.  They  combine  with  bydrogen,  metals  and 
compound  radicals  which  replace  hydrogen  atom  for  atom,  and 
with  the  members  of  the  chlorine  group,  by  preference,  in  the 
proportion  of  1  atom  to  3  ;  they  also  combine  with  oxygen  and 
the  members  of  the  sulphur  group,  by  preference,  in  the  propor- 
tions of  2  atoms  to  3,  or  2  atoms  to  5. 

When  the  qualities  of  the  corresponding  compounds  which  the 
members  of  the  nitrogen  group  form  with  other  elements  are 
duly  taken  into  account,  it  will  be  apparent  that  the  relative 
chemical  power  of  each  element  of  the  group  may  be  inferred 
from  its  position  in  the  series  of  elements  :  — 

N  =  14,    P  =  31,    As  =  75,    Sb  =  122,    Bi  =  210. 
The  chemical  energy  of  these  five  elements,  broadly  considered, 
follows  the  opposite  order  of  their  atomic  weights. 


284  CARBON. 

CHAPTER    XX. 

CARBON. 

365.  Carbon  is  an  extremely  important  and  a  very  abundant 
element.     All  organic  substances,  all  things   which  have  life, 
contain  it.     Large  quantities  of  it  occur  in  the  mineral  kingdom 
as  well,  both  in  the  free  state,  and  in  combination  with  oxygen 
and  with  other  elements.     The  various  forms  of  coal,  graphite, 
petroleum,  asphaltum,  and  all  the  different  varieties  of  limestone, 
chalk,  marble,  coral,  and  sea  shells,  contain  it  in  large  proportion. 
It  is  found  also  in  the  atmosphere  and  in  the  waters  of  the  globe, 
and  though  existing  therein  in  comparatively  small  proportion,  it 
is  an  ingredient  not  less  essential  than  either  of  their  other  con- 
stituents for  the  maintenance   of  the  actual  balance  of  organic 
nature.     All  vegetable  life  is  directly  dependent  upon  the  pres- 
ence of  the  compound  of  carbon  (carbonic  acid)  which  exists  in 
the  atmosphere. 

366.  Three  distinct  allotropic  modifications  of  carbon  are  dis- 
tinguished, namely,  1.  The  diamond ;  2.  Plumbago  or  graphite  ; 
and  3.  Ordinary  charcoal  or  lamp-black.     There  are  many  sub- 
varieties  of  the  modification  last  named,  but  their  peculiarities 
appear  to  depend  chiefly  upon  physical  conditions  of  aggregation  ; 
whereas   each  of  the  three  principal  varieties  of  carbon  above 
enumerated  is  really  different  from  the  other  two  in  chemical 
quality  or  nature. 

367.  The  element  carbon,  in  each  of  its  modifications,  is  an 
infusible,  non-volatile  solid  devoid  of  taste  and  smell.     But  the 
several  modifications  differ  among  themselves  in  color,  hardness? 
lustre,  specific  gravity,  behavior  towards  chemical  agents,  power 
of  conducting  heat  and  electricity,  and  in  various  other  respects. 
All  the  varieties  of  carbon,  however,  agree  in  this,  that  on  being 
strongly  heated  in  presence  of  oxygen,  they  unite  with  it,  and 
form  carbonic  acid  (CO2).     But  in  the  comparative  readiness 
with  which  this  result  is  attained  great  differences  are  noticeable 
in  the  different  varieties. 


DIAMOND.  285 

Lamp-black  and  charcoal,  as  is  well  known,  readily  combine 
with  oxygen  at  the  temperature  of  an  ordinary  fire  ;  they  burn 
easily  in  the  air.  But  graphite  burns  so  slowly  in  air  that  it  is 
used  for  making  the  crucibles  in  which  the  most  refractory 
metals  are  melted.  (See  Appendix,  §  26.)  On  being  heated  to 
a  very  high  temperature,  however,  in  oxygen  gas,  graphite 
slowly  undergoes  combustion,  and  the  same  remark  is  true  of  the 
diamond.  Both  graphite  and  diamond  can  be  consumed  by  nas- 
cent oxygen,  as  when  heated  in  the  condition  of  fine  powder  with 
a  mixture  of  bichromate  of  potassium  (a  substance  rich  in  oxy- 
gen) and -sulphuric  acid.  They  can  be  oxidized  also  by  heating 
them  with  nitrate  or  with  chlorate  of  potassium. 

368.  Diamond.  Of  this  first  variety  of  carbon,  little  need 
here  be  said.  The  physical  properties  which  render  it  so  valu- 
able, its  high  refractive  power  as  regards  light,  and  its  extreme 
hardness,  are  familiar  to  all.  It  is  the  hardest  known  substance, 
being  capable  of  scratching  all  other  substances ;  the  name  dia- 
mond is  a  mere  corruption  of  the  word  adamant. 

Of  the  chemistry  of  the  diamond  very  little  is  known.  It 
consists  of  pure  or  nearly  pure  carbon,  crystallized  in  octohe- 
drons  and  other  forms  of  the  first  or  regular  system  ;  its  specific 
gravity  is  about  3.55,  and  its  specific  heat  0.1469.  It  conducts 
electricity  and  heat  but  slowly ;  and  yet  it  conducts  heat  so  much 
better  than  glass,  that  this  property  is  sometimes  made  use  of  as 
a  test  to  distinguish  false  from  real  diamonds.  Its  refractive 
power  on  light  as  compared  with  that  of  glass  or  rock-crystal  is 
as  2.47  to  1.6. 

Chemists  are  as  yet  unable  to  prepare  this  variety  of  carbon 
artificially ;  the  only  source  of  it  is  the  natural  mineral.  It  was 
thought,  at  one  time,  that  if  there  could  but  be  devised  means  of 
fusing  carbon,  crystals  of  the  diamond  modification  might  pos- 
sibly separate  out  from-  the  molten  liquid  as  it  cooled,  but,  at 
present,  all  the  evidence  goes  to  show  that  at  high  temperatures, 
the  second  modification  of  carbon,  —  namely,  graphite,  —  and 
not  diamond,  is  produced.  If  a  diamond  be  heated  white-hot 
between  the  charcoal  points  of  a  powerful  galvanic  battery,  it 
softens,  and  swells  up,  and,  after  cooling,  there  is  found  a  hard 
black  brittle  mass  like  the  coke  obtained  by  heating  bituminous 


286  GRAPHITE. 

coal.  So,  too,  carbon  is  soluble  in  melted  iron,  and  a  portion  of 
it  crystallizes  out  as  the  iron  becomes  cold,  but  the  crystals  thus 
obtained  are  crystals  of  graphite  and  not  of  diamond.  We  can, 
therefore,  only  surmise  that  diamonds  crystallize  at  a  low  tem- 
perature from  some  unknown  solvent, of  carbon,  or,  with  greater 
probability,  that  when  carbon  is  separated  by  the  decomposition 
of  some  one  of  its  compounds  it  is  left  in  the  diamond  condition. 
The  diamond  is  not  attacked  by  the  strongest  acids  or  alka- 
lies, not  even  by  fluorhydric  acid ;  nor  is  it  acted  upon  by  any  of 
the  non-metallic  elements,  with  the  exception  of  oxygen  at  high 
temperatures.  At  the  ordinary  temperature  of  the  air,  diamond 
undergoes  no  appreciable  change  during  the  lapse  of  centuries  ; 
it  appears  to  be  well  nigh  indestructible,  in  the  ordinary  sense 
of  the  term.  Out  of  contact  with  the  air,  or  in  an  atmosphere 
which  has  no  chemical  action  upon  it,  it  suffers  no  alteration  at 
the  highest  furnace  heat.  In  a  covered  crucible  it  can  be  heated 
white-hot  without  undergoing  change. 

369.  Graphite  or  Plumbago,  sometimes  called  "black-lead," 
is  familiarly  known  as  the  material  of  common   "  lead  pencils." 
It  is  found  as  a  mineral  in  nature  in  various  localities.     It  occurs 
both  in  the  form  of  crystals,  —  six-sided  tables  belonging  to  the 
hexagonal  system,  and  in  the  amorphous,  massive  state.     In  both 
forms  it  is  always  opaque,  of  a  black  or  lead-gray  color  and  me- 
tallic  lustre ;   its    specific   gravity  varies  from   1.8  to  2.1  ;  its 
specific  heat  is  0.201. 

It  conducts  electricity  nearly  as  .well  as  the  metals,  and  is,  on 
this  account,  much  used  for  coating  surfaces  of  wood,  wax,  plas- 
ter, and  other  non-conducting  materials  so  as  to  render  them 
capable  of  conducting  the  galvanic  current  and  so  of  receiving  a 
metallic  film  such  as  is  deposited  from  solutions  of  the  metals 
when  subjected  to  the  action  of  the  galvanic  current ;  it  is  an 
important  material  in  the  art  of  electro-metallurgy.  The  lustre 
and  conducting  power  of  graphite  go  far  to  justify  the  term 
which  has  been  often  applied  to  it,  metallic  carbon. 

370.  Graphite  is  very  friable ;  when  rubbed  upon  paper  it 
leaves  a  black  shining  mark,  whence  its  use  for  pencils.     Amor- 
phous graphite  is  much  more  friable  than  the  crystalline  variety  ; 
it  makes  a  blacker  mark  upon  paper,  and  is  consequently  pre- 


PROPERTIES    OF    GRAPHITE.  287 

ferred  for  the  manufacture  of  pencils ;  it  is,  jn  fact,  so  soft  and 
unctuous  to  the  touch  that  it  is  often  used  as  a  lubricant  for 
diminishing  the  friction  of  machinery.  But  in  spite  of  all  this 
the  particles  of  which  the  masses  of  graphite  are  composed  are 
extremely  hard ;  they  rapidly  wear  out  the  saws  employed  to 
cut  these  masses.  By  powerful  pressure  the  dust  of  plumbago 
can  be  forced  into  the  condition  of  a  coherent  solid  similar  to  the 
native  mineral. 

In  the  air,  at  ordinary  temperatures,  graphite  undergoes  no 
change ;  hence  its  use  for  covering  iron  articles  to  prevent  their 
rusting.  By  virtue  of  its  greasy,  adhesive  quality,  it  is  easy  to 
cover  iron  with  a  thin,  lustrous  layer  or  varnish  of  it ;  the  common 
stove-polishes,  for  example,  are  composed  of  powdered  graphite. 
Even  at  very  high  temperatures,  it  is  scarcely  at  all  oxidized  by 
the  air ;  it  is,  moreover,  altogether  infusible ;  hence  it  is  usefully 
applied  in  the  manufacture  of  a  highly  refractory  kind  of  cruci- 
ble, known  as  black-lead  crucibles  or  blue-pots.  (See  Appendix, 
§  26.)  An  analogous  application  of  graphite  is  seen  in  its  use 
as  "  foundry -facings,"  a  term  applied  to  the  infusible  dust  which 
the  iron-founder  sifts  over  his  mould  of  sand  before  pouring  into 
it  the  melted  metal ;  if  the  hot  metal  were  allowed  to  come 
directly  in  contact  with  the  sand,  a  quantity  of  the  latter  would 
melt  and  remain  adhering  to  the  cold  metal  when  the  casting 
was  taken  from  the  mould.  For  this  purpose,  coal-dust  is  an 
inferior  substitute  for  graphite. 

371.  Pure  plumbago  is  never  met  with  in  nature;  when 
burned  in  oxygen  the  mineral  leaves  from  two  to  five  per  cent, 
of  ashes,  composed  mainly  of  oxide  of  iron,  together  with  small 
quantities  of  silica  and  alumina.  The  presence  of  this  impurity 
is  so  unvarying  that  graphite  was  formerly  supposed  to  be  not 
carbon,  but  a  chemical  compound  of  carbon  and  iron,  a  carbide 
of  iron;  this  view  has  now  been  disproved,  and  it  is  known  that 
the  iron  in  the  native  graphite  exists  there  merely  as  a  mechani- 
cal admixture. 

Soft,  fine-grained  plumbago,  suitable  for  the  manufacture  of 
the  best  pencils,  is  rare ;  but  the  coarse,  foliated  crystallized 
variety  is  abundant,  and  this  may  easily  be  made  soft  and  unctu- 
ous by  the  action  of  certain  oxidizing  agents. 


288  GRAPHITIC    ACID. 

Exp.  152. —  Mix  7^grms.  of  coarsely-powdered  crystallized  graphite 
with  0.5  grm.  of  chlorate  of  potassium  in  fine  powder ;  add  the  mixture 
to  14  grms.  of  strong  sulphuric  acid  contained  in  a  porcelain  dish,  and 
heat  the  whole  over  a  water-bath  as  long  as  yellow  vapors  of  hypo- 
chloric  acid  are  evolved.  Wash  the  cooled  mass  with  water,  and  sub- 
sequently dry  it  on  the  water-bath. 

Ignite  a  fragment  of  the  dry  product  upon  a  piece  of  platinum  foil, 
and  observe  the  extraordinary  intumescence.  After  the  graphite  has 
ceased  to  swell  up,  rub  a  little  of  it  upon  a  porcelain  plate  and  note 
the  exquisite  condition  of  softness  to  which  it  has  been  reduced,  and 
the  ease  with  which  it  can  be  moulded  by  pressure  into  any  desired 
form. 

372.  Regarded  from  the  chemical  point  of  view,  graphite 
resembles  the  other  modifications  of  carbon  in  so  far  as  it  is 
converted  into  carbonic  acid  on  being  ignited  in  oxygen,  and  in 
that  it  undergoes  no  alteration  when  heated  in  close  vessels,  but 
it  differs  materially  from  the  other  varieties  of  carbon  in  its 
behavior  towards  several  of  the  oxidizing  agents.  When  graphite 
is  repeatedly  exposed  to  the  action  of  a  mixture  of  strong  nitric 
and  sulphuric  acids,  or  to  that  of  a  mixture  of  chlorate  or  bi- 
chromate of  potassium  and  sulphuric  or  nitric  acid,  it  is  converted 
into  a  peculiar  acid,  called  graphitic  acid. 

This  graphitic  acid  occurs  in  thin  transparent  crystals,  some- 
what soluble  in  water,  but  insoluble  in  water  containing  acids  or 
salts ;  on  being  heated,  it  decomposes  with  explosion  and  evolu- 
tion of  light.  By  analysis,  it  has  been  found  to  contain  carbon, 
hydrogen,  and  oxygen,  in  proportions  corresponding  to  the  com- 
plex formula  CnH4O5 ;  but  some  chemists,  who  regard  this  body 
as  an  analogue  of  an  acid  Si4H4O5 ,  obtained  by  acting  upon  one 
of  the  modifications  of  silicon  with  oxidizing  agents,  have  sug- 
gested that  the  atomic  weight  of  graphite  may  be  different  from 
that  of  ordinary  carbon,  and  that  the  composition  of  graphitic 
acid  could  be  represented  by  the  simpler  formula  Gr4H4O5 ,  in 
which  the  symbol  Gr  stands  for  graphon,  provided  the  atomic 
weight  of  this  graphon  were  assumed  to  be  33,  instead  of  12,  the 
atomic  weight  assigned  to  ordinary  carbon. 

The  graphitic  modification  of  carbon  can  readily  be  obtained 
artificially.  When  charcoal  is  added  to]  melted  iron,  the  iron 
takes  up  a  considerable  quantity  of  it,  and  if  the  iron  be  then 


GAS-CAKBON.  283 

left  to  cool  slowly,  a  portion  of  the  dissolved  carbon  will  crys- 
tallize out  in  the  form  of  graphite ;  the  crystals  can  readily  be 
isolated  by  dissolving  away  their  metallic  envelope  by  means  of 
dilute  chlorhydric  or  sulphuric  acid. 

As  has  been  already  remarked,  the  crystals  of  graphite  are 
six-sided  plates  of  the  hexagonal  system,  altogether  unlike  the 
forms  of  the  regular  system  which  are  seen  in  the  diamond.  In 
carbon,  then,  as  in  sulphur,  we  have  a  striking  example  of 
dimorphism.  (See  §  192.) 

373.  Gas-Carbon.      An    interesting   sub-variety   of    carbon 
somewhat  similar  to  graphite,  and  standing,  as  it  were,  between 
it  and  the  ordinary  modification  of  carbon,  is  obtained  from  the 
retorts  in  which  common  illuminating  gas  is  manufactured.     It 
is  known  as  "  gas-carbon,"  or  "  carbon  of  the  gas-retorts,"  and 
results  from  the  burning  on  of  drops  of  tar  upon  the  interior 
walls  of  the  retort,  arid  the  long-continued  heating  of  the  crust 
thus  formed. 

Gas-carbon  is  very  hard,  compact,  and  dense  ;  it  has  the  me- 
tallic lustre,  and  conducts  electricity  like  a  metal;  its  specific 
gravity  (2.356)  and  specific  heat  (0.2036)  closely  resemble  those 
of  graphite.  On  account  of  its  high  conducting  power,  it  is  em- 
ployed in  the  manufacture  of  galvanic  batteries  and  of  pencils 
for  the  electric  lamp ;  its  infusibility  and  power  of  resisting 
chemical  agents  have  led  to  the  employment,  in  various  scientific 
researches,  of  crucibles  and  tubes  wrought  out  of  it;  it  has  also 
been  sometimes  employed  as  fuel  in  experiments  where  higher 
degrees  of  heat  are  needed  than  can  be  obtained  from  charcoal 
or  coke.  The  intense  heat  developed  by  the  combustion  of  this 
substance  is  referable  to  its  high  specific  gravity;  —  a  very 
considerable  weight  of  carbon  can  here  be  burned  in  a  small 
space ;  as  a  fuel,  it  has  the  further  merit  of  leaving  scarcely  any 
ashes. 

374.  Coke  and  Anthracite  Coal  are  impure  sub-varieties  of 
carbon  which,  from  the  chemical  point  of  view,  may  be  classed 
either  with  graphite   or  charcoal,  or  better  between  the  two. 
They  are  less  like  graphite,  however,  than  gas-carbon  is.     Coke 
is  the  residue  resulting  from  the  destructive  distillation  of  soft  or 

bituminous  coal. 

19 


290 


ILLUMINATING    GAS. 


Exp.  153.  — Put  into  a  tube  of 
hard  glass,  No.  1,  12  or  15  c.  m.  in 
length,  enough  bituminous  coal,  in 
coarse  powder,  to  fill  one-third  of  the 
tube.  Fit  to  this  ignition-tube  a 
large  delivery-tube  of  glass,  No.  4, 
and  support  the  apparatus  upon  the 
iron  stand,  as  shown  in  the  figure. 
Heat  the  coal  in  the  ignition-tube, 
and  collect  in  bottles  the  gas  which 
will  be  evolved.  This  gas  is  a  mix- 
ture of  several  compounds  of  carbon  and  hydrogen  ;  for  the  present,  it 
may  be  regarded  as  carburetted  hydrogen.  It  is,  in  fact,  ordinary 
illuminating  gas. 

It  is  inflammable,  like  hydrogen,  but  burns  with  a  far  more  luminous 
flame.  It  is  very  light  withal ;  hence  many  of  the  experiments  de- 
scribed in  the  chapter  upon  hydrogen  may  be  performed  with  this  gas. 
(See  Chapter  V.) 

As  soon  as  gas  ceases  to  be  given  off  from  the  coal,  take  the  end  of 
the  delivery-tube  out  of  the  water,  and  when  the  ignition-tube  has 
become  cold,  break  it  and  examine  the  coke  which  it  contains.  The 
coke  used  for  domestic  purposes  is  obtained  as  an  incidental  product  in 
the  manufacture  of  illuminating  gas. 

In  Europe,  where  anthracite  is  lacking,  immense  quantities  of  coke 
are  prepared  for  metallurgical  uses,  the  gas  resulting  from  the  decom- 
position of  the  coal  being  usually  thrown  away. 

375.  Bituminous  coal  is  a  substance  of  vegetable  origin,  which 
appears  to  have  been  formed  from  plants  by  a  process  of  slow 
decay  going  on  without  access  of  air  and  under  tne  influence  of 
heat,  moisture,  and  great  pressure.  Like  vegetable  matter  in 
general,  it  is  composed  of  carbon  and  hydrogen,  together  with 
small  proportions  of  oxygen  and  nitrogen,  and  a  certain  quantity 
of  earthy  and  saline  substances,  commonly  spoken  of  as  inor- 
ganic matter.  On  being  heated  in  the  air,  it  burns  away  almost 
completely  after  a  while,  leaving  nothing  but  the  inorganic  com- 
ponents as  ashes.  But  when  heated  out  of  contact  with  the  air, 
that  is  to  say,  when  subjected  to  destructive  distillation,  as  in 
Exp.  153,  the  volatile  hydrogen  is  all  driven  off  in  combination 
with. some  carbon,  either  as  gas  or  as  a  tarry  liquid,  and  there 
remains,  as  a  residue,  only  carbon  contaminated  with  »the  inor- 
ganic matters  originally  present  in  the  coal. 


COKE ANTHRACITE CHARCOAL.          291 

376.  Both  coke  and  anthracite  are  hard  and  lustrous.     As 
compared  with  charcoal,  they  are  rather  difficult  of  combustion, 
but  in  suitable  furnaces  they  burn  readily  with  evolution  of  in- 
tense heat.     Both  anthracite  and  coke,  the  latter  in  spite  of  its 
porosity,  conduct  heat  readily,  as  compared  with  charcoal ;  hence 
one  reason  of  the  difficulty  of  kindling  them.     In  building   a 
charcoal  fire,  the  heat  evolved  by  the  combustion  of  the  kindling 
material  is  almost  all  retained  by  the  portions  of  charcoal  imme- 
diately in  contact  with  the  kindling  agent,  but  in  the  case  of  coke 
or  anthracite,  a  large  proportion  of  this  heat  is  conducted  off  and 
diffused  throughout  the  heap  of  fuel,  so  that  no  portion  of  the 
fuel  Gan  at  once  become  very  hot.     It  follows  that  both  in  light- 
ing and  feeding  fires  of  coke  or  anthracite,  only  small  portions 
of  the  fuel  should  be  added  at  any  one  time,  lest  the  kindling 
material,  or  the  existing  fire,  be  unduly  cooled.     Since  coke  is 
usually  contaminated   with  a   considerable  proportion   of  inor- 
ganic matter,  its  combustion  is  apt  to  be  hindered  by  the  accu- 
mulation of  ashes  and  consequent  exclusion  of  air,  unless  special 
precautions  be  taken.- 

377.  Charcoal  or  Lamp-black  is  commonly  taken  as  the  repre- 
sentative   of  the    third    or   amorphous  modification    of  carbon- 
This   kind  of  carbon  can   be  obtained  in  a  state   of  tolerable 
purity,  either  by  heating  in  a  close  vessel  sugar,  or  starch,  or 
some  other  organic  substance  which  contains  no  inorganic  con- 
stituents,  or  by  burning  oil  of  turpentine  in  a  quantity  of  air 
insufficient    for  its  complete   combustion.      A  convenient  way 
of  obtaining  it  is  to  place  a  vessel  filled  with  ice-water  directly 
in  the  flame  of  a  lamp  fed  with  oil  of  turpentine,  so  that  the 
combustion  of  the  oil  shall  be  impeded,  and  that  soot  may  be 
deposited  upon  the  walls  of  the  vessel.    In  either  event,  however, 
the  product  is  liable  to  be  contaminated  with  traces  of  hydro- 
gen or  of  oxygen,  or  of  both  these  elements,  which  cannot  be 
expelled  even  by  the  application  of  long  continued  and  intense 
heat. 

For  such  illustrations  as  are  required  in  this  manual,  charcoal 
can  readily  be  prepared  from  wood  in  the  same  way  that  it  is 
made  for  manufacturing  and  domestic  uses,  namely,  by  subject- 
ing the  wood  to  a  process  of  incomplete  combustion. 


292  PREPARATION  OF  CHARCOAL. 

Exp.  154. —  Light  one  end  of  a  splinter  of  dry  wood  and  slowly 
push  the  burning  portion  into  the  mouth  of  a  test-tube,  as  shown  in 
Fig.  47.     The  portion  of  the  splinter  which  remains  outside  the  tube 
FIG.  47.  and  in  contact  with  free  air  will  continue  to 

burn  with  flame,  while  that  within  the  tube 
is  either  extinguished  altogether  or  barely 
glimmers  as  the  carbon  slowly  unites  with 
the  small  portion  of  air  which  can  gain 
access  to  it. 

Exp.  155.  —  Repeat  the  foregoing  experi- 
ment, but  instead  of  the  test-tube,  provide 
a  cup  of  sand  and  slowly  thrust  the  burning  splinter  into  this  sand.  The 
flame  will  be  extinguished  as  fast  as  the  splinter  is  cut  off  from  the  air 
by  immersion  in  the  sand,  and  a  residue  of  carbon  will  thus  be  obtained, 
as  before. 

378.  Whenever  wood,  or  any  other  vegetable  or  animal  mat- 
ter, is  not  completely  consumed,  there  is  left  a  residue  of  carbon 
similar  to  that  obtained  in  the  foregoing  experiments.  Incom- 
plete combustion  in  such  cases  is  really  a  process  of  destructive 
distillation,  or,  rather,  in  any  combustion  of  wood,  or  of  bitumi- 
nous coal,  there  is  always  destructive  distillation  at  first.  When 
the  splinter  of  wood,  of  Exp.  154,  is  heated  in  the  lamp,  in  order 
to  set  it  on  fire,  there  will  distil  off  from  it,  in  the  beginning, 
certain  volatile  compounds  of  hydrogen  and  carbon ;  for  wood, 
like  coal  (§  375),  is  composed  of  carbon,  hydrogen,  oxygen, 
nitrogen,  and  inorganic  or  earthy  matters,  and  when  exposed  to 
strong  heat  it  gives  off  in  the  gaseous  form  the  volatile  elements 
hydrogen,  oxygen,  and  nitrogen,  together  with  some  carbon. 
The  products  of  the  destructive  distillation  of  the  splinter  will 
take  fire  and  burn,  and  the  heat  generated  by  their  combustion 
will  be  sufficient,  not  only  to  distil  the  contiguous  portions  of  the 
wood,  but  also  to  bring  the  residual  carbon  to  the  temperature  at 
which  it  unites  with  oxygen.  This  kindling  temperature  of 
carbon,  it  should  be  remarked,  is  considerably  higher  than  that 
at  which  the  volatile  distillate  composed  of  carbon  and  hydrogen 
takes  fire.  Now  if,  as  in  experiments  154, 155,  the  burning  splin- 
ter be  removed  from  the  air  as  soon  as  the  act  of  distillation  has 
been  completed,  but  before  the  combustion  of  the  carbon  has  set 
in,  the  carbon  will  be  preserved,  as  has  been  seen.  So,  too,  when 
burning  wood  is  extinguished  by  pouring  water  upon  it ;  the  dis- 


DISTILLATION    OF    WOOD. 


293 


FIG.  48. 


dilatory  process  has  occurred  and  has  been  more  or  less  thor- 
oughly completed,  but  the  combustion  of  the  carbon  is  cut  short ; 
for  the  water  not  only  excludes  air  but  absorbs  so  much  heat 
that  the  temperature  of  the  fuel  is  reduced  below  the  kindling 
point.  (Compare  Exp.  24.) 

379.  Charcoal  can  be  obtained  also  by  distilling  wood  in 
retorts  in  the  same  way  that  we  have  seen  that  coke  can  be 
procured  from  bituminous  coal.  (See  Exp.  153.) 

Exp.  156. —  Provide  an  ignition-tube  and  a  delivery-tube  similar  to 
those  employed  in  Exp.  153.  Fill  the  ignition-tube  with  shavings  or 
small  fragments  of  wood,  arrange  the  apparatus  as  in  Fig.  48,  and  light 
the  gas-lamp.  Collect  in  bottles  the  gas  which  is  given  off  from  the 
wood,  and  test  it  as  to  its  inflamma- 
bility by  applying  a  lighted  ma'ch. 
After  the  flow  of  gas  has  ceased,  re- 
move the  end  of  the  delivery-tube 
from  the  water,  plug  it  so  that  no  air 
can  enter  the  ignition-tube,  and  lay 
the  apparatus  aside  until  it  has  be- 
come cold.  Finally  remove  the  cork 
from  the  ignition-tube  and  take  out 
the  charcoal  which  is  contained  in  it. 
Heat  a  portion  of  this  charcoal  upon 
platinum  foil  and  observe  the  manner  in  which  it  burns ;  it  will  illus- 
trate the  fact  that  solid  substances  which  are  incapable  of  evolving 
volatile  or  gaseous  matter  do  not  burn  with  flame, —  they  merely  glow. 

Exp.  157.  —  Pack  an  ignition-tube  with  bits  of  wood,  as  in  Exp. 
156,  but  instead  of  the  ordinary  delivery -tube,  insert  in  the  mouth  of 
this  ignition-tube  a  cork  carrying  a  short  piece  of  glass  tubing  drawn 
out  to  a  fine  open  point.  By  means  of  wire,  tie  the  ignition-tube  to  a 
ring  of  the  iron  stand,  and  place  it  over  the  gas-lamp.  The  gases 
evolved  from  the  wood  will  escape  through  the  narrow  tube,  and  on 
being  kindled,  will  burn  with  a  luminous  flame.  As  has  been  already 
stated  (§  57),  flame  is  caused  by  burning  gas. 

This  experiment,  as  well  as  Exps.  153,  156,  illustrates  the  principle 
of  the  manufacture  of  illuminating  gas.  Upon  the  large  scale,  bitumi- 
nous coal,  or  sometimes  dry  wood,  is  distilled  in  large  iron  or  clay  tubes, 
called  gas-retorts,  and  the  gas  evolved  is  freed  from  tar  and  other 
offensive  impurities  by  processes  of  cooling  and  washing  with  water, 
and  by  passing  it  through  layers  of  lime  or  oxide  of  iron  ;  it  is  then 


294  PREPARATION  OF  CHARCOAL. 

collected  in  large  gas-holders,  from  which  it  is  pressed  through  subter- 
ranean pipes,  it  may  be  for  miles. 

380.  For  use  in  the  arts,  charcoal  is  prepared  by  both  the 
methods  above  indicated  ;  it  is  manufactured  both  by  charring  or 
partially  burning  wood  with  little  access  of  air,  and  by  methodi- 
cally distilling  wood  in  actual  retorts.  The  first  method  is 
employed  in  countries  where  wood  is  abundant,  and  is  carried 
on  in  the  forest  itself.  Logs  of  wood  are  piled  up  into  a  large 
mound  or  stack  around  a  central  aperture,  which  subsequently 
serves  as  a  temporary  chimney,  and  also  for  the  introduction  of 
burning  substances  for  firing  the  heap.  The  finished  heap  is 
covered  with  chips,  leaves,  sods,  and  a  mixture  of  moistened 
earth  and  charcoal  dust,  a  number  of  apertures  being  left  open 
around  the  bottom  of  the  heap  for  the  admission  of  air  and  the 
escape  of  the  products  of  distillation  and  combustion.  The  heap 
is  kindled  at  the  centre  and  burns  during  several  weeks.  When 
the  process  is  judged  to  be  complete,  all  the  openings  are  carefully 
stopped,  in  order  to  suffocate  the  fire,  and  the  heap  is  then  left  to 
itself  until  cold.  The  charcoalre  tains  the  form  of  the  wood, — 
the  shape  of  the  knots,  and  the  annual  rings  of  the  wood  being 
still  perceptible  in  it,  —  but  it  occupies  a  much  smaller  volume 
than  the  wood  ;  generally  its  bulk  does  not  amount  to  more  than 
three-fourths  of  that  of  the  wood,  and  its  weight  never  exceeds 
one-fourth  the  weight  of  the  wood. 

Where  charcoal  is  prepared  by  distilling  wood  in  retorts,  the 
liquid  products  of  distillation,  namely,  tar  and  acetic  acid  ("  pyro- 
ligneous  acid  ")  are  saved  and  utilized. 

381.  Lamp-black.  Usually  when  hydrogen  is  removed  from 
a  gaseous  compound  of  carbon  and  hydrogen,  the  carbon  sepa- 
rates in  the  form  of  soft  flakes,  called  lamp-black  or  soot.  In 
experiment  63,  we  have  already  seen  that  lamp-black  is  formed 
when  hydrogen  is  removed  from  carburetted  hydrogen  by  means 
of  chlorine,  and  we  know  well  that  oxygen  is  capable  of  pro- 
ducing the  same  result.  Hydrogen  is  more  combustible  in  oxy- 
gen than  carbon ;  hence  if  carburetted  hydrogen  be  mixed  with 
only  enough  oxygen  to  consume  its  hydrogen,  and  the  mixture 
be  then  inflamed,  the  carbon  contained  in  it  will  be  set  free. 


LAMP-BLACK.  295 

This  is  the  way  in  which  lamp-black  is  commonly  formed  ;  a 
lamp  "  smokes  "  when  the  supply  of  air  is  insufficient  to  furnish 
oxygen  to  both  the  carbon  and  the  hydrogen  of  the  oil  or  other 
combustible. 

Upon  the  large  scale,  lamp-black  is  manufactured  by  heating  or- 
ganic matters,  such  as  tar,  rosin,  or  pine  knots,  which  contain  vola- 
tile ingredients  very  rich  in  carbon,  until  vapors  are  disengaged, 
and  then  burning  these  vapors  in  a  current  of  air  insufficient  for 
their  complete  combustion.  A  dark-red,  very  smoky  flame  is 
thus  obtained ;  a  large  portion  of  the  carbon  of  the  combustible 
does  not  burn,  but  is  deposited  as  a  very  fine  powder  precisely 
similar  to  that  which  constitutes  the  black  portion  of  common 
smoke.  Lamp-black  finds  important  applications  in  the  arts  as 
a  pigment  and  as  the  chief  ingredient  of  printer's  ink. 

Exp.  158.  —  Fill  an  ordinary  spirit-lamp  (Appendix,  §  5)  with  oil  of 
turpentine,  light  the  wick  and  place  over  it  au  inverted  wide-mouthed 
bottle  of  the  capacity  of  a  litre  or  more,  one  edge  of  the  mouth  of  the 
bottle  being  propped  up  on  a  small  block  of  wood,  so  that  some  air  may 
enter  the  bottle.  As  the  supply  of  air  is  insufficient  for  the  perfect 
combustion  of  the  oil  of  turpentine,  a  quantity  of  lamp-black  will  sep- 
arate and  be  deposited  upon  the  sides  of  the  bottle. 

Hydrogen  kindles  at  a  lower  temperature  than  carbon,  hence 
if  the  flame  of  a  burning  compound  of  carbon  and  hydrogen  be 
cooled  down  below  the  temperature  at  which  carbon  takes  fire, 
lamp-black  will  be  formed,  even  if  there  be  present  an  abundant 
supply  of  air. 

Exp.  159.  —  Press  down  upon  the  flame  of  an  oil-lamp  or  candle  an 
iron  spoon  or  a  porcelain  plate  in  such  manner  that  the  flame  shall  be 
almost,  but  not  quite,  extinguished.  The  solid  body  not  only  obstructs 
the  draught  of  air,  and  thereby  interferes  with  the  act  of  combustion, 
but  it  also  cools  the  flame  by  actually  conducting  away  part  of  its  heat ; 
the  temperature  is  thus  .reduced  to  below  the  kindling-point  of  carbon, 
and  a  quantity  of  lamp-black  remains  unconsumed  and  adhering  to  the 
spoon  or  plate.  This  experiment  is,  of  course,  comparable  with  Exps. 
137,  140,  in  which  spots  of  arsenic  and  antimony  were  obtained  upon 
porcelain,  as  products  of  the  incomplete  combustion  of  their  compounds 
with  hydrogen. 

382.    Charcoal  is  altogether  insoluble  in  water ;  it  is  odorless 


296  PROPERTIES    OF    CHARCOAL. 

and  tasteless.  Unlike  the  diamond,  it  is  a  good  conductor  of 
electricity,  but  a  bad  conductor  of  heat,  particularly  when  in  the 
state  of  powder.  It  is  a  better  conductor  of  heat  in  proportion 
as  it  is  denser;  when  strongly  heated  out  of  contact  with  the 
air  it  becomes  heavier,  harder,  and  closer  in  texture  than  common 
charcoal,  and  less  easily  combustible,  txut  a  far  better  conductor 
of  heat  and  of  electricity  than  before.  The  pieces  of  charcoal, 
for  example,  which  sometimes  fall  unconsumed  from  the  bottoms 
of  smelting  furnaces,  after  having  passed  through  a  long  column 
of  intensely  heated  fuel,  are  found  to  be  peculiarly  compact  and 
close-grained,  and  to  conduct  heat  with  comparative  facility. 

As  has  been  seen,  in  §  376,  the  combustibility  of  a  fuel  is 
diminished  in  proportion  as  the  fuel  is  a  good  conductor  of  heat, 
and  since,  as  has  just  been  stated,  charcoal  conducts  heat  the 
more  readily  in  proportion  as  it  is  denser,  it  follows  that,  other 
things  being  equal,  a  given  sample  of  charcoal  will  take  fire 
more  quickly  if  it  be  light  than  if  heavy.  It  will  appear,  more- 
over, from  the  foregoing  that  the  combustibility  of  charcoal 
should  be  less,  according  as  the  temperature  at  which  the  char- 
coal prepared  is  higher.  If,  for  example,  linen  or  cotton  rags  be 
carbonized  at  low  temperatures,  there  will  be  obtained  a  very 
light  variety  of  charcoal,  called  tinder,  which  takes  fire  with 
especial  ease.  So,  too,  a  very  light  and  easily  inflammable  char- 
coal is  prepared  for  the  manufacture  of  gunpowder  by  distilling 
light  woods,  such  as  willow  and  alder,  at  low  temperatures. 
Wood  charcoal  takes  fire  easily,  as  compared  with  coke ;  coke  as 
compared  with  anthracite,  and  anthracite  as  compared  with  gas- 
carbon.  But,  inversely,  when  the  charcoal  has  once  been  thor- 
oughly lighted,  the  intensity  of  the  heat  obtained  from  a  fire  of 
it  will  be  greater  in  proportion  as  the  charcoal  is  more  dense. 
As  has  been  already  stated  (§  373),  a  better  fire  can  be  obtained 
by  burning  gas-carbon  than  can  be  had  from  coke  or  charcoal, 
for  in  any  given  space,  if  the  supply  of  air  be  ample,  more  of 
the  dense  than  of  the  light  fuel  can,  of  course,  be  burned  in  a 
given  time.  The  specific  gravity  of  charcoal  is  about  1.6,  but 
an  ordinary  fragment  of  it  readily  floats  upon  water,  owing  to 
the  air  within  its  pores ;  if  this  air  be  removed,  as  when  the 
fragment  is  powdered,  the  charcoal  will  sink  at  once.  (See  Exp. 
164.)  It  is  infusible  and  non-volatile. 


CHARCOAL    A    REDUCING   AGENT. 


297 


383.  In  all  its  varieties,  charcoal  is  a  very  important  chemi- 
cal agent,  chiefly  because  of  the  readiness  and  energy  with  which 
it  combines  with  oxygen  at  high  temperatures.  It  might  almost 
be  said  that  the  art  of  metallurgy,  as  it  now  exists,  is  based  upon 
the  affinity  of  carbon  for  oxygen. 

Exp.  160. —  Mix  five  grammes  of  litharge  (oxide  of  lead)  with  quar- 
ter of  a  gramme  of  powdered  charcoal ;  place  a  portion  of  the  mixture 
in  an  ignition-tube  made  of  No.  3  glass,  and  heat  it  strongly  in  the  gas- 
lamp.  The  charcoal  will  unite  with  the  oxygen  of  the  oxide  of  lead, 
and  the  compound  thus  formed  will  escape  in  the  form  of  gas,  while 
metallic  lead  will  remain  in  the  tube. 

This  experiment  is  analogous  to  Exp.  128,  where  arsenious 
acid  was  reduced  by  means  of  charcoal.  Both  experiments  are 
typical  of  the  manner  in  which  hot  charcoal  acts  upon  metallic 
oxides.  At  a  white  heat  it  removes  oxygen  from  its  combina- 
tions with  some  elements  which  hold  it  with  great  force,  such 
as  the  oxides  of  sodium  and  potassium  and  phosphoric  acid. 
Even  water  is  decomposed,  with  liberation  of  hydrogen,  when 
brought  into  contact  with  red-hot  charcoal. 

Exp.  161.  —  Fill  a  piece  of  iron  gas-pipe,  about  35  c.  m.  long  and 
1  c.  m.  or  more  in  internal  diameter,  with  fragments  of  charcoal; 
adapt  to  it  a  delivery-tube,  as  represented  in  Fig.  49,  and  support  it 

FIG.  49. 


upon  a  ring  of  the  iron-stand  over  one  or  two  wire-gauze  gas-lamps. 
Attach  to  the  other  end  of  the  tube  a  thin-bottomed  glass  flask  half 
full  of  water  and  supported  upon  the  ring  a  second  iron-stand.  Light 
the  lamps  beneath  the  tube  full  of  charcoal,  and  wait  until  it  has  become 
red-hot,  then  heat  the  water  in  the  flask  and  cause  it  to  boil  slowly. 
The  steam  will  react  upon  the  hot  carbon  in  a  manner  which  may  be 
formulated  as  follows  :  — 

C  -f  H2O  =  CO  +  2H, 


298  DEFLAGRATION. 

and  there  will  be  formed  a  mixture  of  a  compound  of  oxygen  and  car- 
bon called  carbonic  oxide,  and  free  hydrogen.  Collect  the  mixed  gas 
in  bottles  upon  the  water-pan  and  test  it  as  regards  its  inflammability 
by  applying  a  lighted  match. 

A  certain  quantity  of  water  may  even  be  decomposed  by  thrusting 
pieces  of  brightly  glowing  charcoal  into  water.  If  the  experiment  be 
performed  beneath  an  inverted  bottle  of  Water  held  near  the  surface 
of  the  water-pan,  a  quantity  of  gas  large  enough  to  be  inflamed  can 
readily  be  obtained. 

At  a  red-heat  charcoal  deoxidizes  bodies  which  are  rich  in 
oxygen,  so  readily  that  there  occurs  a  peculiarly  rapid  and 
sparkling  combustion  known  as  deflagration.  Deflagration  dif- 
fers from  explosion  only  in  degree ;  it  is  less  violent  than  explo- 
sion, because  the  combustion  is  less  rapid. 

Exp.  162.  —  Mix  10  grms.  of  nitrate  of  potassium  and  5  grms.  of 
charcoal,  both  in  fine  j>owder ;  place  the  mixture  upon  a  brick,  in  a 
current  of  air,  or  in  any  place  where  the  volatile  products  of  the  re- 
action can  occasion  no  inconvenience,  and  touch  it  with  a  lighted  stick 
or  red-hot  wire.  The  charcoal  will  burn  violently,  with  brilliant  scin- 
tillations, at  the  expense  of  the  oxygen  contained  in  the  nitrate  of 
potassium. 

As  a  modification  of  this  experiment,  heat  a  couple  of  pieces  of 
charcoal  to  redness  in  the  fire  and  sprinkle  upon  the  one  a  small  quan- 
tity of  powdered  nitrate  of  potassium,  and  upon  the  other  a  little 
powdered  chlorate  of  potassium. 

384.  This  deoxidizing  power  of  charcoal,  above  illustrated, 
is  exhibited  only  at  high  temperatures.  At  the  ordinary  tem- 
perature of  the  air,  the  chemical  energy  of  charcoal  is  exceed- 
ingly feeble.  Charcoal  is,  in  fact,  one  of  the  most  durable  of 
substances.  Specimens  of  it  have  been  found  at  Pompeii  and 
upon  Egyptian  mummies,  to  all  appearance  as  fresh  as  if  just 
prepared ;  the  action  of  the  air  continued  through  centuries  has 
exerted  no  appreciable  influence  upon  it.  It  is  on  this  account 
that  many  wooden  articles  which  are  to  be  placed  in  situations 
peculiarly  liable  to  cause  their  decay,  are  covered  with  a  layer 
or  coating  of  charcoal  by  charring  them  superficially ;  the  car- 
bonization of  the  interior  of  casks  destined  to  hold  liquids,  and 
of  those  portions  of  fence-posts,  &c.,  which  are  to  be  sunk  a 
short  distance  in  the  ground,  or  to  remain  near  the  surface  of  the 
ground,  are  familiar  instances  of  this  custom. 


STABILITY    OF    CHARCOAL.  299 

Charcoal  is  not  only  unacted  upon  by  air  or  water  at  the  ordi- 
nary temperature,  but  there  are  few  chemical  substances  which 
have  any  action  upon  it  unless  they  be  hot ;  neither  the  neutral 
solvents,  such  as  alcohol  and  ether,  nor  corrosive  agents,  such  as 
chlorine  or  fluorhydric  and  chlorhydric  acids,  attack  it  in  any 
way.  It  is  slowly  oxidized,  however,  by  nitric  acid,  and  rapidly 
by  perchloric  acid,  and  it  dissolves,  to  a  slight  extent,  in  cold 
concentrated  sulphuric  acid.  At  the  ordinary  temperature,  no, 
one  of  the  elements  combines  with  it  directly,  but  at  high  tem- 
peratures it  unites  directly,  not  only  with  oxygen,  as  we  know, 
but  with  sulphur  as  well,  forming  bisulphide  of  carbon  (CS2). 
At  very  high  temperatures,  as  when  a  powerful  galvanic  battery 
is  discharged  from  carbon  points  immersed  in  an  atmosphere  of 
hydrogen,  carbon  combines  directly  with  hydrogen  also,  and  there 
is  formed  a  gas  called  acetylene  (C2H2).  With  nitrogen  it 
unites  to  form  cyanogen  (CN),  when  a  current  of  nitrogen  gas 
is  passed  through  a  column  of  ignited  charcoal  which  has  pre- 
viously been  charged  with  carbonate  of  potassium  by  soaking  it 
in  a  solution  of  this  salt ;  but  it  will  not  unite  with  nitrogen 
without  the  intervention  of  the  alkaline  carbonate,  or  of  some 
other  substance.  With  chlorine  and  the  other  members  of  the 
chlorine  group  it  does  not  unite  except  indirectly,  but  with  sev- 
eral of  the  metals  it  unites  directly  4o  form  badly-defined  com- 
pounds called  carburets  or  carbides.  Upon  the  chlorides  and 
their  analogues  carbon  has  no  action,  and  the  same  remark  is 
true  of  some  refractory  oxides,  such  as  silicic  acid  and  oxide  of 
aluminum,  which  are  not  decomposed  by  charcoal,  even  at  a 
white  heat ;  but  at  a  strong  red-heat,  it  reduces  most  of  the  sul- 
phates to  the  condition  of  sulphides,  for  example :  — 

CaS04  +  20  =  CaS  +  2C02; 

and  the  nitrates,  chlorates,  and  perchlorates  to  the  state  of  car- 
bonates :  — 

2(K20,N205)  +  5C  ==  2(K20,C02)  +  3CO2  +  4N. 

385.  A  physical  property  of  charcoal,  which  is  of  great  prac- 
tical importance,  is  its  power  of  absorbing  and  condensing  within 
its  pores  a  great  variety  of  gases  and  vapors  ;  it  absorbs  color- 


300  CHARCOAL  ABSORBS  GASES. 

ing  matters  also,  and  various  other  substances  as  well,  abstracting 
them  from  solutions  in  which  they  are  contained. 

Exp.  163.  —  Take  from  the  fire  a  live  coal  (charcoal)  as  large  as  a 
hen's  egg,  extinguish  it  by  covering  it  up  tightly  in  a  small  metallic 
vessel  or  by  covering  it  with  sand,  and  wait  until  it  has  become  cold  ; 
weigh  it  carefully  upon  a  delicate  balance,  and  record  the  weight. 
Place  the  weighed  coal  in  such  position,  in  a  damp  cellar,  for  example, 
that  it  shall  be  freely  exposed  to  moist  air  during  twenty-four  hours ; 
again  weigh  it,  and  note  the  increase.  Boxwood  charcoal  has  been 
found  to  increase  in  weight  14  per  cent,  in  the  course  of  a  single  day, 
and  any  ordinary  charcoal  will  usually  increase  in  weight  from  10  to 
12  per  cent. 

Exp.  164.  —  Take  from  the  fire,  as  before,  a  piece  of  charcoal  which 
has  been  heated  to  full  redness  for  some  time ;  thrust  it  under  water  so 
that  it  may  be  suddenly  cooled,  and  observe  that  it  sinks  in  the  water 
and  that  few  or  no  bubbles  of  gas  escape  from  its  pores. 

Take  another  piece  of  charcoal  which  has  long  been  exposed  to  the 
air  and  has  not  recently  been  heated,  attach  to  it  a  quantity  of  sheet 
lead  sufficient  to  sink  it  in  water,  and  immerse  the  whole  in  a  large 
beaker-glass  two-thirds  full  of  hot  water.  The  mobile  water  will  im- 
mediately enter  the  pores  of  the  charcoal,  and  a  portion  of  the  air 
which  had  previously  been  absorbed  by  these  pores  will  be  driven  out, 
and  can  be  seen  escaping  in  bubbles  through  the  water,  chiefly  from 
the  broken  ends  of  the  coal. 

In  a  similar  way,  if  a  piece  of  old  charcoal  be  placed  in  a  bottle  full 
of  water  standing  inverted  upon  the  water-pan,  a  quantity  of  air  will 
gradually  be  expelled  from  it  as  the  water  enters  its  pores  and  will  col- 
lect at  the  top  of  the  bottle. 

The  air  can  at  any  time  be  quickly  removed  by  sinking  a  piece  of 
the  charcoal,  by  means  of  lead,  in  water  of  the  ordinary  temperature, 
placing  the  vessel  which  contains  the  water  under  the  receiver  of 
air-pump  and  pumping  out  the  air  from  this  receiver ;  as  the  pressui 
of  the  atmosphere  is  removed  from  the  surface  of  the  water  a  mult 
tude  of  bubbles  of  air  will  be  seen  to  issue  from  the  pores  of  the  coal. 

To  the  presence  of  air  and  aqueous  vapor,  which  has  been 
thus  absorbed,  is  to  be  attributed  the  snapping  and  crackling  of 
old  charcoal  when  it  is  thrown  upon  a  hot  fire.  Charcoal  which 
has  been  recently  burned,  and  which  has  not  yet  absorbed  air 
and  moisture,  does  not  thus  snap  and  crackle  ;  moreover,  it  kin- 
dles much  more  easily  than  charcoal  which  has  long  been  exposed 


CHARCOAL    ABSORBS    GASES.  301 

to  the  air,  for  from  the  latter  a  quantity  of  gas  and  vapor  must  be 
expanded  and  driven  out  before  the  coal  can  ignite,  and  this  ex- 
pansion being,  of  course,  attended  with  absorption  of  heat,  keeps 
down  the  temperature  of  the  coal  below  the  kindling  point. 

It  is  not  even  necessary  that  the  coal  shall  have  been  recently 
burned,  in  order  that  it  may  kindle  readily,  if  only  it  be  kept  in 
tightly-closed  vessels,  and  thus  prevented  from  absorbing  mois- 
ture. In  the  laboratory  it  is  well  to  throw  the  remnants  of 
charcoal  fires  into  an  iron  kettle,  furnished  with  a  tightly-fitting 
cover,  in  order  to  have  always  a  store  of  easily  inflammable  coal 
for  starting  new  fires  in  small  hand-furnaces. 

The  absorptive  power  of  charcoal  for  gases  can  readily  be 
shown  directly  by  placing  recently  burned  charcoal  in  a  small 
confined  volume  of  almost  any  gas. 

Exp.  165. —  Fill  a  glass  cylinder  of  200  or  300  c.  c.  capacity  and 
closed  at  one  end  with  dry  sulphurous  acid  gas  (Exp.  100),  over  the 
mercury  trough  ;  take  from  the  fire  a  red-hot  piece  of  charcoal  as  large 
as  can  be  introduced  into  the  mouth  of  the  cylinder  ;  thrust  it  beneath 
the  surface  of  the  mercury  and  hold  it  there  for  a  moment,  in  order 
that  it  may  be  extinguished,  then  pass  it  up  into  the  jar  of  sulphurous 
acid.  Mercury  will  rise  in  the  cylinder  in  proportion  as  the  gas  is 
absorbed,  and  will  soon  completely  fill  it. 

386.  As  one  result  of  the  enormous  condensation  to  which 
gases  are  subjected  when  thus  absorbed  by  coal,  heat  is  neces- 
sarily developed ;  the  temperature  of  the  charcoal  rises  as  the 
gas  is  condensed  in  it,  and  to  such  an  extent  that  heaps  of 
recently -burned  finely-divided  charcoal  often  take'  fire  on  being 
exposed  to  the  air. 

Different  gases  are  absorbed  by  charcoal  in  very  different 
proportions.  It  has  been  found,  by  experiment,  that  one  cubic 
centimetre  of  dry,  compact  charcoal,  such  as  that  from  boxwood 
will  absorb  in  the  course  of  twenty-four  hours, 


90  c.  c.  of  Ammonia  gas. 
85     "     "  Chlorhydric  acid  gas. 
65     "     "  Sulphurous  acid  gas. 
55     "     "  Sulphydric  acid  gas. 
40     "     "  Nitrous  Oxide  gas. 
35     "     "  Carbonic  acid  gas. 


35  c.  c.  of  Olefiant  gas. 

9.4  "     "  Carbonic  Oxide. 
9.3   "     "  Oxygen. 

7.5  "     "  Nitrogen. 
5.0   "     "  Marsh  gas. 
1.8    "     "  Hydrogen. 


302  COMBINATION    THROUGH    CHARCOAL. 

As  one  consequence  of  this  diversity  of  absorbing  power,  it  follows 
that,  from  a  mixture  of  several  gases,  charcoal  can  remove  some 
gases  more  readily  than  others.  Thus,  when  recently-ignited 
charcoal,  which  has  been  cooled  under  mercury,  as  in  the  pre- 
ceding experiment,  is  passed  up  into  a  jar  of  common  air,  it 
absorbs  oxygen  from  it  more  rapidly  than  nitrogen.  But,  on  the 
other  hand,  it  is  found  that,  after  the  charcoal  has  become  satu-" 
rated  with  one  kind  of  gas,  it  can  still  take  up  a  certain  quantity 
of  any  other  gas  which  may  be  presented  to  it. 

387.  This  power  possessed  by  charcoal  of  absorbing  gases  is 
evidently  a  particular  case  of  the  physical  force  called  adhesion 
or  capillary  attraction,  whose  manifestations  are  familiar  to  us 
when  seen  in  the  drinking  up  of  water  by  a  sponge,  or  of  oil  by 
a  lamp-wick ;  if  the  charcoal  be  moistened  with  water,  that  is  to 
say,  if  its  pores  be  clogged  by  the  interposition  of  a  liquid,  its 
absorbing  power  for  gases  will  be  largely  diminished. 

The  subject  is  chiefly  interesting  to  chemists,  because  of  its 
intimate  connection  with  the  power  of  causing  various  gases  to 
combine  with  one  another,  which  is  possessed  by  charcoal  as  well 
as  by  finely-divided  platinum  (§§  224,  240)  and  several  other 
substances.  In  the  same  way  that  spongy  platinum  causes  hy- 
drogen and  oxygen,  or  sulphurous  acid  and  oxygen,  to  unite  with 
one  another,  chemical  combination  ensues  when  mixtures  of 
various  gases  are  brought  into  contact  with  charcoal.  Though 
the  absorbent  power  of  platinum,  as  regards  gases,  can  hardly 
be  compared  with  that  of  charcoal,  its  power  of  causing  com- 
bination is  very  much  greater ;  platinum  appears  to  possess, 
moreover,  in  a  marked  degree,  the  faculty  of  attracting  and 
attaching  to  itself  small  quantities  of  most  gases. 

Charcoal  can  be  made  more  efficient  as  an  agent  for  causing 
combination,  by  covering  it  with  a  film  of  platinum.  If  coarsely- 
powdered  charcoal  be  thoroughly  impregnated  with  bichloride  of 
platinum  by  boiling  it  for  some  time  in  an  aqueous  solution  of 
this  salt,  and  if  it  be  then  heated  to  redness  in  a  close  crucible,  in 
order  that  the  platinum  salt  may  be  decomposed,  there  will  be 
left  a-  residue  of  platinum  everywhere  attached  to  the  sides  of 
the  pores  of  the  coal.  Such  platinized  coal  is  very  effective, 
both  as  an  absorbent  of  gases  and  as  an  agent  for  producing 


CHARCOAL    A    DISINFECTANT.  303 

combination.  But  even  by  itself  charcoal  has  a  very  decided 
influence  upon  the  combination  of  gases. 

If  recently-ignited  charcoal  be  allowed  to  become  charged 
with  dry  sulphydric  acid  gas  and  then  introduced  into  an  at- 
mosphere of  oxygen,  the  elements  of  the  sulphydric  acid  will 
combine  with  the  oxygen  so  quickly  that  an  explosion  ensues, 
and  both  water  and  sulphurous  acid  are  produced. 

If  the  coal  charged  with  sulphydric  acid  be  brought  in  con- 
tact with  air,  instead  of  pure  oxygen,  combination  will  occur  as 
before,  only  more  slowly  ;  in  this  case,  however,  the  hydrogen 
alone  will  be  consumed,  the  sulphur  of  the  sulphydric  acid  being 
set  free  in  the  solid  state. 

Charcoal  is  much  employed  as  a  disinfecting  agent.  It  is 
capable  of  removing  many  offensive  odors  from  the  air,  such,  for 
example,  as  the  fetid  products  given  off  during  the  putrefaction 
of  animal  and  vegetable  substances.  Animal  matter  in  an  ad- 
vanced stage  of  putrefaction  loses  all  offensive  odor  when  covered 
with  a  layer  of  charcoal,  and  the  flesh  of  a  dead  animal  buried 
beneath  a  thin  layer  of  charcoal  will  gradually  waste  away  and 
be  consumed  without  exhaling  any  unpleasant  smell. 

Exp.  166. —  Place  a  small  quantity  of  powdered  charcoal  in  a  bottle 
containing  sulphydric  acid  gas  and  shake  the  bottle.  The  odor  of  the 
sulphydric  acid  will  quickly  disappear.  In  the  same  way,  an  aqueous 
solution  of  sulphydric  acid  (Exp.  90)  can  be  deodorized  by  filtering  it 
through  a  layer  of  charcoal. 

Exp.  167.  —  In  a  shallow  open  basket  or  in  a  box,  through  the  bot- 
tom of  which  numerous  holes  have  been  bored,  spread  a  layer  of 
coarsely-powdered  bone-black  about  5  c.  m.  thick,  placing  a  sheet  of 
filtering  paper  below,  if  need  be,  to  prevent  the  powder  from  sifting 
through  the  holes  in  the  box.  Place  the  body  of  a  rat  or  of  some  other 
small  ahimal  upon  the  layer  of  bone-black,  and  pour  on  more  bone- 
black  until  the  rat  is  covered  with  a  layer  of  it  about  5  c.  m.  deep. 
Hang  up  the  basket  or  box  in  a  warm  room,  so  that  air  may  have  free 
access  to  itr  and  leave  it  at  rest.  After  the  lapse  of  several  weeks 
it  will  be  found,  on  examination,  that  all  the  putrescible  portions  of 
the  animal  have  disappeared,  and  that  nothing  is  left  but  a  mass 
of  hair  and  bones;  but  in  the  interim  no  odor  will  have  been  de- 
tected arising  from  the  decomposing  animal,  excepting  a  faint  odor 
of  ammonia. 


304  CHARCOAL    REMOVES    COLORS. 

In  the  same  way,  water  can  be  preserved  untainted  in  casks  which 
have  been  charred  internally,  and  the  quality  of  some  kinds  of  wine 
is  improved  if  it  be  stored  in  such  casks. 

In  all  these  cases  the  use  of  charcoal  as  a  disinfectant  depends 
not  merely  upon  its  mechanical  ability  to  absorb  offensive  gases, 
but  also  and  mainly  upon  the  fact  that  the  absorbed  gases  are 
chemically  destroyed  within  the  pores  of  the  coal  by  the  oxygen 
which  is  sucked  into  these  spaces  from  the  air.  The  purifying 
action  depends  upon  oxidation,  upon  the  burning  up  of  the  offen- 
sive gases  as  fast  as  they  are  formed.  The  charcoal  is  in  no 
sense  an  antiseptic  or  preservative  agent  proper  to  prevent 
decay ;  on  the  contrary,  it  actually  hastens  the  destruction  of 
putrescible  organic  matters.  Under  ordinary  circumstances, 
while  in  contact  with  the  air,  the  pores  of  charcoal  are,  of  course, 
always  charged  with  oxygen  by  virtue  of  their  absorptive  power. 
Whenever,  therefore,  any  new  gas  is  dragged  in,  and  forced  into 
intimate  contact  with  this  oxygen,  it  is  precisely  as  if  the  gas  had 
been  carefully  collected  and  then  subjected  to  the  action  of  some 
corrosive  chemical  agent.  A  great  merit  of  charcoal  as  a  disin- 
fectant is,  that  it  constantly  draws  in  to  destruction  the  offensive 
matters  around  it ;  pans  of  charcoal  placed  about  a  room,  —  the 
wards  of  a  hospital,  for  example,  —  the  air  of  which  is  offensive, 
soon  remove  the  unpleasant  smell.  Sieves  of  charcoal,  placed 
across  the  air-vents  of  sewers  in  such  manner  that  the  out-going 
air  may  be  filtered  through  the  charcoal,  are  found  to  be  most 
efficient  instruments  for  destroying  the  noxious  effluvia  which 
commonly  escape  from  these  openings.  In  this  case,  where  a 
current  of  air  is  constantly  passing  through  the  charcoal  filter, 
the  latter  will  preserve  its  efficiency  for  an  indefinite  length  of 
time,  if  only  it  be  kept  dry,  for  the  action  of  the  coal  consists 
merely  in  bringing  about  oxidation  and  destruction  of  the  offen- 
sive gases  of  the  sewer,  and  as  fast  as  one  portion  of  these  is 
consumed  a  new  portion  can  be  taken  in  to  destruction. 

388.  Charcoal  not  only  destroys  odors,  but  it  removes  colors 
as  well,  and  for  this  purpose  it  has  long  been  employed  in  the 
purification  of  sugar  and  of  many  chemical  and  pharmaceutical 
preparations.  Almost  any  coloring  matter  can  be  removed  from 
a  solution  by  filtering  the  liquid  through  a  layer  of  charcoal. 


CHARCOAL  REMOVES  COLORS.  305 

Exp.  168. —  Provide  four  small  bottles  of  the  capacity  of  100  or  200 
c.  c.,  and  place  in  each  of  them  a  table-spoonful  of  bone-black  (§  389)  ; 
into  the  first  bottle  pour  a  quantity  of  the  blue  compound  of  iodine 
and  starch  obtained  in  Exp.  73;  into  the  .second,  a  decoction  of 
cochineal ;  into  the  third,  a  diluted  portion  of  the  blue  liquor  obtained 
by  dissolving  indigo  in  Nordhausen  sulphuric  acid,  and  into  the  fourth 
a  solution  of  permanganate  of  potassium  ;  enough  of  the  solution  being 
taken  in  each  instance  to  nearly  fill  the  bottle.  Cork  the  bottles  and 
shake  them  violently,  then  pour  the  contents  of  each  upon  a  filter  (see 
Appendix,  §  14),  and  observe  that  the  filtrate  is  in  each  instance  color- 
less or  nearly  so.  In  case  the  first  portions  of  the  filtrate  happen  to 
come  through  colored,  they  may  be  poured  back  upon  the  filter  and 
allowed  to  again  pass  through  the  coal. 

In  the  purification  of  brown  sugar  the  coloring  matters  are  removed 
in  a  manner  similar  to  the  foregoing,  the  colored  syrup  being  filtered 
through  layers  of  bone-black.  Many  crystallizable  organic  acids  and 
alkaloids  are  purified  in  the  same  way  in  the  chemical  laboratory.  But 
it  must  never  be  forgotten  that,  besides  the  coloring  matters,  charcoal 
can  absorb  many  other  substances ;  sulphate  of  quinine,  for  example, 
is  removed  from  its  solutions,  to  a  very  considerable  extent,  by  char- 
coal, and  the  same  remark  applies,  with  perhaps  still  more  force,  to 
strychnine.  The  bitter  principle  of  the  hop,  "  lupulin,"  may  be  en- 
tirely removed  from  ale  by  filtering  the  latter  through  bone-black. 
The  removal  of  metals,  like  gold  and  silver,  from  their  dilute  solutions, 
by  means  of  charcoal,  appears  to  be  a  phenomenon  of  the"  same  order. 

In  all  these  cases  where  coloring  matters,  and  the  like,  are  re- 
moved from  solutions,  the  action  of  the  coal  appears  to  depend 
in  the  main  directly  upon  the  physical  property  of  adhesion ;  the 
subsequent  oxidizing  action  being  here  far  less  clearly  marked 
than  in  the  instances  previously  studied  (§  387)  where  gases  are 
acted  upon.  Much  of  the  absorbed  color  or  other  matter  will 
usually  be  found  attached  to  the  surfaces  of  the  coal,  undecom- 
posed  and  unaltered.  Thus,  if  the  coal  which  has  been  charged 
with  indigo,, in  Exp.  168,  be  digested  in  a  solution  of  caustic 
soda,  the  latter  will  dissolve  the  indigo  and  remove  it  from  the 
\Joal.  The  alkaloids  above  mentioned,  which  have  been  removed 
from  their  aqueous  solutions  by  means  of  charcoal,  can  be  again 
recovered  by  boiling  the  coal  in  alcohol,  and  the  metals  can  be 
dissolved  again  by  means  of  strong  acids. 

When  employed   to  remove  coloring  matters,  charcoal  soon 


306        DECOLORIZING  POWER  OF  CHARCOAL. 

becomes  saturated  with  the  color  and  ceases  to  absorb  any  more 
of  it ;  if  the  spent  coal  be  then  collected  and  redistilled,  it  will 
be  found  that  it  has  regained  but  little  of  its  decolorizing  power, 
for  its  pores  are  filled  with  charcoal,  resulting  from  the  carboniza- 
tion of  the  absorbed  coloring  matters,  and  this  charcoal  is  not 
porous,  but,  on  the  contrary,  compact  and  glistening,  like  the 
charcoal  obtained  from  sugar  or  glue,  and  is  almost  entirely  des- 
titute of  decolorizing  power.  In  order  to  revivify  their  coal,  the 
sugar-refiners  first  digest  it  in  chlorhydric  acid,  then  allow  it  to 
ferment  in  order  that  the  absorbed  matters  may  be  decomposed, 
and,  finally,  after  washing  and  drying,  subject  it  to  distillation. 

389.  As  obtained  from  different  sources,  charcoal  exhibits 
very  different  degrees  of  decolorizing  power ;  but  of  the  varieties 
commonly  met  with  and  to  be  procured  in  commerce,  bone-black 
is  the  most  efficient.  Bone-black  is  prepared  for  the  use  of 
sugar-refiners,  by  subjecting  bones  to  destructive  distillation  in 
large  iron  cylinders  and  carefully  cooling  the  charcoal  out  of 
contact  with  the  air. 

Exp.  169. —  Repeat  Exp.  153,  p.  290,  but  instead  of  bituminous 
coal,  charge  the  ignition-tube  with  coarsely-powdered  bone.  Distil  as 
long  as  gas  is  evolved,  then  remove  the  delivery-tube  from  the  water, 
plug  it  by  means  of  a  bit  of  caoutchouc  tube  and  a  glass  rod,  and  wait 
until  the  ignition-tube  is  cold,  before  opening  it  to  examine  the  bone- 
black. 

Dry  bones  contain  only  about  one-third  their  weight  of  organic 
matter,  nearly  66  per  cent,  of  them  consisting  of  phosphate  of 
calcium,  in  the  interstices  of  which  the  animal  matter  is  dis- 
tributed ;  hence  it  happens  that  the  charcoal  obtained  by  distill- 
ing bones  is  in  an  exceedingly  porous  and  divided  condition. 
The  decolorizing  power  of  bone  charcoal  may,  moreover,  be  in- 
creased by  digesting  it  in  dilute  chlorhydric  acid,  which  dissolves 
out  a  portion  of  the  phosphate  of  calcium  and  leaves  the  charcoal 
even  more  porous  than  before. 

But  because  bone-black  is  most  commonly  employed  for  de- 
colorizing, it  must  not,  therefore,  be  inferred  that  it  is  the  most 
powerful  decolorizer  of  the  several  varieties  of  charcoal.  A 
more  efficient  coal  can  easily  be  prepared  by  mixing  nitrogenized 
organic  matters,  such  as  blood,  hoofs,  horns,  or  scraps  of  leather, 


DECOLORIZING  POWER  OF  CHARCOAL. 


307 


with  carbonate  of  potassium,  distilling  the  mixture,  and  finally 
leaching  the  product  with  water.  Charcoal  of  peculiar  decolor- 
izing power  may  be  prepared  by  igniting  a  mixture  of  4  parts  of 
fresh  blood  and  1  part  of  carbonate  of  potassium  in  an  iron  cru- 
cible, so  long  as  vapors  are  evolved,  then  washing  the  product 
with  water  and  boiling  it  with  chlorhydric  acid,  and  again  wash- 
ing, drying,  and  igniting  in  a  close  vessel.  A  solution  of  horn- 
filings  in  caustic  potash,  evaporated  to  dryness  and  ignited,  also 
yields  a  very  effective  charcoal.  As  with  bone-black,  the  effi- 
ciency of  this  coal,  prepared  with  caustic  or  carbonated  potash, 
appears  to  depend  upon  the  minute  subdivision  of  its  particles  and 
the  porosity  resulting  from  the  mixture  of  the  inorganic  matter. 
The  decolorizing  power  of  the  charcoal  obtained  from  vegetable 
substances  can  be  in  like  manner  increased  by  mixing  these  sub- 
stances with  lime  or  clay  before  the  carbonization.  A  mixture 
of  100  parts  pipe-clay,  stirred  up  with  water  to  the  consistence 
of  a  thin  paste,  20  parts  of  tar  and  .500  parts  of  powdered  bitu- 
minous coal,  yields  a  charcoal  which  decolorizes  almost  as  well 
as  bone-black.  It  is,  however,  no  very  easy  matter  to  determine 
which,  of  a  number  of  varieties  of  charcoal,  is  the  most  power- 
ful decolorizer,  since  the  decolorizing  power  differs  with  the 
nature  of  the  coloring  matter  as  well  as  according  to  the  quality 
of  the  charcoal.  It  has  been  found,  for  example,  that  with 


THE    RELATIVE     DECOLORIZING 


EQUAL    WEIGHTS    OF     CHARCOAL 
PREPARED   BY 

POWER 

Upon  a  solution  of 
indigo  in  sulphu- 
ric acid  was 

Upon  a  solution  of 
brown  sugar  ivas 

Igniting  a  mixture  of  blood  and  car- 

bonate of  potassium 

50 

20 

Igniting  a  mixture  of  blood  and  car- 

bonate of  calcium     .... 

18 

11 

Igniting  a  mixture  of  blood  and  phos- 

phate of  calcium      .... 

12 

10 

Igniting  a  mixture  of  glue  and  carbon- 

ate of  potassium       .... 

36 

16 

Igniting  acetate  of  sodium    . 

12 

9 

Digesting  bone-black  with  chlorhydric 

1.9 

1.3 

1.9 

1.6 

Ordinary  bone-black  .... 

1 

1 

390.    Compounds  of  Carbon  and  Hydrogen  are  exceedingly 


08 


ORGANIC    CHEMISTRY. 


numerous.  There  are  many  different  series  of  them,  each  mem- 
ber of  either  of  these  series  differing  but  slightly  from  its  next 
neighbors  in  composition  and  properties.  But  all  these  sub- 
stances are  commonly  classed  as  organic  compounds ;  they  con- 
stitute a  special  branch  of  chemical  science  called  organic 
chemistry,  and  are  not  discussed  in  -works  which  treat  of  all 
the  elements  commonly  met  with,  without  special  reference  to 
any  one.  This  subdivision  of  the  general  subject  is  resorted  to 
merely  for  convenience ;  there  is  no  inherent  reason  why  the 
compounds  of  hydrogen  and  carbon  should  not  be  studied  in  this 
manual  as  well  as  the  compounds  of  hydrogen  and  oxygen.  But 
since  carbon  is  capable  of  uniting  with  hydrogen,  oxygen,  or 
nitrogen,  or  with  two  of  these  elements,  or  with  all  three  of 
them,  in  the  most  varied  proportions,  there  are  formed  so  many 
different  compounds,  that  it  has  been  found  advantageous  to  study 
them  by  themselves. 

The  best  definition  of  the  so-called  organic  chemistry  which 
can  be  given  to-day,  is,  that  it  is  the  Chemistry  of  the  Com- 
pounds of  Carbon.  The  department  of  organic  chemistry  has 
grown  out  of  ordinary  chemistry  solely  because  of  the  fact  that 
the  compounds  of  carbon  with  hydrogen,  oxygen,  and  nitrogen 
are  more  numerous,  and  often  of  more  complex  composition,  than 
the  compounds  formed  by  any  of  the  other  elements.  These 
compounds  of  carbon  with  hydrogen,  and  with  the  other  ele- 
ments, are  all  definite  chemical  compounds  conforming  to  the  law 
of  multiple-proportions  (§  76),  but  they  count  by  thousands,  and 
the  mere  enumeration  of  their  names  and  properties  would  fill  a 
volume. 

391.  As  examples  of  the  series  of  compounds  of  carbon  and 
hydrogen,  to  which  allusion  was  just  now  made,  the  following 
lists  may  be  given :  • — 

Boils  at  °  C, 


Soils  at  °  C. 


Soils  at  °  C. 


CH2 

CH4 

C2H4 

C2H6 

CgHfi 

C6H6    . 

.      80° 

C3H8 

.  -—30° 

C4H8 

.       5o 

C7H8    . 

.    110° 

C4H10 

GO 

C5H10 

.     35° 

C8H10  . 

.    140o 

C5H12 

.       30° 

C6H12 

.     65° 

C9H12  . 

.    170° 

.          60°          : 

C7H14 

.     95° 

C7Hia 

.       90° 

.  125° 

HOMOLOGOUS    SERIES.  309 

The  first  series  of  the  compounds,  above  formulated,  is  found  in 
petroleum,  and  in  the  oil,  known  as  coal-oil,  obtained  by  distilling 
highly  bituminous  coals  and  shales  at  comparatively  low  tem- 
peratures ;  the  petroleum  used  for  lighting  contains  all  these 
different  compounds,  together  with  others  of  the  same  class. 
The  second  series  may  generally  be  obtained  by  the  destructive 
distillation  of  various  organic  compounds ;  and  the  members  of 
the  third  series  are  found  in  the  most  volatile  portion  of  coal- 
tar,  —  the  tar  obtained  by  distilling  bituminous  coal  at  high 
temperatures,  as  in  the  manufacture  of  illuminating  gas. 

It  will  be  observed  that  there  is  a  constant  difference  of  one 
atom  of  carbon  and  two  atoms  of  hydrogen  (CH2)  between 
any  two  contiguous  compounds  enumerated  in  the  lists.  The 
boiling-points  of  the  several  members  exhibit  a  constant  differ- 
ence of  30°  for  each  increment  of  CH2 ,  as  we  go  down  the  lists, 
so  far  as  has  been  determined  ;  and  so  with  all  the  other  physical 
properties,  such  as  specific  gravity,  mobility,  expansibility  by 
heat,  and  the  like ;  the  intensity  of  these  properties  increases  or 
decreases  regularly,  in  a  constant  ratio,  as  we  pass  from  one 
member  of  the  series  to  the  members  next  in  order.  Such  series 
are  called  "  homologous  "  series  (having  the  same  proportion)  ; 
they  nre  clearly  analogous  to  the  groups,  families,  or  series  of 
elements  which  we  have  already  studied  in  chlorine,  bromine, 
and  iodine  (§  152)  ;  in  oxygen,  sulphur,  selenium,  and  tellurium 
(§  257)  ;  in  nitrogen,  phosphorus,  arsenic,  antimony,  and  bis- 
muth (§  364),  and  are  now  proceeding  to  study  in  carbon,  boron, 
and  silicon.  Just  as  in  these  groups  of  elements,  the  student  has 
seen  a  true  serial  arrangement  of  the  different  members,  and  has 
observed  that  the  different  terms  of  each  series  differ  from  one 
another  in  atomic  weight  and  exhibit  parallel  differences  in  the 
intensity  of  their  properties,  so  here  in  each  of  the  homologous 
series  of  hydrocarbons  there  have  been  observed  similar  constant 
differences.  In  the  series  of  hydrocarbons,  we  know  that  there  is 
a  constant  difference  of  composition  of  CH2 ,  and  this  difference 
of  composition  we  believe  to  be  at  the  bottom  of  the  constant 
difference  of  physical  properties,  but  to  the  cause  of  the  constant 
differences  in  the  homologous  series  of  elements  we  have,  as  yet? 
no  clew. 


310  MARSH-GAS. 

The  power  of  arranging  numerous  allied  compounds  into 
groups  or  series,  like  those  enumerated  upon  page  308,  has  been 
of  great  service  to  chemists  in  facilitating  the  study  of  the  com- 
pounds of  carbon.  For  every  such  series,  a  general  algebraic 
expression  has  been  devised  which  serves  as  the  name  of  the 
series.  Like  any  other  name,  this  concise  expression  brings 
before  the  mind  of  the  chemist  the  general  properties  of  the 
series.  Thus  the  expression  CnHza  +  2  is  general  for  the  first 
series  above  mentioned,  CnH2a  for  the  second,  and  Cu  qp  3H2n 
for  the  third. 

In  this  manual  we  shall  describe  only  one  of  the  compounds 
of  carbon  and  hydrogen,  a  compound  which  occurs  ready  formed 
in  nature. 

392.  Marsh-gas,  or  Light  Carburetted  Hydrogen  (CH4),  is  a 
permanent  gas  which  constitutes  a  large  proportion  of  the  ordi- 
nary illuminating  gas  obtained  from  coal.  It  is  disengaged  in 
large  quantities  from  some  sorts  of  bituminous  coal,  even  at  the 
ordinary  temperature  of  the  air,  and  more  rapidly  at  higher  tem- 
peratures. In  coal  mines,  the  gas  thus  given  off  is  known  as 
"  fire-damp  "  ;  by  mixing  with  air,  in  the  galleries  of  badly-ven- 
tilated mines,  it  forms  explosive  mixtures,  which  frequently  occa- 
sion frightful  accidents,  when  ignited  through  carelessness. 

The  gas  is  evolved  also,  in  large  quantities,  from  the  mud  at 
the  bottom  of  stagnant  pools ;  it  is  one  of  the  products  of  the 
putrefaction  of  vegetable  matter  under  water,  where  the  supply 
of  air  is  insufficient  to  oxidize  the  matter  completely  to  carbonic 
acid  and  water ;  hence  the  name  marsh-gas. 

In  hot  weather  marsh-gas  can  be  obtained  by  thrusting  a  pole  into 
the  mud  at  the  bottom  of  a  pond  and  collecting  the  bubbles  of  gas  as 
they  arise,  by  holding  over  them  an  inverted  bottle  full  of  water. 
When  the  bottle  has  been  filled  with  gas  it  should  be  corked  tightly 
under  water,  and  carried  to  the  laboratory  for  examination.  The  gas 
thus  obtained  is  contaminated  with  nitrogen  and  with  a  large  quantity 
of  carbonic  acid,  these  gases  being  set  free,  together  with  marsh-gas 
during  the  process  of  putrefaction.  Before  the  gas  thus  collected  wil 
burn,  it  is  usually  found  necessary  to  remove  from  it  the  carbonic  acid 
this  can  be  done  by  pouring  into  the  bottle  a  small  quantity  of  a  solu- 
tion of  caustic  soda,. or  some  milk  of  lime,  closing  and  shaking  the 
bottle,  and  finally  removing  the  stopper  under  water.  The  bottle  ma) 


PREPARATION    OF   MARSH-GAS. 


311 


Fio.  50. 


then  be  placed  upright  and  a  lighted  match  applied  to  the  gas  ;  it  will 
take  fire  and  burn  with  a  blue  flame,  the  size  of  which  may  be  in- 
creased by  pouring  water  into  the  bottle  so  that  the  gas  shall  be  driven 
out  into  the  air. 

In  order  to  prepare  the  gas  artifi- 
cially, a  mixture  of  1  part  by  weight 
of  acetate  of  potassium  and  two  or 
three  parts  of  caustic  potash,  may  be 
heated  in  an  ignition-tube  provided 
with  a  delivery-tube  and  arranged  as 
in  Fig.  50.  The  carburetted  hydro- 
gen is  given  off  at  a  temperature 
below  redness,  and  carbonate  of  po- 
tassium remains  in  the  ignition-tube. 
Instead  of  caustic  potash,  slaked  lime 
may  be  used.  The  reaction  may  be  represented  as  follows :  — 

C2H3K02        +        KHO        =         CH4  +          K2C03. 

Acetate  of  Potassium.      Hydrate  of  Potassium.       Marsh-gas.     Carbonate  of  Potassium. 

393.  Marsh-gas  is  transparent,  colorless,  and  little  more  than 
half  as  heavy  as  air.     Next  to  hydrogen  it  is  the  lightest  known 
substance,  its  specific  gravity  being  only  8.     It  takes  fire  readily 
when   touched  with   a  lighted  match,  but  is  nevertheless  more 
difficult   of  inflammation  than    most  of  the  other  combustible 
compounds  of  hydrogen.     While  free  hydrogen  and  sulphuretted 
hydrogen  can  be  lighted  by  a  glass  rod  which  has  been  heated  to 
dull  redness,  the  rod  must  be  raised  to  the  temperature  of  bright 
redness,  or  even   to  a  white  heat,  in  order  that  it  may  kindle 
marsh-gas.     As  prepared  from  acetate  of  potassium,   the   gas 
burns  with  a  pale  yellowish-blue  flame.     It  is  rather  more  solu- 
ble in  water  than  oxygen  ;  at  0°  one  volume  of  water  dissolves 
0.055  volume  of  it.     It  has  never  been  condensed  to  the  liquid 
condition. 

394.  We  have,  thus  far,  observed  three  different  proportions 
in  which  the  other  elements  unite  with  hydrogen  ;  the  members 
of  the  chlorine  group  unite  with  hydrogen,  by  preference,  in  the 
proportion  of  one  volume  to  one  volume ;  one  volume  of  any 
member  of  the  sulphur  group  combines,  by  preference,  with  two 
volumes  of  hydrogen  ;  one  atom  of  any  member  of  the  nitrogen 
group  unites,  by  preference,  with  three  atoms  of  hydrogen.     The 


312  ANALYSIS    OP   MARSH-GAS. 

condensation  increases  in  direct  ratio  to  the  increasing  proportion 
of  hydrogen,  so  that,  in  every  case,  two  volumes  only  of  the 
resultant  compound  are  produced.  We  have  thus  become  famil- 
iar with  the  fact,  that  the  space  occupied  by  the  molecule  of  a 
compound  gas  is  always  two  unit-volumes  (§  258).  An  exam- 
ination of  the  molecule  of  marsh-gas  will  reveal  a  fourth  kind  of 
hydrogen-compound,  —  a  compound  containing  in  the  product- 
volume  (§  260)  four  volumes  of  hydrogen  condensed.  It  is  not 
difficult  to  prove  experimentally  that  two  volumes  of  marsh-gas 
yield,  on  decomposition,  four  volumes  of  hydrogen,  but  unfortu- 
nately it  is  not  within  our  power  to  demonstrate,  by  experiment, 
the  volume  of  carbon  with  which  these  four  volumes  of  hydro- 
gen are  combined,  for  carbon  is  a  solid  incapable  of  volatilization 
by  the  intensest  heat  at  our  present  command.  We  were  able 
to  determine  the  combining  volume  of  each  constituent  of  chlor- 
hydric  acid,  steam,  and  ammonia,  but  we  have  no  positive 
knowledge  whatever  concerning  the  manner  in  which  carbon 
enters  into  combination  by  volume.  Its  combining  proportion  by 
weight  can  be  ascertained,  but  it  must  be  carefully  observed  that 
all  views  respecting  the  volumetric  composition  of  the  very 
numerous  compounds  of  carbon  are  purely  speculative,  so  far  as 
the  carbon  is  concerned,  until  carbon  shall  have  been  actually 
volatilized  and  its  vapor  weighed. 

That  marsh-gas  really  contains  hydrogen  and  carbon  may  be 
readily  proved  by  bringing  into  play,  under  appropriate  condi- 
tions, the  strong  affinity  of  chlorine  for  hydrogen.  Chlorine  will 
set  free  carbon  from  marsh-gas,  just  as  it  liberates  nitrogen  from 
ammonia  (Exp.  67). 

Exp.  1 70.  —  Fill  a  tall  bottle  of  at  least  one  litre  in  capacity  with 
warm  water,  invert  it  over  the  \vater-pan,  and  pass  marsh-gas  into  it, 
until  a  little  more  than  one-third  of  the  water  i  s  displaced  ;  cover  the 
bottle  with  a  thick  towel,  to  exclude  the  light,  and  theii  fill  the  rest  of 
the  bottle  with  chlorine.  Cork  the  bottle  tightly,  and  shake  it  vigor- 
ously, in  order  to  mix  the  gases  together,  keeping  the  bottle  always 
covered  with  the  towel.  Finally,  open  the  bottle  and  apply  a  light  to 
the  mixture.  Ignition  takes  place,  chlorhydric  acid  is  produced,  while 
the  sides  and  mouth  of  the  bottle  become  coated  with  solid  carbon  in 
•the  form  of  lamp-black.  The  presence  of  the  acid  may  be  proved  by 


ATOMIC    WEIGHT    OF    CARBON.  313 

the  smell,  by  its  reaction  with  moistened  blue  litmus-paper,  and  by  the 
white  fumes  which  are  generated  when  a  rod  moistened  with  ammonia 
water  is  brought  in  contact  with  the  escaping  acid  gas. 

Carbon  and  hydrogen  are,  therefore,  elementary  constituents 
of  marsh-gas.  We  should  be  glad  to  add  the  synthetical  to  the 
analytical  demonstration,  and  make  marsh-gas  out  of  carbon  and 
hydrogen,  but  no  means  are  at  present  known  by  which  these 
two  elements  can  be  directly  combined  to  form  marsh-gas.  As 
in  the  case  of  ammonia,  we  are  obliged  to  rely  upon  the  assur- 
ance of  the  balance  that  the  sum  of  the  weights  of  the  two 
constituents,  separated  from  a  given  quantity  of  marsh-gas,  is 
precisely  equal  to  the  weight  of  the  marsh-gas  submitted  to 
analysis.  The  experimental  process  by  which  this  fact  is  demon- 
strated is  too  complex  to  be  profitably  studied  at  this  stage  of 
progress,  and  the  fact  must,  therefore,  be  accepted  as  the  result 
of  experience. 

395.  It  remains  to  show  how  the  investigation  of  the  com- 
position of  the  product-volume  of  marsh-gas  leads  to  the  knowl- 
edge of  the  atomic  weight,  or  least  combining  weight,  of  the 
element  carbon.  Marsh-gas  is  the  compound  best  suited  for 
ascertaining  the  atomic  weight  of  carbon,  because  experience 
has  proved  that  it  contains  proportionally  less  carbon  than  any 
other  hydride  of  the  element.  We  determined  the  atomic 
weight  of  oxygen  from  steam,  the  hydrogen  compound  which 
contains  the  smallest  proportion  of  oxygen ;  of  chlorine,  from 
chlorhydric  acid,  the  only  hydride  of  chlorine ;  of  nitrogen,  from 
ammonia,  the  hydride  which  contains  the  smallest  proportion  oi 
nitrogen.  So  the  atomic  weight  of  carbon  is  the  weight  of  car- 
bon which  experiment  proves  to  be  contained  in  two  unit- vol- 
umes of  marsh-gas.  By  physical  determinations,  the  specific 
gravity  of  marsh-gas  has  been  shown  to  be  8 ;  in  other  words, 
one  unit-volume  of  marsh-gas  weighs  8 ;  then  two  unit-volumes, 
or  the  product-volume,  must  weigh  16.  How  much  of  this 
weight  of  1 6  is  hydrogen  ?  This  question  can  be  answered  ex- 
perimentally by  ascertaining  how  many  unit-volumes  of  hydrogen 
are  locked  up  in  two  unit-volumes  of  marsh-gas. 

When  a  series  of  electric  sparks  begin  to  traverse  a  measured 
volume  of  marsh-gas  contained  in  a  U-tube,  arranged  like  the 


314  ATOMIC    WEIGHT    OF    CARBON. 

U-tube  of  figure  11,  but  without  the  jacket,  b  c,  and  its  accom- 
paniments, the  gas  is  found  to  expand,  and  in  a  few  minutes  a 
light  deposit  of  carbon  appears  in  the  vicinity  of  the  points  of 
the  platinum  wires.  •  The  decomposition  of  the  marsh-gas  pro- 
ceeds slowly,  so  that  a  considerable  time  is  required  for  the 
execution  of  the  experiment.  If  the  mercury  in  the  U-tube  be 
finally  brought  to  a  level  in  the  two  limbs,  it  will  be  seen  that 
the  original  volume  of  gas  has  very  nearly  doubled.  When 
this  point  is  once  attained,  the  continued  transmission  of  sparks 
produces  no  further  increase  of  the  volume  of  the  gas.  The 
expanded  gas  may  be  shown  by  the  usual  tests  to  be  hydrogen. 

This  experiment  is  rather  difficult  to  perform,  and  does  not 
yield  perfectly  exact  results,  for  a  minute  portion  of  marsh-gas 
escapes  decomposition ;  nevertheless  it  establishes,  beyond  rea- 
sonable doubt,  the  fact  that  marsh-gas  contains  twice  its  volume 
of  hydrogen.  Two  unit-volumes  of  marsh-gas,  weighing  16, 
must,  therefore,  contain  four  unit-volumes  of  hydrogen  ;  but  four 
unit-volumes  of  hydrogen  weigh  4 ;  therefore,  the  quantity  of 
carbon  contained  in  the  product-volume  of  marsh-gas  must  weigh 
12,  which  number  we  admit  as  the  atomic  weight  of  carbon. 

But  it  may  be  said,  —  what  means  have  we  of  knowing  that 
12  represents  the  least  combining  weight  of  carbon,  for  the  atom 
is  by  definition  the  least  quantity  of  an  element  which  can  be 
conceived  to  exist  in  combination.  If  it  was  possible  to  demon- 
strate that  the  proportional  quantity  by  weight  of  carbon,  which 
is  represented  by  the  number  of  12,  was  just  the  quantity  re- 
quired to  fill  one  unit-volume  when  converted  into  vapor,  we 
should  have  the  same  reason  for  believing  12  to  be  the  weight  of 
one  atom  of  carbon  that  we  have  for  considering  16  to  be  the 
weight  of  one  atom  of  oxygen,  or  35.5  the  weight  of  one  atom 
of  chlorine.  As  we  cannot  convert  carbon  into  vapor  at  all,  it 
is  impossible  to  ascertain  with  certainty  how  many  atoms,  or 
how  many  volumes,  12  proportional  parts  by  weight  of  carbon 
really  represent ;  the  quantity  of  carbon  which  is  combined  with 
four  atoms  of  hydrogen  in  marsh-gas  may  be  four  atoms,  each 
weighing  3,  or  two  atoms,  each  weighing  6,  or  one  atom  weighing 
12.  As  it  is  necessary  to  assume  some  number  of  atoms,  as 
represented  by  the  proportional  weight  12,  that  assumption 


TYPICAL   HYDROGEN    COMPOUNDS.  315 

which  will  conveniently  formulate  the  simplest  and  most  familiar 
compounds  of  carbon  will  be  the  best.  Accordingly  it  has,  of 
late,  been  assumed  that  12  proportional  parts  by  weight  of  car- 
bon constitute  the  least  quantity  of  this  element  which  is  con- 
ceived to  enter  into  chemical  combination,  or,  in  other  words, 
that  the  atomic  weight  of  carbon  is  12.  The  atom  of  carbon, 
thus  understood,  can  then  fix,  or  combine  with,  four  atoms  of 
hydrogen  or  of  any  member  of  the  chlorine  group,  and  two 
atoms  of  oxygen  or  of  any  other  member  of  the  sulphur  group. 
The  following  substances,  familiar  in  common  life,  or  subjects  of 
discussion  in  this  chapter,  may  be  mentioned  in  illustration  of 
this  principle  :  — 

Marsh-gas    .     .     .     CH4  Carbonic  acid    .     .     .     COa 

Chloroform  .     .     .     CHC13        Bisulphide  of  carbon  .     CS2 
Chloride  of  carbon     CC14 

If  it  were  assumed  that  12  proportional  parts  by  weight,  the 
relative  quantity  of  carbon  in  each  of  the  above-mentioned  com- 
pounds, represent  two  or  four  atoms  or  volumes  of  carbon,  instead 
of  one,  every  one  of  these  formulae  would  become  less  simple. 
There  are  a  great  multitude  of  compounds  which  contain  a  larger 
proportional  quantity  of  carbon  than  the  compounds  cited  above, 
but  the  greater  proportional  quantity  is  always  some  multiple  of 
12  proportional  parts,  or,  in  other  words,  is  always  an  integral 
number  of  carbon  atoms,  each  weighing  12. 

In  marsh-gas,  we  have  thus  found  a  new  term  in  the  series  of 
hydrogen  compounds.  Marsh-gas  is  an  example  and  type  of  the 
hydrides  richest  in  hydrogen  ;  so  far  as  we  yet  know,  hydrogen 
does  not  form  with  any  element  whatsoever,  any  compound 
whereof  the  product-volume  contains  more  than  four  atoms  of 
hydrogen  united  with  one  atom  of  the  other  element.  The  fol- 
lowing brief  table  comprehends  all  the  principal  types  of  hydro- 
gen compounds,  beginning  with  chlorhydric  acid,  the  poorest 
hydride,  and  passing  through  water  and  ammonia,  intermediate 
compounds,  to  marsh-gas,  the  hydride  richest  in  hydrogen :  — 

Chlorhydric  acid  f  HC1  =   2  volumes. 

Water H2O  =    2         ** 

Ammonia H3N  =   2         " 

Marsh-as UC  =2        " 


316  ILLUMINATING    GAS. 

Each  of  these  types  of  hydrogen-compounds  characterizes  a 
group  of  chemical  elements,  —  the  first  type  is  characteristic  of 
the  chlorine  group ;  the  second,  of  the  sulphur  group ;  the  third, 
of  the  nitrogen  group,  and  the  fourth,  of  the  group  which  we 
shall  soon  know  as  the  carbon  group. 

396.  The  compounds  of  carbon  and  hydrogen  are  of  great 
practical  interest,  since  the  flame  of  all  ordinary  lamps  and  fires 
results  from  their  combustion.  Any  allusion  to  their  properties 
at  once  suggests  the  influence  which  these  properties  exert  in 
the  usual  methods  of  obtaining  light  and  heat,  and  necessitates  a 
more  complete  discussion  of  the  subjects  of  flame  and  combus- 
tion than  we  have  had  hitherto.  We  shall  recur  to  this  subject 
in  a  subsequent  section,  after  having  studied  the  oxides  of  car- 
bon. For  the  elucidation  of  the  subject  of  combustion,  ordinary 
illuminating  gas,  which  is  a  mixture  of  many  hydrides  of  carbon, 
will  serve  as  well  as  any  pure  hydrocarbon.  A  brief  description 
of  this  product  may  here  be  given. 

About  94-100ths  of  the  volume  of  purified  coal-gas  consists 
of  a  mixture  of  marsh-gas,  free  hydrogen,  and  carbonic  oxide, 
—  the  marsh-gas  usually  amounting  to  about  one-third  part  of 
the  whole  gas.  These  non-luminous,  or  very  feebly  luminous 
gases,  serve  as  carriers  of  the  six  or  seven  per  cent,  of  real 
light-producing  ingredients  which  are  contained  in  the  gas. 
This  mixture  of  light-giving  ingredients  is  exceedingly  complex. 
The  vapor  of  benzole  is,  no  doubt,  one  of  the  most  important  of 
these  ingredients.  Some  of  the  higher  homologues  of  marsh- gas 
lend  their  aid,  and  a  hydrocarbon  of  composition  C2H2 ,  called 
acetylene,  is  important  and  very  generally  present.  Sometimes 
a  little  olefiant  gas  (C2H4)  is  present,  as  well  as  other  compounds 
of  the  same  homologous  series,  but  the  old  view,  that  this  sub- 
stance constitutes  the  chief  luminiferous  ingredient  of  coal  gas,  is 
no  longer  admitted. 

For  all  practical  purposes,  we  can  here  consider  this  mixture 
of  gases  as  carburetted  hydrogen.  That  it  contains  hydrogen, 
can  readily  be  shown  by  holding  a  cold,  dry  bottle  over  a  burn- 
ing jet  of  it,  and  observing  that  water  is  a  product  of  the  com- 
bustion ;  and  that  it  contains  carbon  can  be  seen  by  holding  in 
the  flame  a  piece  of  cold  porcelain,  and  noting  the  deposition  of 


CARBON   AND    OXYGEN.  317 

soot.  Coal-gas  is  only  about  half  as  heavy  as  air ;  in  many 
respects  it  resembles  hydrogen,  and  most  of  the  experiments 
which  were  performed  with  hydrogen  can  be  equally,  or  nearly, 
as  well  performed  with  this  gas.  The  student  will  do  well  to 
repeat,  as  an  example,  Exp.  24,  substituting,  for  the  hydrogen, 
common  gas  drawn  from  the  street  mains.  In  the  same  way  he 
may  repeat  Exp.  29,  mixing  1  volume  of  coal-gas  with  from  8 
to  12  volumes  of  air.  If,  instead  of  coal  gas,  pure  light  car- 
buretted  hydrogen  be  taken,  the  explosion  will  be  most  violent, 
\yith  8  to  10  volumes  of  air ;  with  only  3  or  4  volumes  of  air, 
or  more  than  15  volumes,  the  mixture  is  not  explosive ;  either 
too  much  or  too  little  air  prevents  the  explosion. 

Compounds  of  Carbon  and  Oxygen.  There  are  two  of  these 
compounds,  —  carbonic  acid  (CO2),  and  carbonic  oxide  (CO). 

397.  Carbonic  Acid  (CO2)  is  always  formed  when  carbon  or 
any  of  its  compounds  is  burned  in  an  excess  of  air  or  of  oxygen 
gas,  or  in  contact  with  substances,  gaseous,  liquid,  or  solid,  which 
are  rich  in  oxygen  and  yield  it  readily  to  other  bodies. 

Exp.  171. —  Place  a  live  coal  (charcoal)  upon  a  deflagrating  spoon, 
and  thrust  it  into  a  bottle  full  of  air,  or,  better,  oxygen  gas ;  cover  the 
bottle  closely  and  set  it  aside  for  examination.  Or,  invert 
an  empty  bottle  over  a  burning  lamp  or  candle,  so  that  the 
products  of  the  combustion  of  the  lamp  may  be  received 
in  it ;  the  bottle  will  immediately  become  clouded  upon 
the  inside  from  deposition  of  water  resulting  from  the 
combustion  (see  §  56),  and  will  also  be  filled  with  carbona- 
ceous and  other  gaseous  products,  simultaneously  formed. 
Cover  the  bottle  and  test  its  contents  in  the  manner  de- 
scribed in  the  succeeding  experiment. 

Exp.  172.  —  Pour  some  lime-water  —  a  solution  of  common  slaked 
lime  in  water  —  into  the  bottles  filled  with  gaseous  products  of  com- 
bustion in  Exp.  171,  and  shake  the  bottles.  The  liquid  will  become 
milky  and  turbid,  and,  when  left  at  rest,  will  deposit  a  white  powder 
(carbonate  of  calcium).  The  presence  of  carbonic  acid  can  readily  be 
detected  by  means  of  lime-water,  since  this  insoluble  precipitate  of 
carbonate  of  calcium  is  formed  when  the  two  substances  are  brought 
together. 

The  bottles  of  gas  obtained  in  Exp.  171,  will,  of  course,  con- 
tain, besides  carbonic  acid,  a  quantity  of  nitrogen  derived  from 


318  PREPARATION    OP    CARBONIC    ACID. 

the  air  which  took  part  in  the  combustion,  unless,  indeed,  as  was 
suggested,  the  charcoal  be  burned  in  pure  oxygen.  It  is  to  be 
observed  that,  in  all  ordinary  cases  of  combustion,  whether  of 
wood,  coal,  wax,  or  oil,  there  result  these  same  gaseous  products, 
—  carbonic  acid  and  nitrogen ;  they  ascend,  as  invisible  aerial 
currents,  from  every  well-regulated  flame  or  fire,  and  are  con- 
tinually issuing  from  the  chimneys  of  ouc  houses,  though,  in  the 
absence  of  the  particles  of  solid  carbon,  or  of  condensed  aqueous 
vapor,  which  constitute  smoke,  we  can  see  no  product  of  the 
combustion. 

Exp.  173. —  As  was  just  now  said,  carbonic  acid  may  be  produced 
also  by  heating  carbon  in  contact  with  solid  bodies  which  contain  oxy- 
gen, such,  for  example,  as  the  red  oxide  of  mercury.  Mix  11  grammes 
of  red  oxide  of  mercury  with  0.33  grm.  of  charcoal ;  place  the  mix- 
ture in  an  ignition-tube,  arranged  as  in  figure  50  ;  heat  the  tube  and 
collect  over  water  the  gas  which  is  evolved.  Test  the  product  with 
lime-water,  as  in  Exp.  172.  The  reaction  between  the  charcoal  and 
the  oxide  of  mercury  may  be  written  as  follows :  — 

2HgO  -f-  C  =  CO2  -f  2Hg. 

The  metallic  mercury  set  free  condenses  in  droplets  upon  the  cold 
upper  portions  of  the  ignition-tube.  Here,  again,  as  in  Exps.  128,  160, 
the  metallic  oxide  is  reduced  by  the  charcoal. 

398.  Carbonic  acid  may  readily  be  obtained  from  certain  com- 
pounds called  carbonates,  several  of  which  are  abundant  min- 
erals. Common  chalk,  marble,  and  limestone,  for  example,  are 
composed  of  carbonate  of  calcium,  and  carbonic  acid  can  readily 
be  obtained  by  strongly  heating  them,  or  by  subjecting  them  to 
the  action  of  strong  acids. 

Exp.  1 74.  —  Place  two  or  three  grammes  of  coarsely-powdered  mar- 
ble in  an  ignition-tube  provided  with  a  gas  delivery-tube  bent  at  a 
right  angle  ;  place  the  ignition-tube  upon  the  iron-stand  over  the  gas- 
lamp,  and  dip  the  outer  opening  of  the  delivery-tube  into  a  small  bottle 
containing  liine- water ;  heat  the  marble  strongly,  and  observe  the  white 
precipitate  which  forms  in  the  lime-water  as  the  carbonic  acid  gas 
comes  in  contact  with  it.  The  carbonate  of  calcium,  thus  precipitated 
by  bringing  together  carbonic  acid  and  oxide  of  calcium,  is  chemically 
identical  with  the  chalk  or  marble  from  which  the  acid  was  expelled. 

In  actual  practice  enormous  quantities  of  carbonic  acid  are 


PREPARATION    OF    CARBONIC    ACID. 


319 


FIG.  52. 


expelled  from  limestone  in  this  way  for  the  sake  of  the  quick- 
lime which  is  left  as  a  residue,  but  the  carbonic  acid,  thus  ex- 
pelled by  heat,  is  rarely  collected,  for  a  more  convenient  method 
of  procuring  it  is  to  treat  the  limestone  with  some  acid  capable 
of  expelling  carbonic  acid. 

Exp.  175. —  In  a  gas-bottle  of  500  or  600  c.  c.  capacity,  arranged 
precisely  as  for  generating  hydrogen  (see  Exp.  19),  place  10  or  12 
grms.  of  chalk  or  marble  in  small 
lumps ;  cover  the  chalk  with  water, 
and  pour  in  through  the  thistle- 
tube  concentrated  ehlorhydric 
acid,  by  small  portions,  in  such 
quantity  as  shall  insure  a  con- 
tinuous and  equable  evolution  of 
gas.  Collect  several  bottles  of 
the  gas  over  water,  then  replace 
the  anterior  portion  of  the  de- 
livery-tube with  a  straight  tube 
and  collect  one  or  two  bottles  of 
the  gas  by  displacement ;  carbonic 
acid  gas  is  half  as  heavy  again  as 
air.  The  reaction  between  the  carbonate  of  calcium  and  the  ehlorhy- 
dric acid  may  be  thus  formulated :  — 

CaC03  +  2HC1  =  CaCl2  +  H2O  +  CO2. 

When  chalk  is  the  material  operated  upon,  sulphuric  acid  may  be  sub- 
stituted with  advantage  for  the  ehlorhydric  acid ;  for  the  latter,  being 
rather  easily  volatile,  is  liable  to  be  carried  forward  by  the  current  of 
carbonic  acid,  and  to  contaminate  the  product.  When  carbonic  acid  is 
prepared  for  commercial  purposes  by  the  action  of  an  acid  upon  car- 
bonate of  calcium,  sulphuric  acid  is  almost  always  the  acid  employed, 
and  powdered  chalk  the  substance  acted  u-pon  :  — 

CaC03  +  H2S04  =  CaS04  +  H2O  +  CO2. 

With  a  porous  material,  like  chalk,  this  action  occurs  readily,  but  in 
attempting  to  operate  upon  compact  varieties  of  carbonate  of  calcium, 
such  as  marble,  difficulties  are  encountered ;  the  sulphate  of  calcium, 
which  is  a  product  of  the  reaction,  is  a  rather  difficultly  soluble,  sub- 
stance, and,  being  deposited  upon  the  surface  of  the  marble,  soon  covers 
it  with  a  coating  so  thick  and  impermeable  that  the  action  of  the  sul- 
phuric acid  upon  the  marble  is  well-nigh  completely  arrested. 

Carbonate  of  calcium  being  cheaper  than  most  other  carbonates,  is 


320  PROPERTIES    OF    CARBONIC    ACID. 

more  commonly  employed  than  any  other  as  a  source  of  carbonic  acid, 
but  it  is  sometimes  convenient  to  substitute  for  it  the  carbonate  of  so- 
dium, or  of  potassium  (saleratus),  or  even  of  ammonium,  since  carbonic 
acid  is  given  off  from  these  compounds  very  quickly  and  abundantly. 
A  self-regulating  gas-generator,  such  as  represented  in  Fig.  xxviii.  of 
the  Appendix,  charged  with  large  solid  lumps  of  the  commercial  car- 
bonate of  ammonium  and  chlorhydric  acid,  is,  perhaps,  the  most  con- 
venient apparatus  which  can  be  employed  for  preparing  the  gas  upon 
the  lecture-table. 

399.  At  the  ordinary  atmospheric  temperature  and  pressure, 
carbonic  acid  is  a  transparent,  colorless  gas,  of  *a  slightly  acid 
smell  and  taste.     It  is  incombustible,  being  already  the  product 
of  the  complete  combustion  of  carbon,  and  is,  moreover,  incapa- 
ble of  supporting  the  combustion  of  most  other  bodies,  since  the 
oxygen   contained  in   it  is  very  firmly  held;  like  nitrogen,  it 
immediately  extinguishes   burning  bodies  which  are  immersed 
in  it. 

Exp.  176.  —  Thrust  into  a  bottle  of  the  gas,  obtained  in  Exp.  175,  a 
lighted  candle,  or,  better,  a  large  flame  of  alcohol  burning  upon  a  tuft 
of  cotton  ;  in  either  case  the  flame  will  be  instantly  extinguished. 

The  specific  heat  of  gaseous  carbonic  acid,  between  10°  and 
200°,  is  0.2169.  Its  specific  gravity  is  22  ;  being  thus  1.53  times 
heavier  than  air,  it  can  be  poured  from  one  vessel'  to  another 
almost  as  readily  as  if  it  were  water. 

Exp.  111.  —  Invert  a  bottle  filled  with  carbonic  acid  upon  another 
bottle  of  equal  size  filled  with  air,  in  such  manner  that  the  mouth  of  the 
upper  inverted  bottle  shall  rest  upon  the  mouth  of  the  lower  bottle. 
After  the  lapse  of  several  minutes,  thrust  a  burning  splinter  of  wood 
into  each  of  the  bottles  ;  in  the  upper  bottle  the  splinter  will  continue 
to  burn,  for  into  this  bottle  the  air  from  the  lower  bottle  has  ascended, 
while  in  the  lower  bottle,  now  full  of  carbonic  acid,  the  splinter  will 
be  extinguished. 

Exp.  178.  —  From  a  large  bottle  full  of  the  gas,  pour  a  quantity  of 
carbonic  acid  upon  the  flame  of  a  lamp  or  candle  ;  that  is  to  say,  hold 
the  mouth  of  the  open  bottle  of  carbonic  acid  obliquely  over  the  candle 
flame  so  that  the  gas  shall  fall  like  water  upon  it ;  the  flame  will  imme- 
diately be  extinguished. 

400.  Owing  to  the  great  weight  of  carbonic  acid,  it  often  fails 
to  rise  out  of  wells,  and  other  cavities  in  the  earth,  in  which  it  is 


VENTILATION    OF    WELLS.  321 

generated  by  the  decomposition  or  decay  of  organic  substances. 
Before  allowing  workmen  to  descend  into  any  such  place,  where 
there  is  reason  to  suspect  the  presence  of  carbonic  acid,  a  burn- 
ing candle  should  first  be  lowered ;  if  the  candle  is  extinguished, 
or  even  if  it  burn  feebly,  the  noxious  character  o^  the  air  is 
indicated,  and  measures  should  at  once  be  taken  to  purify  the 
locality  by  ventilation  or  otherwise.  One  way  of  removing  the 
carbonic  acid  is  to  absorb  it -by  means  of  some  chemical  agent, 
such  as  slaked  lime  (hydrate  of  calcium),  or  potash-lye.  The 
slaked  lime  is  most  efficient  as  an  absorbent  when  neither  very 
wet  nor  very  dry ;  it  should  not  be  dry  enough  to  be  dusty, 
nor  yet  noticeably  moist.  If  a  quantity  of  it  be  suspended  in 
the  well,  or  thrown  upon  the  floor  of  the  cellar  containing  car- 
bonic acid,  this  gas  will  quickly  combine  with  the*  lime  to  form 
carbonate  of  calcium,  as  in  Exp.  172.  Another  good  method  is 
to  lower  down  a  chafing-dish  full  of  brightly  glowing  charcoal ; 
if  the  carbonic  acid  be  present  in  such  quantity  that  the  test 
candle  has  been  extinguished  by  it,  the  charcoal  will,  in  like 
manner,  be  immediately  extinguished,  and,  in  cooling,  will  rapidly 
absorb  this  gas  (see  §  385),  together  with  any  nitrogen  which 
may  be  present ;  a  current  of  fresh  air  will,  in  this  way,  be  in- 
duced to  flow  into  the  well.  If,  by  the  candle-test,  but  little 
carbonic  acid  be  found  in  the  well,  it  would,  of  course,  be  best  to 
extinguish  the  charcoal  before  lowering  it  into  the  impure  air ; 
this  can  readily  be  done  by  covering  it  up  tightly  in  an  iron 
kettle. 

It  should  be  distinctly  understood  that  the  accumulation  of 
carbonic  acid  in  wells  and  caves  is  a  consequence  of  the  low 
diffusive  power  of  the  gas  (see  §  53)  ;  carbonic  acid  mixes  with 
air  very  slowly,  but  when  once  mixed  with  air  it  has  no  further 
tendency  to  settle  down,  or  to  separate  by  itself  in  any  way. 

Exp.  179.  —  Over  a  bottle  filled  with  carbonic  acid  gas,  irrvert  an- 
other bottle  full  of  air,  in  such  manner  that  the  mouth  of  the  air-bottle 
shall  rest  upon  that  of  the  upright  bottle  full  of  carbonic  acid.  After 
some  hours,  pour  lime-watei;  into  each  of  the  bottles  and  shake  them ; 
a  precipitate  of  carbonate  of  calcium  will  be  formed  in  both  cases,  for 
a  part  of  the  carbonic  acid  will,  by  this  time,  have  ascended  out  of  the 
lower  into  the  upper  bottle.  The  two  gases  have,  in  fact,  become  inti- 
21 


322  DIFFUSION    OF    GASES. 

mately  mixed  or  blended  ;  the1  heavy  carbonic  acid  has  diffused  upwards 
into  the  air,  and  the  lighter  atmospheric  air  has  diffused  downwards  into 
the  carbonic  acid,  just  as,  in  a  previous  experiment,  we  have  seen  hy- 
drogen and  oxygen  diffuse  into  each  other.  (Fig.  16,  p.  40.) 

The  great  importance  of  this  diffusion  of  gases,  in  the  economy  of 
nature,  is  well  illustrated  by  the  case  now  under  consideration.  When- 
ever, as  in  the  processes  of  respiration  and  combustion,  oxygen  is 
withdrawn  from  and  carbonic  acid  thrown  into  the  air,  the  carbonic 
acid,  and  the  nitrogen  with  which  it  is  accompanied,  immediately  mix 
with  the  surrounding  air  and  distribute  themselves  through  the  atmos- 
phere. The  composition  of  the  atmosphere  is  thus  maintained  uniform 
all  over  the'globe,  in  spite  of  the  constant  removal  from  it  of  oxygen 
in  some  localities,  and  the  addition  of  carbonic  acid  in  others.  It  is 
only  in  confined  places,  where  nearly  pure  carbonic  acid  is  produced 
more  rapidly  than  it  can  pass  off  by  diffusion,  that  the  gas  accumulates 
to  any  appreciable  extent. 

The  singular  facility  with  which  gases,  and  particularly  hydrogen, 
traverse  porous  bodies,  is  very  strikingly  illustrated  by  the  following 
experiment,  which  our  acquaintance  with  carbonic  acid  now  enables 
us  to  perform.  Through  a  large  glass  tube,  a  smaller  tube  of  porous, 
unglazed  earthenware  is  passed,  and  the  ends  of  the  glass-tube  are 
tightly  closed  by  the  corks  which  hold  the  porous  tube.  By  the  tube 

FIG.  53. 


a,  a  rather  rapid  stream  of  carbonic  acid  is  brought  from  a  self-regu- 
lating generator  into  the  annular  space  between  the  two  tubes,  while, 
by  the  tube  &,  a  slower  current  of  hydrogen  is  introduced  into  the  inm 
porous  tube.     It  would  be  expected  that  hydrogen  should  be  found  ii 
the  cylinder  d,  and  carbonic  acid  in  the  cylinder  c ;  but,  on  the  con- 
trary, an  inflammable  gas  is  f6und  in  c,  and  in  the  cylinder  d  carbonic 
acid  pure  enough  to  extinguish  a  burning  splinter.     The  hydrogen  dif- 
fuses almost  instantaneously  into  the  annular  space,  and  the  carbonic 
acid  enters  the  inner  tube  to  replace  the  issuing  hydrogen.] 


SOLUBILITY    OF    CARBONIC    ACID.  323 

401.  When  pure,  or  nearly  pure,  carbonic  acid  is  irrespirable, 
it  produces  spasms  in  the  respiratory  passages,  and  is  thus  pre- 
vented from  entering  the  lungs.     When  so  far  diluted  with  air 
as  to  admit  of  being  respired,  it  acts  as  a  narcotic  poison ;  it  is, 
however,  far  less  poisonous  than  the  other  oxide  of  carbon,  car- 
bonic oxide,  directly  to  be  described. 

402.  Carbonic  acid  gas  is  soluble  in  water  to  a  considerable 
extent.     One  measure  of  water,  at  the  ordinary  temperature  and 
pressure,  will  dissolve  one  measure  of  carbonic  acid  gas,  but  its 
solubility  increases  if  the  pressure  be  increased. 

Exp.  180. —  Into  a  long-necked  flask  or  phial  filled  with  carbonic 
acid,  pour  a  quantity  of  water,  close  the  bottle  with  the  finger,  and 
shake  it ;  immerse  the  mouth  of  the  bottle  in  water,  and  remove  the 
finger,  water  will  rush  into  the  bottle  to  supply  the  place  of  the  gas 
which  has  been  dissolved.  Again  place  the  finger  upon  the  mouth  of 
the  bottle,  shake  the  bottle  as  before,  and  subsequently  open  it  beneath 
the  surface  of  the  water,  a  fresh  portion  of  water  will  flow  into  the 
bottle  to  supply  the  new  vacuum ;  in  this  way,  by  repeated  agitation 
with  water,  all  of  the  carbonic  acid  in  the  bottle  can  be  absorbed. 

Owing  to  this  solubility  in  water,  some  carbonic  acid  is  always  lost 
when  the  gas  is  collected  over  water,  as  in  Exp.  1 75,  but  since  consid- 
erable time  is  required  to  absorb  all  the  gas,  there  is  little  objection  to 
collecting  it  over  water. 

403.  When  subjected  to  increased  pressure,  carbonic  acid  gas 
dissolves  in  water  much  more  abundantly  than  at  the  ordinary 
pressure  of  the  air ;  under  a  pressure  of  two  atmospheres,  one 
measure  of  water  will  dissolve  two  measures  of  the  gas ;  under 
a  pressure  of  three  atmospheres,  it  will  dissolve  three  measures, 
and  so  on.     Water  thus  surcharged  with  carbonic  acid   has  an 
agreeable,  acid,  pungent  taste,  and  effervesces  briskly  when  the 
compression  is  suddenly  removed,  as  when  the  liquid  is  allowed 
to  flow  out  into  the  air ;  such  carbonic  acid  water,  or  "  mineral 
water,"  as  it  is  then  called,  flows  from  the  earth  in  many  locali- 
ties, as  at  Seltzer  and  Saratoga ;  it  is  also  prepared,  artificially, 
in  large  quantities,  and  sold  as  a  beverage  under  the  meaningless 
name   of  soda-water.      Carbonic   acid   water   possesses    solvent 
powers  far  greater  than  those  of  pure  water ;  few  minerals  are 
capable  of  resisting  its  long-continued  action ;  hence  the  waters 
of  the  springs  from  which  it  issues  are  usually  highly  charged 


324  FERMENTATION. 

with  saline  and  mineral  ingredients,  and  are  often  of  medicinal 
value. 

404.  The  effervescent  qualities  of  fermented  liquors,  such  as 
cider,  champagne  and  beer,  are,  in  like  manner,  dependent  upon 
the  presence   of  compressed   carbonic  acid  gas.     In   all  these 
cases  the  carbonic  acid  is  of  value,  not  only  on  account  of  the 
agreeable  pungency  which  it  imparts  to  the  beverage,  but  also 
because  of  the  fact  that  in  escaping  from  solution  and  assuming 
the  gaseous  condition,  it  absorbs  a  very  considerable  amount  of 
heat,  and  so  cools  the  liquid  which  contained  it. 

405.  Carbonic  acid  is  widely  diffused  in  nature.     Traces  of  it 
occur  in  the  air  and  in  water  everywhere,  and  there  are  many 
localities,  besides  the  mineral-springs  before-mentioned,  where  it 
issues  from  the  earth  in  large  quantities,  notably  in  several  vol- 
canic districts.     It  is  produced,  not  only  in  the  actual  combustion 
of  all  substances  which  contain  carbon,  but  also  during  the  decay 
and  putrefaction  of  all  animal  and  vegetable  substances.     During 
fermentation  it  is  evolved  in  large  quantities,  and  it  is  continually 
given  off  during  the  respiration  of  animals. 

Exp.  181.  —  Put  one  or  two  table-spoonfuls  of  coarse  meal  into  a 
bottle  of  about  250  c.  c.  capacity;  cover  the  meal  with  water,  and  con- 
nect the  bottle,  by  means  of  a  cork  and  glass-tube,  with  a  second  bottle 
filled  about  three  c.  m.  deep  with  lime-water ;  the  delivery -tube  must 
reach  into  tlie  lime-water.  Out  of  the  top  of  the  second  bottle  carry 
a  second  bent  glass-tube,  whose  open  end  dips  into  -water.  The  bottle 
containing  the  lime-water  will,  of  course,  be  closed  by  the  cork  through 
which  pass  the  two  tubes  above  described.  Keep  the  apparatus  in  a 
warm  place  ;  bubbles  of  gas  will  pass  from  the  first  into  the  second 
flask ;  they  contain  carbonic  acid,  as  may  be  seen  from  the  precipi- 
tate of  carbonate  of  calcium  which  they  produce. 

Exp.  182.  —  Dissolve  2  grammes  of  honey  or  molasses  in  200  c.  c.  of 
water  ;  fill  a  large  test-tube  with  the  mixture,  and  add  to  it  a  few  drops 
of  baker's  or  brewer's  yeast ;  close  the  open  mouth  of  the  test-tube 
with  the  thumb,  and  invert  it  in  a  small  saucer  or  porcelain  capsule 
filled  with  the  diluted  syrup.  Place  the  saucer  and  tube,  with  their 
contents,  in  a  warm  place,  having  a  temperature  of  about  20°  or  30°, 
and  leave  them  there  during  24  hours.  In  a  short  time  fermentation 
Bets  in,  and  the  sugar  of  the  syrup  is  gradually  converted  into  alcohol 
and  carbonic  acid. 


RESPIRATION.  325 


CGH1206  =  2C2HeO  +  2C02. 
Sugar.          Alcohol. 

The  carbonic  acid  thus  formed  rises  in  minute  bubbles,  causing  a  gentle 
effervescence  in  the  liquid,  and  collects  in  the  upper  part  of  the  tube, 
while  the  alcohol  remains  dissolved  in  the  liquid.  That  the  gas  pb- 
tained  in  this  experiment  is  really  carbonic  acid,  may  be  proved  by 
transferring  some  of  it  into  a  clean  tube  at  the  water-pan,  and  then 
testing  with  lime-water. 

Exp.  183.  —  Provide  two  test-glasses  or  small  bottles;  place  in  each 
15  or  20  c.  c.  of  lime-water;  through  a  glass-tube,  blow  into  the  lime- 
water  of  one  of  the  bottles  air  coming  from  the  lungs.  By  means  of 
bellows,  to  the  nozzle  of  which  a  gas-delivery  tube  has  been  attached, 
force  through  the  lime-water  of  the  second  bottle  a  quantity  of  fresh 
air.  The  clear  liquid  of  the  first  bottle  will  quickly  become  turbid 
through  deposition  of  carbonate  of  calcium,  while  the  lime-water  of  the 
second  bottle  will  remain  clear  for  a  long  while. 

This  experiment  may  be  modified  by  constructing  an  apparatus  with 
valves,  in  such  manner  that  the  air  drawn  into  the  lungs  shall  be  made 
to  pass  through  one  bottle  of  lime-water,  while  the  air  expired  goes 
out  through  another  bottle  of  lime-water.  The  contents  of  the  first 
bottle  will  remain  clear,  while  the  liquid  in  the  other  immediately 
becomes  turbid. 

Ordinary  fresh  air  contains  only  about  l-2000th  part  of  carbonic 
acid,  while  air  expired  from  the  lungs  contains  as  much  as  3  or  4  per 
cent,  of  it.  In  breathing,  animals  inhale  oxygen  from  the  air  ;  this 
oxygen  combines  with  carbon  within  their  bodies  and  is  exhaled  as 
carbonic  acid.  A.  crowd  of  men  consume  the  oxygen  of  the  air,  just 
as  lamps  or  fires  consume  it  ;  to  carry  off  this  product  of  animal  com- 
bustion is  one  of  the  objects  of  systems  of  ventilation.  Air  which  con- 
tains even  as  little  as  1  or  2  per  cent,  of  carbonic  aid,  exerts  a  very 
depressing  effect  when  breathed  for  any  length  of  time. 

406.  From  the  foregoing  statements  it  appears  that,  in  the 
several  processes  called  "  life,"  "  fermentation,"  "  decay,"  and 
"  combustion,"  there  is  involved  chemical  action,  —  in  all  of  these 
processes  oxygen  from  the  air  unites  with  carbon,  while  carbonic 
acid  is  set  free  and  thrown  into  the  atmosphere.  The  question 
now  arises,  what  becomes  of  all  this  vast  amount  of  carbonic 
acid  which  is  constantly  coming  into  our  atmosphere  from  the 
respiration  of  animals,  from  fires,  from  decaying  and  ferment- 
ing substances,  from  volcanic  fissures,  and  from  various  other 


326  LIQUID    CARBONIC    ACID. 

sources  ?  If  this  carbonic  acid  remained  in  the  air,  the  latter 
would  quickly  become  unfit  to  support  animal  life.  But  it  is  not 
found  that  the  average  proportion  of  carbonic  acid  in  the  air 
does  increase,  —  on  the  contrary,  all  the  evidence  goes  to  show 
that  there  was,  at  certain  earlier  geological  epochs,  more  of  it  in 
the  air  than  now.  .Many  geologists  believe  that,  in  the  early 
history  of  our  globe,  there  was  much  more  carbonic  acid  in  the 
air  than  at  present ;  hence  immense  forests  arose,  whose  carbon 
is  now  stored  away  in  the  form  of  coal ;  hence,  also,  the  forma- 
tion of  enormous  beds  of  limestone  covering  many  parts  of  the 
earth's  surface,  —  processes  of  which  the  faint  continuations  are 
now  seen  in  the  formation  of  peat-bogs  and  coral-reefs.  On  the 
other  hand,  it  is  not  found  that  the  proportion  of  oxygen  in  the 
atmosphere  undergoes  any  appreciable  change,'  in  spite  of  the 
enormous  volume  of  it  which  is  absorbed  in  the  processes  of 
breathing,  combustion,  and  decay  above  enumerated.  For,  un- 
like animals,  plants,  in  breathing,  take  in  carbonic  acid  and  give 
out  oxygen.  The  leaves  of  plants  are  so  constructed  that  they 
can  decompose  carbonic  acid,  fix  carbon  for  the  building  up  of 
the  plant,  and  set  oxygen  free.  This  reciprocal  action  of  plants 
and  animals,  tending  to  maintain  unchanged  the  constitution  of 
the  atmosphere,  is  one  of  the  most  wonderful  adjustments  of 
nature. 

407.  Carbonic  acid  can  be  liquefied  by  pressure,  and  the 
liquid  thus  obtained  can  be  solidified  by  exposure  to  cold.  When 
the  gas  is  generated  in  a  confined  space  in  a  strong  vessel,  it  soon 
exerts  so  powerful  a  pressure  that  a  large  portion  of  it  condenses 
to  a  transparent,  colorless,  mobile  liquid,  somewhat  resembling 
water,  thought  it  refracts  light  less  powerfully  ;  or  it  may  be 
liquefied  by  mere  cooling,  to  — 106°,  under  the  atmospheric 
pressure.  A  better  way  of  preparing  the  liquid  acid  is  to  pump 
the  gas,  by  means  of  a  forcing  syringe,  into  a  strong  wrought- 
iron  vessel  surrounded  with  pounded  ice.  The  pressure  can 
thus  be  regularly  and  methodically  increased  and  the  receiver 
finally  filled  with  the  liquid.  At  0°  a  pressure  of  3G  atmos- 
pheres is  required,  in  order  that  the  acid  shall  remain  in  the 
liquid  state.  Liquid  carbonic  acid  does  not  mix  readily  with 
water  or  with  the  fixed  oils,  but  with  alcohol,  ether,  petroleum, 


SOLID    CARBONIC    ACID.  327 

and  similar  liquids,  it  is  miscible  in  all  proportions.  Its  specific 
gravity  is  0.83  at  0°,  but  it  expands  to  such  an  extent  on  being 
heated,  that  at  30°  its  specific  gravity  is  only  0.6.  It  expands  to  a 
greater  extent,  on  the  application  of  heat,  than  any  known  sub- 
stance, even  to  a  greater  extent  than  the  gases ;  20  volumes  of  it 
at  0°  become  29  volumes  when  the  temperature  is  raised  to  30°  ; 
150  volumes,  at  30°,  shrink  to  100  volumes  when  the  tempera- 
ture is  reduced  to  — 20°.  The  liquid  acid  well  illustrates  the 
general  fact  that  liquids  expand  proportionally  much  more  when 
heated  under  a  high  pressure  than  under  a  low  one. 

408.  If  the  stop-cock  of  a  vessel  containing  liquid  carbonic 
acid  be  opened,  in  such  manner  that  a  stream  of  the  liquid  shall 
be  forced  out  into  the  air,  a  portion  of  it  at  once  assumes  the 
gaseous  state,  and  in  so  doing  absorbs  so  much  heat  from  the 
remainder,  that  the  latter  solidifies,  and  is  deposited  in  the  form 
of  white  flakes  like  snow.     This  snow-like  substance  is  slowly 
converted  into  gas,  when  exposed  at  the  ordinary  pressure  of  the 
air,  and  so  disappears.     Though  its  temperature  is  lower  than 
— 78°,  it  may  be  handled  lightly  without  exciting  any  special 
sensation  of  coldness  or  pain,  for  the  gas  which  it  is  constantly 
emitting  is  a  bad  conductor  of  heat,  and  prevents  it  from  coming 
into  intimate  contact  with  the  skin.     When,  however,  the  solid 
acid  is  forcibly  pressed  between  the  fingers,  it  produces  painful 
blisters,  as  if  it  were  red-hot  iron.     In  order  to  use  the  solid 
acid,  for  producing  low  temperatures,  it  is  best  to  mix  it  with  a 
small  quantity  of  ether ;    in  such   a   mixture  quicksilver   can 
readily  be  frozen,  and  many  gases  can  be  liquefied  or  even  solidi- 
fied.    If  this  mixture  be  placed  in  the  vacuum  of  an  air-pump, 
a  temperature  as  low  as  — 100°  can  be  obtained  ;  and  if  a  tube 
containing  liquid  carbonic  acid  be  then   placed  in  the  mixture, 
the  liquid  will  speedily  be  frozen  to  a  clear  transparent  mass  like 
ice. 

409.  Carbonic  acid  gas,  on  being  heated  from  0°  to  100°,  does 
not  expand  at  the  same  rate  as  air  and  the  other  permanent 
gases,  but  increases  in  volume  to  a  greater  extent  than  any  of 
them.     Upon  being  heated  one  degree,  it  expands  0.003688  its 
volume  at  0°.     This  behavior  is  in  accordance  with  the  general 
rule,  that  those  gases  expand  the  most  which  are  most  readily 


DECOMPOSITION    OF    CARBONIC    ACID. 

condensible  to  the  liquid  state,  while  those  gases  which  have  re- 
sisted all  efforts  to  liquify  them  scarcely  show  any  appreciable 
differences  in  the  rate  of  expansion. 

In  the  same  way,  carbonic  acid,  like  the  other  easily  condensi- 
ble gases  (see  §  221),  does  not  conform  precisely  to  the  law  of 
Mariotte.  At  pressures  greater  than  one-third  of  the  pressure 
of  our  atmosphere,  its  volume  diminishes  more  rapidly,  with  in- 
creasing pressure,  than  would  be  the  case  with  air  and  the  other 
permanent  gases  under  the  same  conditions. 

410.  Carbonic  acid  is  one  of  the  compound  gases  which  can 
be  split  by  heat  alone  into  its  proximate  constituents,  —  in  other 
words,  it  exhibits  the  phenomena  of  dissociation  (§  300).    When 
the  gas  is  passed  through  a  strongly-heated  porcelain  tube,  the 
gaseous  mixture  which  escapes  from  the  tube  contains,  besides 
undecomposed  carbonic  acid,  notable  quantities  of  carbonic  oxide 
(CO)  and  oxygen. 

411.  As  has  been  already  stated  (§  399),  the  oxygen  in  carbonic 
acid  is  so  strongly  held  that  it  cannot  be  withdrawn  by  combusti- 
bles  under  ordinary  circumstances,  but  at  high  temperatures, 
carbonic  acid  is  decomposed  by  carbon  and  by  several  of  the 
metals,  such  as  iron,  zinc,  and  manganese,  besides  potassium, 
sodium,  and  the  other  metals  of  the  alkalies  and  alkaline  earths. 
By  the  alkali-metals  the  oxygen  is  completely  removed  from 
carbonic  acid,  and  carbon  "is  set  free;  but  by  the  other  agents 
above-mentioned,  only  half  the  oxygen  of  the  acid  is  taken  away 
while  carbonic  oxide  gas  is  formed :  — 

C02  +  C  =  2CO. 

Phosphorus,  also,  at  high  temperatures  and  in  presence  of  a  fixed 
alkali,  decomposes  carbonic  acid  in  the  same  way  and  abstracts 
part  of  its  oxygen.  So,  too,  if  a  mixture  of  equal  volumes  of 
hydrogen  and  carbonic  acid  be  passed  into  one  end  of  a  red-hot 
tube,  steam  and  carbonic  oxide  gas  will  escape  at  the  other :  — 

CO2  +  2H  =  CO  +  H2O . 

The  decomposing  power  of  the  alkali-metals,  above  alluded  to, 
furnishes  us  one  means  of  partially  analyzing  carbonic  acid. 

Exp.  184. —  To  a  gas-bottle  in  which  carbonic  acid  is  being  steadily 
evolved,  according  to  Exp.  1 75,  attach  a  chloride  of  calcium  tube,  and 


COMPOSITION    OF    CARBONIC    ACID. 


329 


beyond  this  drying-tube,  a  short  tube  of  hard  glass,  from  which  an 
exit-tube  leads  into  a  small  open  bottle,  as  shown  in  Fig.  54.     When 


FIG.  54. 


the  extinction  of  a  lighted  match  in  the  open  bottle  proves  the  appa-  * 
ratus  to  be  full  of  carbonic  acid,  thrust  into  the  hard  glass- tube  a  bit  of 
potassium  as  big  as  a  pea,  previously  dried  between  folds  of  blotting- 
paper,  then  gently  heat  the  potassium  with  a  lamp.  The  potassium 
will  take  fire  and  burn  at  the  expense  of  the  oxygen  of  the  carbonic 
acid,  and  black  particles  of  carbon  will  be  deposited  upon  the  walls  of 
the  tube.  After  the  reaction  has  ceased,  and  the  tube  has  been  allowed 
to  become  cold,  place  it  in  a  bottle  of  water  so  that  the  saline  mass 
(carbonate  of  potassium)  may  dissolve ;  the  particles  of  carbon  will  then 
be  seen  more  clearly,  floating  in  the  liquid  ;  they  may  be  collected  upon 
a  filter.  The  potassium  in  this  experiment  may  be  replaced  by  sodium, 
but  in  this  case  a  somewhat  higher  temperature  is  required. 

412.  The  quantitative  composition  of  carbonic  acid  may  be 
readily  ascertained  by  the  method  of  synthesis.  When  a  quan- 
tity of  carbon  is  burned  in  a  confined  and  measured  volume  of 
oxygen,  it  is  found  that  the  volume  of  carbonic  acid  gas  produced 
has  sensibly  the  same  bulk  as  the  original  oxygen.  Hence  we 
conclude  that  the  normal  or  product-volume  of  the  molecule  of 
carbonic  acid  gas  contains  two  volumes  of  oxygen. 

Now,  two  volumes  of  carbonic  acid  weigh  44.152,  since  the 
weight  of  the  unit- volume,  or  the  specific  gravity  of  carbonic 
acid,  has  been  found  to  be  22.076. 

Subtracting  from  this  weight  of  the  product-volume  of  the  gas     44.152 
The  weight  of  two  unit-volumes  of  oxygen  (15.969  X  2)        .     31.938 

There  remains  as  the  weight  of.  the  carbon  in  the  product-vol- 
ume of  the  gas 12.214 

The  weight  of  one  atom  of  carbon  is  12,  as  we  have  seen  above 


330  SYNTHESIS    OF    CARBONIC    ACID. 

(§  395),  and  it  follows  that  the  formula  of  carbonic  acid  is  CO2  • 
From  these  figures  the  following  percentage  composition  of  car- 
bonic acid  may  be  deduced  :  — 

Carbon 27.66 

Oxygen       ........     72.34 

100.00 

But  these  results  must  be  regarded  merely  as  approximations  to 
the  truth,  since  the  deviation  of  carbonic  acid  from  the  law  of 
Mariotte  (§  409)  renders  it  well-nigh  certain  that  we  have  not 
yet  been  able  to  precisely  determine,  by  experiment,  the  true 
weight  of  a  unit-volume  of  this  gas. 

413.  The  composition  of  carbonic  acid  has,  however,  been  de- 
termined with  very  great  accuracy,  by  burning  a  known  weight 
of  pure  carbon  in  a  stream  of  oxygen  gas  and  carefully  collect- 
ing and  weighing  the  carbonic  acid  produced. 

In  order  to  do  this,  the  product  of  the  combustion  of  the  carbon 
together  with  the  excess  of  oxygen,  is  made  to  flow  through  U-tubes, 
(Appendix,  §  15)  filled  with  fragments  of  hydrate  of  potassium,  a  sub- 
stance which  absorbs  only  the  carbonic  acid ;  the  weight  of  these  tubes 
is  determined  before  the  commencement  of  the  experiment  and  again  at 
its  close  ;  the  increase  of  weight  during  the  experiment  being,  of  course, 
referable  to  the  carbonic  acid  absorbed.  Knowing,  then,  the  weight  of 
the  carbon  taken  and  the  weight  of  the  carbonic  acid  into  which  it  has 
been  converted  by  uniting  with  oxygen,  a  very  simple  calculation,  as 
before,  gives  us  the  percentage  composition  of  the  acid.  Experiments 
of  this  kind  have  yielded  the  following  result :  — 

Carbon        .         .         .  •      .        ."      ;        .         .     27.27 
Oxygen       .         .         .'*•;•..         .     72.73 

100.00 

It  appears,  therefore,  that  in  uniting  to  form  carbonic  acid,  the 
elements  carbon  and  oxygen  combine  in  the  proportion  of  3  parts 
by  weight  of  carbon  to  8  parts  by 'weight  of  oxygen,  or  in  the 
proportion  of  12  to  32.  If  the  number  72.73  be  divided  by  16, 
the  number  representing  the  weight  of  a  unit-volume  of  oxygen ; 
and  if  the  number  100  be  divided  by  22,  the  number  which,  in 
accordance  with  the  weight  of  all  the  evidence  thus  far  accumu- 
lated must  be  regarded  as  the  true  unit-volume  weight  of  car- 


CARBONATES.  331 

bonic  acid,  there  will  be  obtained  in  each  case  the  same  quotient, 
namely,  4.545  volumes ;  whence  we  conclude,  as  before,  that  any 
volume  of  carbonic  acid  contains  the  same  volume  of  oxygen. 

414.  Carbonic  acid  unites  with  the  protoxides  of  most  of  the 
metals  to  form  well-defined  salts  called  carbonates.  The  greater 
number  of  the  best  defined  carbonates  contain  only  one  molecule 
of  base ;  but  besides  the  normal  carbonates  of  the  general  for- 
mula M2O,CO2  or  M2CO3,  there  are  sesqui-carbonates  of  the 
formula  2M2O,3CO2,  basic  carbonates  containing  two,  three,  or 
more  molecules  of  the  base  to  one  of  the  acid,  and  bicarbonates, 
M20,H2O,2CO2  or  MHC03.  Strictly  speaking,  the  class  of 
compounds  last-named  should,  perhaps,  be  regarded  as  double 
salts  of  the  metal  and  hydrogen;  the  appellation,  bicarbonate, 
ordinarily  applied  to  them,  being  a  name  of  very  doubtful  cor- 
rectness. The  normal  carbonate,  and  the  so-called  bicarbonate 
of  sodium,  for  example,  differ  only  in  that  half  the  sodium  in  the 
normal  salt  has  been  replaced  by  hydrogen  in  the  bicarbonate  :  — 

Normal  Carbonate  of  Sodium.  Bicarbonate  of  Sodium. 

Na2C03.  NaHC03. 

Carbonic  acid  is  an  exceedingly  weak  acid ;  it  fails  to  neu 
tralize  (§  65)  completely  the  causticity  of  oxides,  such  as  those 
of  the  alkaline  metals  ;  the  normal  carbonate  of  sodium,  for 
example,  is  decidedly  alkaline  in  its  reaction  and  properties. 
The  so-called  bicarbonate  of  sodium  is  also  slightly  alkaline, 
and  even  the  solution  of  carbonate  of  calcium  in  carbonic  acid 
water  exhibits  an  alkaline  reaction  when  tested  with  turmeric 
paper.  Almost  all  the  carbonates  are  readily  decomposed  by 
acids, —  even  by  very  weak  acids,  —  with  an  effervescence  caused 
by  the  escape  of  carbonic  acid.  Most  of  them  are  decomposed 
also  on  being  heated,  but  from  the  normal  salts  of  sodium  and 
potassium,  carbonic  acid  cannot  be  expelled  by  heat  alone,  how- 
ever intense. 

415.  Carbonic  Oxide  (CO).  As  has  been  stated  in  §  411, 
carbonic  oxide  can  be  prepared  by  acting  upon  carbonic  acid 
with  hot  charcoal. 

Exp.  185.  — In  the  middle  of  a  tube  of  hard  glass  No.  2  or  21,  about 
35  c.  m.  long,  pack  a  column  15  c.  in.  iii  length  of  coarsely-powdered 


332 


PREPARATION    OF    CARBONIC    OXIDE. 


charcoal.     Place  the  tube  upon  a  sheet-iron  trough  on  a  ring  of  the 
iron-stand  abo've  wire-gauze  lamps,  as  shown  in  the  figure.     Connect 


FIG.  55. 


the  tube  either  with  a  gas-holder  containing  carbonic  acid,  or  with  a 
bottle  in  which  the  gas  is  being  generated.  Heat  the  charcoal  in- 
tensely, and  from  time  to  time  test  the  gas  which  is  delivered  at  the 
water-pan,  as  to  its  inflammability.  Carbonic  oxide  takes  fire  on  being 
touched  with  a  lighted  match  and  burns  with  a  bluish  flame.  In  place 
of  the  charcoal,  small  fragments  of  iron  or  of  zinc  may  be  employed  in 
this  experiment. 

Instead  of  gaseous  carbonic  acid,  the  solid  compounds  called 
carbonates  can  be  conveniently  employed  for  preparing  carbonic 
oxide. 

Exp.  186. —  Mix  powdered  chalk  (carbonate  of  calcium)  with  an 
equal  weight  of  iron  or  zinc  filings,  place  the  mixture  in  an  ignition- 
tube,  provided  with  a  gas-delivery  tube,  and  heat  it  to  redness  over  the 
gas-lamp.  The  metal  will  abstract  an  atom  of  oxygen  from  the  car- 
bonate of  calcium,  oxide  of  iron  or  of  zinc  will  be  formed,  while 
carbonic  oxide  will  pass  off  through  the  delivery-tube  to  be  collected 
at  the  water-pan  :  — 

CaCO3  -f  Fe  =  FeO  -f  CaO  +  CO. 

In  the  same  way,  when  a  mixture  of  chalk  and  finely-powdered 
charcoal  is  heated  to  full  redness,  carbonic  oxide  gas  is  given  off:  — 

CaCO3  -f  C  =  CaO  -f  2CO . 

It  should  be  mentioned,  however,  that  in  all  these  cases  the  carbonic 
oxide  obtained  is  more  or  less  contaminated  with  carbonic  acid,  por- 
tions of  which  escape  reduction  by  the  metal  and  carbon ;  the  carbonic 
acid  may  always  be  readily  removed  by  causing  the  gas  to  pass  through 
a  strong  solution  of  caustic  soda  or  through  a  U-tube  filled  with  bits  of 
pumice-stone  saturated  with  soda-lye. 


PREPARATION    OF    CARBONIC    OXIDE. 


333 


Carbonic  oxide  can  be  obtained  also  by  heating  charcoal  with 
other  solid  oxygen  compounds,  such  as  the  phosphate  of  calcium, 
already  mentioned  (§  270),  or  the  oxides  of  almost  any  of  the 
metals,  provided  the  charcoal  be  in  excess. 

Exp.  187.  —  Heat  in  an  ignition-tube,  as  before,  a  mixture  of  1  grm. 
of  finely-powdered  charcoal  and  8  grms.  of  red  oxide  of  iron ;  collect  the 
gas  over  water,  pour  into  the  bottle  a  little  soda-lye,  close  the  mouth  of 
the  bottle  tightly  and  shake  it,  then  open  the  mouth  of  the  bottle  under 
water,  an(J  finally  test  the  gas  with  a  lighted  match. 

416.  Another  easy  method  of  preparing  carbonic  oxide  is  to 
decompose  oxalic  acid  by  means  of  oil  of  vitriol ;  this  is  the 
method  usually  employed  in  the  laboratory.  Oxalic  acid  is  a 
solid,  vegetable  acid,  to  be  procured  of  the  druggists ;  its  com- 
position may  be  represented  by  the  formula  H2CA-  On  being 
heated  with  concentrated  sulphuric  acid,  it  suffers  decomposition 
in  a  manner  which  may  be  formulated  as  follows :  — 

HACA  +  HAS03  =  2HAS03  +  CO  +  CO2. 
The  elements  of  water  are  taken  away  from  the  oxalic  acid  and 
united  with  the  sulphuric  acid,  while  the  remainder  of  the  oxalic 
acid  breaks  up  into  carbonic  acid  and  carbonic  oxide. 

Exp.  188.  —  Place  in  a  flask  of  about  350  c.  c.  capacity,  9  grms.  of 
common  oxalic  acid  and  53  Fia.  56. 

grms  of  concentrated  sulphu- 
ric acid ;  connect  the  flask  with 
a  bottle  filled  with  fragments  of 
pumice-stone  saturated  with  a 
strong  solution  of  caustic  soda, 
as  shown  in  the  figure ;  heat 
the  contents  of  the  flask  gent- 
ly, and  collect  the  gas  which 
is  evolved  over  water  in  the 
usual  way.  The  carbonic  acid 
resulting  from  the  reaction 
will  all  be  absorbed  by  the 
soda-lye,  and  carbonic  oxide 
will  aloue  be  delivered  at  the 
water-pan. 

None  of  the  methods  heretofore  given  yield  pure  carbonic 
oxide  directly ;  in  each  of  the  experiments  we  are  compelled  to 


334  PROPERTIES    OF    CARBONIC    OXIDE. 

wash  out  carbonic  acid  from  the  gas  obtained,  if  an  absolutely 
pure  product  is  desired,  but  there  are  methods  by  which  pure 
carbonic  oxide  may  be  prepared  without  need  of  any  process  of 
purification.  One  of  the  best  of  these  is  as  follows  :  — 

Exp.  189.  —  In  a  thin-bottomed  flask  of  about  250  c.  c.  capacity  and 
provided  with  a  suitable  gas-delivery  tube,  heat  a  mixture  of  5  grammes 
of  finely-powdered  ferrocyanide  of  potassium  (yellow  prussiate  of 
potash)  and  40  or  50  grms.  of  strong  sulphuric  acid.  Collect  the  gas 
over  water  and  test  it  as  to  its  inflammability.  Thrust  also  a  lighted 
splinter  into  the  gas  and  observe  that  it  will  be  extinguished.  The 
reactions  which  occur  between  the  chemicals  employed  may  be  expressed 
as  follows  :  — 

Ferrocyanide  of  Potassium.  Water.  Sulphuric  Acid. 

K,FeC6N6H603       +     3H2O       +     6H2SO4     = 
6CO      -f       2K\,SO4     +       FeSO4   -f        3(NH4)2SO4. 

Carbonic  Sulphate  of  Sulphate  of  Sulphate  of 

Oxide.  Potassium.  Iron.  Ammonium. 


417.  Carbonic  oxide  is  a  transparent,  colorless  gas,  having 
little,  if  any,  odor  ;  it  has  never  yet  been  liquefied.  It  is  some- 
what lighter  than  air,  its  specific  gravity  being  14,  while  that 
of  air  is  14.5.  It  is  but  little  soluble  in  water,  and  may  be 
collected  and  preserved  over  water  without  much  loss.  It  ex- 
tinguishes combustion  just  as  hydrogen  does,  and  destroys  ani- 
mal life.  Unlike  hydrogen  and  nitrogen,  however,  it  is  a  true 
poison.  It  destroys  life,  not  negatively  by  mere  suffocation  or 
exclusion  of  oxygen,  but  by  direct  noxious  action.  Even  when 
largely  diluted  with  air,  it  is  still  poisonous,  producing  giddiness, 
insensibility,  and  finally  death.  It  is  the  presence  of  this  gas 
which  occasions  the  peculiar  sensation  of  oppression  and  head- 
ache which  is  experienced  in  rooms  into  which  the  products  of 
combustion  have  escaped  from  fires  of  charcoal  or  anthracite. 
Carbonic  oxide  is  very  much  more  poisonous  than  carbonic  acid. 
Much  o£  the  ill  repute  which  attaches  to  carbonic  acid  really 
belongs  to  carbonic  oxide,  for  since  both  these  gases  are  produced 
by  burning  charcoal,  many  persons  are  liable  to  confound  them  ; 
but  carbonic  acid  is,  comparatively  speaking,  almost  innocuous. 
Carbonic  acid,  it  is  true,  is  somewhat  poisonous  ;  it  does  not 
merely^  suffocate,  like  water,  or  nitrogen,  or  hydrogen,  but  it  is 


COMBUSTION    OF    CARBONIC    OXIDE.  335 

very  much  less  poisonous  than  carbonic  oxide.  It  has  been  found, 
by  experiment,  that  an  atmosphere  containing  only  1-1 00th  of 
carbonic  oxide  is  as  fatal  to  a  bird  as  one  containing  1-2 5th  part 
of  carbonic  acid. 

Carbonic  oxide  exhibits  neither  an  acid  nor  an  alkaline  reac- 
tion, when  tested  with  vegetable  colors,  and,  in  general,  has  but 
little  tendency  to  combine  with  other  substances.  With' oxygen, 
however,  it  combines  readily  at  comparatively  low  temperatures ; 
an  iron  wire  heated  to  dull  redness  is  sufficient  to  inflame  it  in 
the  air.  Unlike  most  other  combustible  gases,  it  contains  no 
hydrogen,  and,  therefore,  produces  no  water  when  burned; 
nothing  but  carbonic  acid  results  from  its  burning. 

Exp.  190.  —  To  the  apparatus  employed  for  evolving  carbonic  oxide 
in  Exp.  188,  attach  a  piece  of  small  glass  tubing  drawn  out  at  the  end 
to  a  fine  point,  and  bent  in  such  manner  that  a  stream  of  gas  may  be 
delivered  upwards  from  this  point.  Light  the  gas  as  it  flows  out  of  the 
tube  and  hold  over  the  pale-blue  flame  a  clean,  dry  bottle.  No  moist- 
ure will  be  deposited  upon  the  sides  of  the  bottle.  That  carbonic  acid 
has  been  produced  by  the  combustion,  may  be  proved  by  pouring  a 
little  lime-water  into  the  bottle,  and  shaking  it  about  in  the  gas  therein 
contained. 

418.  Carbonic  oxide  is  a  very  powerful  deoxidizing  agent. 
At  high  temperatures  it  is  capable  of  taking  oxygen  away  from 
many  of  the  compounds  which  contain  that  element.     Hence  it 
plays  a  very  important  part  in  metallurgical  operations.     Much 
of  the  reducing  action  which  is,  commonly  speaking,  attributed 
directly  to   carbon,  is  really  effected  in  practice  through  the 
mediation  of  carbonic  oxide  gas. 

Exp.  191.  — In  the  middle  of  a  tube  of  hard  glass,  No.  3,  about  20 
c.  m.  long,  place  a  gramme  of  black  oxide  of  copper  (CuO)  ;  support 
the  tube  upon  a  ring  of  the  iron-stand  over  the  gas-lamp,  and  connect 
it  at  one  end  with  a  flask  in  which  carbonic  oxide  is  being  evolved,  as 
in  Exp.  188,  and  at  the  other,  with  a  tube  bent  at  a  right  angle  and 
dipping  into  a  bottle  which  contains  lime-water.  After  the  tube,  which 
contains  the  oxide  of  copper,  has  become  full  of  carbonic  oxide,  heat 
it  and  observe  that  the  oxide  of  copper  is  reduced,  that  metallic  copper 
alone  remains  in  the  tube,  and  that  the  carbonic  acid  formed  has  made 
turbid  the  lime-water  in  the  bottle. 

419.  The   specific   heat   of   carbonic   oxide   is   considerably 


336  HEAT    FROM    CARBONIC    OXIDE. 

greater  than  that  of  carbonic  acid,  being  0.245,  while  that  of 
carbonic  acid  is  only  0.2103. 

When  a  mixture  of  carbonic  oxide  and  oxygen,  in  the  pro- 
portions of  two  volumes  of  the  former  to  one  of  the  latter  gas 
is  lighted,  it  explodes  with  about  the  same  degree  of  violence  as 
a  mixture  of  hydrogen  and  oxygen  (§  58),  a  very  considerable 
amount  of  heat  being  evolved  in  the  act  of  combination. 

Though  considerable  heat  is  evolved  during  the  union  of  car- 
bonic oxide  and  oxygen,  the  amount  is  much  less  than  that  which 
results  from  the  complete  combustion  of  charcoal  to  carbonic 
acid.  One  gramme  of  carbonic  oxide  disengages  in  burning 
2403  units  of  heat  (§  55),  while  one  gramme  of  wood  charcoal, 
in  burning  to  carbonic  acid,  yields  8080  units.  The  same 
amount  of  heat  (2403  units)  is  reabsorbed  when  the  carbonic 
acid,  obtained  by  burning  one  gramme  of  carbonic  oxide,  is  again 
reduced  to  the  state  of  carbonic  oxide.  (Compare  Exp.  185.) 

The  gramme  of  hydrogen ,  yields,  as  it  unites  with  oxygen, 
34,462  units  of  heat ;  but  since  carbonic  oxide  is  28  times 
heavier  than  hydrogen,  it  is  obvious  that  more  heat  will  be  de- 
veloped by  the  complete  combustion  of  a  given  volume  of  carbonic 
oxide  than  by  that  of  the  same  volume  of  hydrogen. 

420.  Carbonic  oxide,  unlike  carbonic  acid,  is  not  decomposed 
when  heated  to  redness  in  contact  with  hydrogen,  charcoal,  iron, 
or  zinc.  Sodium  and  potassium,  however,  abstract  the  oxygen 
from  this  gas  as  they  do  from  carbonic  acid. 

It  unites  with  chlorine  directly,  under  the  influence  of  sun- 
light, forming  a  gaseous  compound,  the  composition  of  which 
may  be  represented  by  the  formula  COC12.  When  left  for 
some  time  in  contact  with  caustic  potash,  at  the  temperature  of 
100°,  it  combines  with  it,  and  there  is  produced  a  compound 
known  as  formiate  of  potassium  :  — 

KHO  +  CO  —  CHKO2.* 

It  is  absorbed  readily  by  solutions  of  the  salts  of  dinoxide  of 
copper  (Cu2O)  in  ammonia  water,  and  by  a  solution  of  dichloride 
of  copper  (Cu2Cl2)  in  strong  chlorhydric  acid,  and  can  thus  be 
separated  from  a  mixture  with  other  gases.  Melted  metallic 
potassium  also  absorbs  a  certain  amount  of  carbonic  oxide  and 
combines  with  it. 


DISSOCIATION    OF    CARBONIC    OXIDE.  337 

421.  Carbonic  oxide  may  be  resolved  into  carbon  and  oxygen 
by   heat   alone,   but  this   dissociation    occurs    only   under   very 
peculiar  circumstances. 

A  porcelain  tube  is  placed  in  a  furnace  where  it  can  be  raised  to  a 
very  high  temperature ;  the  ends  of  this  tube  project  beyond  the  fur- 
nace and  are  closed  by  corks ;  through  these  corks  passes,  in  the  axis 
of  the  porcelain  tube,  a  very  thin  brass  tube,  and  each  cork  carries  also  a 
small  glass  tube ;  by  one  of  these  tubes  carbonic  oxide  enters  the  porce- 
lain tube,  and  by  the  other  the  products  of  the  reaction  escape  from 
the  apparatus.  Two  little  screens  of  porcelain  divide  internally  that 
part  of  the  porcelain  tube  which  lies  in  the  furnace  and  is  to  be  heated, 
from  the  parts  which  project  beyond  the  furnace  and  remain  cool.  A 
rapid  current  of  cold  water  is  made  to  flow  through  the  thin  brass  tube, 
in  such  quantity  that  in  traversing  the  tube  while  the  furnace  is  in 
full  action  the  water  shall  not  be  sensibly  warmed. 

The  apparatus  being  thus  disposed,  and  the  porcelain-tube  heated,  a 
slow  and  regular  current  of  pure  and  dry  carbonic  oxide  is  passed  into 
the  hot  tube.  The  gas,  as  it  issues  from  the  tube,  passes  immediately 
through  a  strong  solution  of  caustic  potash,  which  will  absorb  the  car- 
bonic acid  formed  so  that  the  experimenter  can  weigh  the  quantity  of 
acid  produced,  or  through  lime-water,  which  will  demonstrate  the  pres- 
ence of  carbonic  acid  by  becoming  turbid.  Carbonic  acid  is  formed 
whenever  the  porcelain  tube  is  bright  red-hot.  A  portion  of  the  car- 
bonic oxide  is  decomposed  into  oxygen,  which  unites  with  another  por- 
tion of  carbonic  oxide  to  make  carbonic  acid,  and  carbon,  which  is 
deposited  in  the  condition  of  lamp-black  upon  the  cold  brass  tube  which 
traverses  the  hot  porcelain  tube  from  side  to  side.  The  first  action  of 
the  heat  is  to  set  free  particles  of  carbon  from  the  carbonic  oxide,  and 
all  such  particles  which  happen  to  fasten  upon  the  brass  tube  are  in-* 
stantly  chilled  down  below  the  temperature  at  which  they  will  either 
unite  with  free  oxygen  on  the  one  hand,  or  reduce  carbonic  acid  on 
the  other. 

We  have  repeatedly  used  the  electric  spark  as  a  means  of  decom- 
posing compound  gases,  such,  for  example,  as  ammonia  (§  89)  and 
marsh-gas  (§395).  It  is  supposed  that  it  is  the  intense  heat  of  the  spark 
which  effects  such  decomposition,  and,  in  the  light  of  the  experiment 
just  described,  it  seems  probable  that  the  efficacy  of  the  spark-current 
is  due  to  the  fact  that  the  few  particles  of  gas  which  each  spark  heats 
intensely  arc  immediately  in  contact  with  an  atmosphere  of  gas  which 
s  in  constant  motion  and  is  relatively  very  cold. 

422.  The  composition  of  carbonic  acid  being  known,  that  of 

22 


338 


COMPOSITION    OF    CARBONIC    OXIDE. 


carbonic  oxide  can  readily  be  determined  by  burning  a  definite 
volume  of  this  gas  with  an  excess  of  oxygen  in  a  eudiometer. 
FIG.  57.     If  there  be  introduced  into  the  eudiometer 

100  volumes  of  carbonic  oxide, 
and  100       "          "   oxygen, 


and  if  through  the  200  "  "  mixed  gases 
an  electric  spark  be  made  to  pass,  combination  will 
veccur,  and  the  gas  which  remains  will  occupy  only  150 
volumes.  If  a  small  quantity  of  a  solution  of  caustic 
soda  be  now  introduced  into  the  eudiometer,  all  the,  carbonic 
acid  which  has  been  formed  will  be  absorbed  ;  there  will  remain 
only  50  volumes  of  gas,  .which,  upon  examination,  will  be  found 
to  be  pure  oxygen. 

If  only  50  volumes  of  the  original  oxygen  are  thus  left  free, 
50  volumes  of  oxygen  must  have  been  absorbed.  It  appears, 
then,  that  100  volumes  of  carbonic  oxide  have  united  with  50 
volumes  of  oxygen  to  form  100  volumes  of  carbonic  acid;  and 
that  the  original  bulk  of  the  carbonic  oxide  taken  has  remained 
unchanged.  Since  it  is  admitted,  as  we  have  already  seei 
(§  412),  that  the  product-volume  of  carbonic  acid  contains 
volumes  of  oxygen,  it  follows  that  the  double  volume  of  carbor 
oxide  can  only  contain  1  volume  of  oxygen  ;  or,  in  other  words 
that  2  volumes  of  this  gas  contain  1  volume  of  carbon-vapor 
1  volume  of  oxygen  united  without  condensation  :  — 


CO 

28 

It  will  be  noticed  that  the  addition  of  a  certain  quantity  of  oxy- 
gen to  a  measured  quantity  of  carbonic  oxide  converts  it  into 
carbonic  acid  without  changing  the  original  measured  volume  of 
gas.  We  have  often  prepared  compound  gases  from  elementary 
gases  by  this  method,  and  in  such  cases  there  is  generally  a 
change  of  volume.  We  are  here,  however,  converting  one  com- 
pound gas  into  another  compound  gas,  and  the  product-volumes 
of  all  compound  gases  are  the  same. 

The  specific  gravity  or  unit-volume  weight  of  carbonic  oxidt 
has  been  found,  by  experiment,  to  be  13.97. 


COMBUSTION.  339 

From  the  weight  of  two  unit-volumes  of  carbonic  oxide  . 
Deduct  the  weight  of  one  unit-volume  of  oxygen 

There  remains  the  weight  of  the  atom  of  carbon      .         .         .     11.94 

This  result  accords  very  well  with  the  previously  given  atomic 
weight  of  carbon.  It  will  be  noticed  that  the  specific  gravity  of 
carbonic  oxide  is  the  same  as  that  of  nitrogen. 

423.  Combustion.     Now   that  we   have   become   acquainted 
with  carbon,  hydrogen,  and  oxygen,  and  with  some  of  the  more 
important  compounds  formed  by  the  union  of  these  elements, 
the  subject  of  combustion  can  be  more  fully  discussed  than  has 
been  possible  hitherto.     Unlike  most  of  the  chemical  processes 
employed  by  man,  which  have  for  their  object  the  preparation  of 
some  tangible    chemical   compound  or  product,   combustion   is 
resorted  to  merely  for  the  sake  of  the  heat  or  light  which  inci- 
dentally accompanies  the  chemical  action. 

As  a  general  rule,  only  the  compounds  of  carbon  and  hydrogen 
are  employed  as  combustibles,  though  there  are  some  exceptions 
to  this  rule,  as  when  the  metal  magnesium  is  burned  for  light, 
or  the  heating  of  a  sulphuretted  ore  is  effected  by  the  combus- 
tion of  its  own  sulphur.  In  the  Bessemer  process  of  making 
steel  from  cast-iron  (see  the  Chapter  upon  Iron),  intense  heat  is 
evolved,  partly  by  the  combustion  of  the  carbon  which  cast-iron 
contains,  but  partly  also  by  the  combustion  of  iron.  The  carbon 
compounds  are  peculiarly  well  adapted  to  the  purpose,  since  the 
products  of  their  combustion  are  gaseous,  and  can/therefore,  be 
readily  removed ;  new  portions  of  the  combustible  are-  thus 
continually  laid  bare,  and  a  way  opened  for  the  admission  of 
fresh  air. 

424.  In  almost  all  cases  artificial  light  results  from  the  incan- 
descence of  particles  of  solid  matter.     When  the  heat,  which  is 
an  invariable  accompaniment  of  chemical  combination,  can.  play 
directly  upon   such   solid  particles  with  force  enough  to  ignite 
them,  an  exhibition  of  light  will  accompany  the  chemical  change. 
The  hydrogen  flame  affords  no  light,  or  as  good  as  none,  because 
in  it  nothing  but  a  gas  is  heated.     But  when  a  solid  body,  such 
as  the  platinum  wire  or  the  piece  of  lime  employed  in  Exps.  26 
and  27,  is  placed  in  this  non-luminous  hydrogen  flame,  intense 
light  is  radiated  *rom  the  heated  solid. 


340  LUMINOSITY    OF   FLAMES. 

Exp.  192. —  Sprinkle  fine  iron  filings  into  the  flame  of  an  alcohol 
lamp,  or  into  the  non-luminous  flame  of  the  gas-lamp,  and  observe  the 
light  given  off  by  the  particles  of  metal  as  they  become  incandescent 
while  passing  through  the  flame.  Or  rub  together  two  pieces  of  char- 
coal above  a  non-luminous  flame,  in  such  manner  that  charcoal  powder 
shall  fall  into  the  flame.  Or  rub  the  coat-sleeve  beneath  a  non-lumi- 
nous flame,  or  even  beneath  the  luminous  flame  of  an  ordinary  Argand 
gas-burner,  and  observe  that  the  particles  of  dust  detached  become 
incandescent  and  luminous  as  they  pass  upward  through  the  flame. 
Other  things  being  equal,  the  hotter  the  flame  the  more  intense  will 
be  the  light  emitted  by  the  ignited  solid. 

425.  In  ordinary  luminous  flames,  such  as  those  of  candles, 
lamps,  and  illuminating  gas,  the  solid  matter  heated  is  carbon. 

Exp.  193. —  By  means  of  caoutchouc  tubing,  attach  to  any  small 
gas-burner,  a  piece  of  hard  glass  tubing,  No.  4,  about  20  c.  m.  long,  the 
outer  end  of  which  has  been  drawn  to  a  fine  open  point.  Open  the 
cock  of  the  gas-burner  so  that  gas  may  flow  into  and  through  the  glass 
tube,  and  light  this  gas  as  it  escapes.  When  the  last  traces  of  air  have 
been  expelled  from  the  tube,  heat  the  middle  of  the  latter  intensely 
with  the  flame  of  a  lamp.  Part  of  the  transparent  and  colorless  car- 
buretted  hydrogen,  of  which  the  illuminating  gas  consists  (§  396),  will 
be  decomposed  by  the  heat  as  it  passes  through  the  tube,  just  as  sul- 
phuretted hydrogen  (Exp.  95),  phosphuretted  hydrogen,  arseniuretted 
hydrogen  (Exp.  137),  and  antimoniuretted  hydrogen  (Exp.  140),  are 
decomposed  under  like  conditions,  and  a  ring  of  carbon  will  be  depos- 
ited in  the  cold  portion  of  the  tube  a  short  distance  in  front  of  the 
flame.  • 

In  the  open  channel  afforded  by  the  tube  it  is  not  easy  to  heat  the 
whole  of  the  carburetted  hydrogen  to  the  temperature  necessary  for 
its  decomposition,  but  by  lighting  the  gas,  as  it  issues  from  the  tube, 
heat  enough  to  decompose  it  can  readily  be  obtained.  Precisely  as  in 
the  combustion  of  wood  (§  378),  after  the  fire  is  once  started  the  com- 
bustible suffers  decomposition  ;  the  easily  inflammable  hydrogen  of  the 
gas  burns  first,  and  solid  particles  of  carbon  are  set  free.  These  par- 
ticles of  carbon  are  heated  to  ignition  by  the  burning  hydrogen,  and  as 
they  pass  up  through  the  flame  emit  light ;  finally  they  are  themselves 
completely  burned  to  carbonic  acid  upon  the  outside  of  the  flame,  pro- 
vided there  be  present  a  sufficient  supply  of  air.  That  there  are 
really  particles  of  free  carbon  in  the  flame  has  already  been  sufficiently 
demonstrated  in  Exp.  159. 

If  the  supply  of  air  furnished  to  the  flame  is  insufficient  to  convert 


THE    BUNSEN    BURNER. 


341 


58)  from  the  gas-lamp,  con- 
FIG.  58. 


all  of  the  component*  of  the  gas  into  carbonic  acid  and  water,  then  a 
number  of  the  particles  of  carbon  will  escape  unburned,  and  a  smoky 
flame  will  be  the  result.  If,  on  the  other  hand,  the  supply  of  air  is 
excessive,  then  all  the  carbon  will  be  burned  at  the  instant  when  it  is 
set  free,  and  no  light  will  be  afforded.  In  the  gas-lamps  commonly  em- 
ployed in  chemical  laboratories  for  purposes  of  heating  (see  Appendix, 
§  5),  illuminating  gas  is  purposely  mixed  with  a  considerable  volume 
of  air  before  it  is  lighted ;  there  is  thus  obtained  an  intensely  hot,  non- 
luminous  flame.  Such  flames  deposit  no  soot  upon  the  vessels  which 
are  heated  in  them ;  moreover,  the  heat  which  would  be  consumed  in 
heating  the  particles  of  carbon,  and  so  producing  light,  is  in  such 
flames  utilized  for  heating  purposes. 

Exp.  194.  —  Unscrew  the  tube  /  (Fi< 
structed  as  described  in  §  5  of  the  Ap- 
pendix, and  light  the  gas  as  it  escapes 
from  the  holes  in  the  face  of  the  screw 
d ;  the  flame  will  be  luminous,  and,  if 
the  holes  are  large  enough  to  permit 
a  rapid  exit  of  gas,  even  smoky.  Ex- 
tinguish the  burning  jet,  screw  on  the 
tube/,  and  relight  the  mixture  of  air 
and  gas  at  the  top ;  the  flame  will  be 
nearly  colorless.  Sometimes,  when 
the  gas-cock  is  too  nearly  closed,  the 
flame  of  the  mixed  gas  and  air  is  lia- 
ble to  pass  down  the  tube/,  and  ignite  the  feeble  jet  of  gas  at  the  aper- 
tures in  d.  The  lamp  then  burns  with  a  sickly  yellow  flame,  which  is 
often  tinged  with  green  coming  from  the  copper  in  the  heated  brass 
tube  /.  The  lamp  must  be  extinguished,  and  relit  at  the  top  of  the 
tube  with  a  freer  supply  of  gas.  When  the  tube  /is  in  place,  the  jets 
of  gas,  issuing  vertically  from  the  face  of  the  screw  d,  draw  in  currents 
of  air  through  the  side  holes  near  the  bottom  of  the  tube  /;  this  air 
mixes  with  the  gas  rising  through  /  and  at  the  top  of  this  tube,  where 
the  mixture  is  inflamed,  the  carburetted  hydrogen  is  in  intimate  contact 
with  air  enough  to  burn  it  at  once  and  completely. 

Between  the  two  extremes  which  a  Bunsen  burner  may  be 
thus  made  to  illustrate,  between  a  smoky  flame,  on  the  one  hand, 
and  a  non-luminous  flame  on  the  other,  there  are  two  points 
which  have  special  significance,  —  the  point  of  most  light,  and 
the  point  of  most  agreeable  light.  The  point  of  most  light  may 
always  be  hit  upon  by  constructing  such  a  bui;ner  as  will  just 
not  allow  the  gas  to  smoke. 


342  QUANTITY   AND    INTENSITY   FLAMES. 

Exp.  195. —  Across  the  top  of  the  chimney  of  an  Argand  gas-burner, 
which  is  burning  with  a  shorter  flame  than  usual,  place  several  narrow 
strips  of  tin  or  of  sheet-iron,  so  as  to  obstruct  the  flow  of  air  through  the 
chimney.  The  small,  low  flame  with  which  the  experiment  began  will 
increase  in  size  as  the  access  of  air  is  diminished,  and,  at  last,  the  whole 
interior  of  the  chimney  will  be  filled  with  a  long,  smoky  flame.  The 
volume  of  gas  burning  at  any  one  moment  of  the  experiment  is  no 
greater  than  at  another,  for  the  cock  which  regulates  the  flow  of  the 
gas  remains  fixed  and  untouched,  but  the  amount  of  light  afforded  by 
the  large  smoky  flame  is  manifestly  greater  than  that  yielded  by  the 
small  bright  flame  with  which  the  experiment  started.  If  any  doubt 
suggest  itself  as  to  this  point,  it  will  quickly  be  dissipated  by  perform- 
ing the  experiment  in  a  darkened  room  and  noting  the  comparative 
visibility  of  the  more  distant  objects  therein  contained,  first  with  the 
one  flame  and  then  with  the  other ;  or  the  observer  may  determine  at 
what  distances  from  the  two  flames  fine  print  can  be  deciphered. 

A  murky  flame,  such  as  was  just  now  obtained,  before  actual  smoking 
begins,  in  which  the  largest  possible  number  of  the  particles  of  carbon 
are  heated,  though  none  of  them  are  heated  very  hot,  yields  the  largest 
amount  of  light  which  the  particular  sample  of  gas  under  examination 
is  capable  of  affording.  Such  flames  are  called,  technically,  quantity 
flames ;  they  are  better  adapted  than  any  others  for  lighting  streets 
and  large  halls.  In  practice,  such  flames  are  obtained  by  burning  the 
gas  at  a  low  pressure,  that  is,  under  such  conditions  that  it  shall  be  very 
gently  pressed  out  into  the  air,  so  that  air  shall  mix  with  it,  and  act 
upon  it,  but  slowly. 

But  besides  this  point  of  the  maximum  amount  of  light,  there 
is  another  of  the  most  agreeable  light ;  and  this  is  something 
which  each  individual  must  determine  for  himself.  Few  persons 
would  choose,  as  a  study-lamp,  either  the  murky  flame  of  Exp. 
195,  or  the  intense  lime-light  of  Exp.  27,  but  between  these  two 
extremes  no  one  light  is  likely  to  suit  many  people  equally  well. 

If  a  bright  intensity  flame  is  required,  we  have  only  to  arrange 
matters  in  such  a  way  that  air  may  come  to  the  gas  so  quickly 
and  abundantly  that  a  portion  of  the  carbon  in  the  gas,  as  well 
as  the  hydrogen,  shall  be  burned  at  once  in  the  lower  part  of  the 
flame,  and  by  the  heat  of  its  combustion  ignite  more  intensely 
the  remaining  particles  of  carbon.  Among  the  very  great  num- 
ber of  gas-burners  which  have  been  devised,  there  may  be  found 
those  adapted  to  meet  almost  any  requirement.  Each  kind 


NO    GAS-BURNER    UNIVERSAL.  343 

of  burner  brings  the  gas  and  the  air  into  contact  with  one  an- 
other in  some  special  way,  producing  a  flame  of  convenient 
shape,  of  peculiar  economy,  or  of  particular  steadiness  or  bril- 
liancy. It  is  obvious  that  the  conditions,  under  which  gas  is 
most  advantageously  burned,  are  different  for  different  uses,  and 
that  no  one  burner  can  be  equally  available  under  such  varying, 
and,  in  some  sense,  antagonistic  conditions.  The  Argand  burner 
may,  perhaps,  be  made  to  fulfil  as  many  of  these  conditions  as 
any  other ;  from  it  there  may  be  obtained,  at  will,  either  an  in- 
tensity or  a  quantity  flame,  as  has  been  shown  in  Exp.  195. 

426.  The  chemical  composition  of  the  gas  to  be  burned  is,  of 
course,  an  important  point  to  be  considered  in  the  construction 
of  the  burner.  A  gas  rich  in  carbon  requires,  for  its  combus- 
tion, far  more  air  than  gas  which  is  less  carboniferous. 

Exp.  196.  —  Place  three  small  tufts  of  cotton  upon  an  earthen  plate ; 
moisten  one  with  alcohol,  another  with  petroleum,  and  the  third  with 
benzin ;  touch  a  lighted  match  to  the  vapor  which  arises  from  each. 
In  the  one  case  there  will  be  seen  hardly  any  luminous  particles  of 
carbon ;  in  the  second,  a  bright  light,  and  in  the  third,  so  much  carbon 
will  be  set  free  that,  under  the  conditions  of  the  experiment,  a  great 
deal  of  it  cannot  find  air  with  which  to  unite,  and  consequently  escapes 
as  smoke. 

The  composition  of  alcohol  may  be  represented  by  the  formula 
C2HGO  ;  it  contains  a  large  proportion  of  hydrogen  and  some  oxygen ; 
hence  steam  is  necessarily  produced  when  it  burns ;  this  steam  spreads, 
or  diffuses,  the  flame  and  promotes  the  prompt  union  of  the  alcohol 
vapor  with  the  oxygen  of  the  air,  so  that  few  particles  of  carbon  have 
time  to  become  incandescent  before  they  are  consumed.  But  in  ben- 
zin, the  formula  of  which  may  be  written  CGH6 ,  there  is  no  oxygen 
and  a  far  larger  proportion  of  carbon  than  in  alcohol ;  hence  the 
necessity  of  supplying  a  large  amount  of  air  to  the  lamps  in  which  its 
vapor  is  burned.  The  best  way  of  consuming  benzin  is  to  mix  its 
vapor  with  air  in  suitable  proportions,  and  to  press  this  mixture  through 
a  gas-burner  as  if  it  were  ordinary  illuminating  gas.  When  thus 
treated,  it  burns  without  smoke,  and  affords  a  brilliant  white  light. 
Petroleum  (C2H6),  like  benzin,  contains  no  oxygen,  but  it  contains 
far  less  carbon  than  benzin,  though  much  more  than  alcohol ;  it  does 
not  smoke  like  benzin,  and  yet  it  smokes  so  much  that  it  cannot  readily 
be  burned  from  a  simple  wick ;  it  is  commonly  burned  in  lamps  pro- 
vided with  a  special  draught  of  air. 


344  ALL    FLAMES    GAS-FLAMES. 

427.  Ordinary  lamps  and  candles  are,  strictly  speaking,  gas- 
lamps.  In  all  cases  their  flames  are  composed  of  burning  gas. 

Exp.  197.  —  Construct  a  lamp  as  follows :  To  a  wide-mouthed  bottle 
of  the  capacity  of  about  50  c.  c.  fit  a  cork  loosely;  bore  a  hole  in  the 
cork  and  place  therein  a  short  piece  of  glass-tubing,  No.  3,  open  at 
both  ends ;  through  this  glass-tube  draw  a  piece  of  lamp-wicking,  or 
any  loose  twine,  long  enough  to  reach  to  the  bottom  of  the  bottle.  It 
is  essential,  either  that  the  cork  should  fit  the  bottle  loosely,  or  that 
there  should  be  a  hole  in  the  cork,  in  order  that  the  pressure  of  the 
external  air  may  act  upon  the  surface  of  the  alcohol,  —  to  this  end  a 
very  small  glass-tube  may  be  inserted  in  the  cork  at  some  distance  from 
the  tube  which  carries  the  wick.  Fill  the  bottle  nearly  full  with  alcohol, 
and,  after  a  few  minutes,  touch  a  lighted  match  to  the  top  of  the  wick. 
The  fluid  alcohol  is  drawn  up  out  of  the  bottle  by  force  of  capillary 
attraction  exercised  by  the  pores  T)f  the  vegetable  fibre  of  which  the 
wick  is  composed.  When  heat  is  applied  to  the  alcohol  at  the  top  of 
the  wick,  some  of  it  is  converted  into  vapor ;  this  vapor  then  takes  fire, 
and,  in  burning,  furnishes  heat  for  the  vaporization  of  new  portions  of 
the  alcohol.  From  the  top  of  the  wick  there  is  constantly  arising  a 
column  of  gas  or  vapor,  and  upon  the  exterior  of  this  conical  column 
chemical  combination  is  all  the  while  going  on  between  its  constituents 
and  the  oxygen  of  the  air.  The  dark  central  portion  of  the  alcohol 
flame  is  nothing  but  gas  or  vapor. 

Exp.  198.  —  Thrust  the  phosphorus  end  of  an  ordinary  friction-match 
directly  into  the  middle  of  the  flame  of  the  alcohol-lamp  of  Exp.  197. 
The  combustible  matter  upon  the  end  of  the  match  will  not  take  fire 
in  the  atmosphere  of  carbonaceous  gases,  of  which  the  centre  of  the 
flame  consists.  The  wood  of  the  match-stick,  of  course,  takes  fire  at 
the  point  where  it  is  in  contact  with  the  outer  edge  of  the  flame, 
for  it  is  there  heated  in  contact  with  air.  In  withdrawing  the  match 
from  the  middle  of  the  flame,  it  is  not  easy  to  prevent  it  from  taking 
fire  as  it  passes  through  the  outer  edge  of  the  flame ;  for  the  materials 
on  the  tip  of  the  match  have  been  so  strongly  heated  by  radiation, 
during  their  sojourn  within  the  circle  of  fire,  that  they  are  now  ready 
to  burst  into  flame  immediately  on  coming  in  contact  with  the  air ;  by 
a  quick  jerk,  however,  the  match  may  often  be  withdrawn  from  the 
flame  without  taking  fire. 

Exp.  199.  —  Hold  a  thin  wire,  best  of  platinum,  though  iron  will 
answer  well  enough,  or  a  splinter  of  wood  across  the  flame  of  an  alco- 
hol-lamp, as  shown  in  Fig.  59.  The  wire  will  be  heated  to  redness, 
and  the  wood  will  burn  only  at  the  outer  edges  of  the  flame  where  the 
gas  and  air  meet;  in  the  interior  of  the  flame  the  wire  will  remain 


OIL-LAMPS   AND    CANDLES.  345 

dark  and  the  wood  unburned,  for  there  is  no  combustion  there,  FIG.  59. 
and  comparatively  little  heat.  If  the  wire  be  successively 
placed  at  different  heights  in  the  flame,  the  size  and  shape 
of  the  internal  cone  of  gas  can  easily  be  made  out ;  it  will 
appear,  moreover,  that  the  hottest  part  of  the  flame  is  just 
above  the  top  of  the  interior  cone  of  gas.  As  a  rule,  when 
glass-tubing,  or  the  like,  is  to  be  heated  in  a  flame,  it  should 
never  be  placed  below  this  point  of  the  greatest  heat. 

428.  The  flame  of  an  ordinary  oil-lamp  or  of  a  petroleum- 
lamp,  in  the  same  way  as  the  flame  of  an  alcohol-lamp,  is  com- 
posed of  an  inner  cone  of  gas,  or  vapor  of  hydrocarbons,  and  an 
envelope  where  chemical  combination  is  going  on ;  and  a  candle 
flame  is  really  the  flame  of  an  oil-lamp  (Exp.  200).  The  pres- 
ence of  vapor  in  the  candle-flame  can  be  readily  shown  (Exps. 
201,  202).  In  the  candle-flame,  as  in  that  of  the  alcohol-lamp, 
there  is-  a  cone  of  unburnt  gas  surrounded  by  a  shell  of  burning 
substances  (Exps.  203,  204). 

Exp.  200.  —  Touch  a  lighted  match  to  the  wick  of  a  new  candle ; 
the  cotton  of  which  the  wick  is  composed  takes  fire  and  is  at  once  con- 
sumed for  the  most  part,  but,  in  burning,  the  cotton  gives  off  consider- 
able heat,  and  some  of  the  wax  or  tallow  of  which  the  candle  is 
composed  is  thereby  melted  and  converted  into  oil.  The  liquid  oil 
ascends  the  wick  by  virtue  of  capillary  attraction,  and  is  converted 
into  vapor  or  gas  by  the  heat  of  the  cotton  still  burning  at  the  stump 
of  the  wick :  this  gas  then  burns  precisely  like  the  alcohol  vapor  in 
Exp.  197  or  the  illuminating  gas  in  Exp.  193,  and  by  the  heat  thus 
disengaged  new  portions  of  wax  or  tallow  are  continually  melted. 
There  is  always  a  little  cup  of  oil  at  the  top  of  the  rod  of  wax  or  tal- 
low of  which  the  candle  consists,  and  the  apparatus  is  as  truly  an  oil- 
lamp  as  if  the  oil  were  held  in  a  vessel  of  glass  or  metal. 

Exp.  201.  —  Let  a  candle,  best  of  tallow,  burn  until  the  snuff  has 
become  long  ;  blow  out  the  flame,  and  observe  the  cloud  of  vapor  which 
ascends  from  the  hot  wick.  Touch  a  lighted  match  to  this  column  of 
vapor  and  notice  that  it  takes  fire  at  some  little  distance  from  the  wick. 
After  the  flame  has  been  extinguished,  the  wick  retains  heat  enough 
for  a  few  moments  to  distil  off  a  quantity  of  gas,  although  there  is  not 
heat  enough  generated  to  inflame  this  gas.  To  the.  gas  or  vapor  thus 
evolved  is  to  be  referred  the  disagreeable  odor  which  is  observed  when 
a  candle  is  blown  out. 

Exp.  202.  —  Draw  a  glass-tube,  No.  5  or  6,  10  or  15  c.  m.  long  to  a 
moderately  fine  open  point ;  with  a  piece  of  wire,  bind  this  tube  in  an 


346  FORM    OF    LUMINOUS    FLAMES. 

inclined  position  to  a  ring  of  the  iron-stand,  and  place  the  lower  end 
of  the  tube  in  the  middle  of  a  candle  flame,  just  below  the  centre,  so 
that  a  portion  of  the  gas  of  the  inner  cone  of  the  flame  may  escape 
through  the  tube ;  light  the  gas  at  the  point  at  the  top  of  the  glass-tube 
and  observe  that  it  will  burn  there  steadily,  if  the  experiment  is  per- 
formed in  a  quiet  place  where  there  are  no  draughts  of  air. 

Exp.  203.  —  Press  down  a  piece  of  white  letter-paper,  for  an  instant, 
upon  the  flame  of  a  candle  until  it  almost  touches  the  wick,  then 
quickly  remove  the  paper  before  it  takes  fire,  and  observe  that  its  upper 
FIG.  60.  surface  is  charred  in  the  manner  shown  in 

Fig.  60.  There  will  be  obtained,  in  fact, 
burned  into  the  paper,  a  diagram  of  the 
cylindrical  column  of  unburned  gas  and  of 
the  shell  of  burning  matter  which  surrounds 
it.  Within  the  charred  ring  the  paper  is  unacted  upon,  for  that  part 
of  it  was  in  contact  only  with  the  unburnt  gas  in  the  centre  of  the 
flame. 

Exp.  204.  —  Replace  the  paper  of  Exp.  203  with  a  strip  of  glass,  so 
held  that  the  conical  flame  of  the  candle  shall  be  cut  across  horizon- 
tall}'  by  the  glass  as  it  was  by  the  paper,  in  Exp.  203.  Look  down 
from  above  through  the  glass  into  the  hollow  cylinder  of  unburnt  gas 
within  the  circle  of  combustion. 

FIG.  61.  429.  In  any  flame,  which  is  rendered  luminous  by 
particles  of  hot  carbon,  three  portions  can  be  distin- 
guished. 1st.  The  dark  interior  cone  of  gas,  a,  Fig. 
61 ;  2d.  The  zone  of  intense  chemical  action,  b,  where 
the  hydrogen  is  burning  and  the  particles  of  carbon 
are  heated  to  whiteness ;  and,  finally,  upon  the  very 
outside  a  thin,  scarcely  perceptible  film  of  burning 
carbonic  oxide,  c. 

430.  From  the  study  of  luminous  flames  we  pass 
to  a  consideration  of  flames  employed  only  as  sources 
of  heat.  In  the  experiments  (25-27)  with  the  oxy- 
hydrogen  blow-pipe,  it  has  been  already  shown  that  a 
very  intense  heat  may  be  obtained  by  throwing  oxygen  into  the 
hydrogen  flame,  and  so  localizing  the  chemical  action  and  the 
heat  with  which  this  action  is  accompanied.  The  subject  may 
be  here  conveniently  studied  by  employing  coal-gas  and  air  in 
place  of  hydrogen  and  oxygen. 
Exp.  205.  —  Fill  one  gas-holder  with  air,  and  screw  to  it  a  metallic 


THE    BLAST-LAMP.  347 

jet,  such  as  is  shown  in  Fig.  62.     Fill  another  gas-holder  with  ordi- 
nary illuminating  gas  and  connect  the  opening  of  this  gas-holder  with 

FIG.  62. 


the  lower  opening  of  the  metallic  jet.  Open  the  cock  of  the  holder 
which  contains  the  coal-gas  and  inflame  the  gas  at  the  point  of  the 
metallic  jet.  There  will  be  thus  obtained  a  long  stream  of  gas  burn- 
ing at  the  expense  of  the  air  which  bathes  its  surface.  The  chemical 
action  between  the  oxygen  of  the  air  and  the  constituents  of  the  coal- 
gas,  and  the  heat  resulting  from  this  action,  are  diffused  over  the  entire 
surface  of  this  long  flame.  Without  touching  the  cock  of  the  holder 
which  contains  the  coal-gas,  or  in  any  way  altering  the  amount  of  gas 
which  flows  out  of  this  holder,  open  the  cock  of  the  holder  which  con- 
tains air,  so  that  air  may  be  thrown  into  the  middle  of  the  coal-gas 
flame.  The  latter  will  be  immediately  shortened  down  to  almost  noth- 
ing. The  constituents  of  the  coal-gas  will  now  all  combine  with  oxy- 
gen in  a  very  small  space,  and  the  heat  of  combination  which  was 
diffused  before  will  be  correspondingly  concentrated.  It  is  much  the 
same  as  if  the  coal-gas  and  the  air  had  been  mixed  together  beforehand 
and  then  lighted.  Indeed,  in  one  of  the  first  forms  of  the  oxyhydro- 
gen  blow-pipe,  a  mixture  of  the  two  gases,  such  as  we  have  exploded 
in  Exp.  30,  was  first  prepared,  and  then  forced  out  of  a  single  gas- 
holder of  peculiar  construction,  provided  with  an  exceedingly  minute 
orifice,  at  the  mouth  of  which  the  mixed  gases  were  burned.  This 
apparatus  was  inconvenient  and  dangerous,  and  has  long  since  been 
superseded,  but  it  well  illustrated  the  local  concentration  of  heat  now 
under  discussion. 

431.  The  principle  of  the  common  mouth  blow-pipe,  of  the 
glass-blower's  lamp  (Appendix,  §  6),  and  of  all  blasts  and  blow- 
ers, is  identical  with  that  of  the  oxyhydrogen  blow-pipe,  —  which, 
as  has  been  already  stated  (§  55),  is  the  simplest  case  of  all. 
Air,  or  more  strictly  speaking,  oxygen,  is  thrown  into  the  com- 
bustible gas  or  fuel,  in  order  that  the  combustion  may  go  on  in 
a  small  space. 

The  mouth  blow-pipe  may  be  used  with  a  candle,  or  with  any  hand- 


348  THE    BLOW-PIPE. 

lamp  proper  for  burning  oil,  petroleum,  or  any  of  the  so-called  burning 
fluids,  provided  that  the  form  of  the  lamp  below  the  wick-holder  is 
such  as  to  permit  the  close  approach  of  the  object  to  be  heated  to  the 
side  of  the  wick.  "When  a  lamp  is  used,  a  wick  about  1.2  c.  m.  long 
and  0.5  c.  m.  broad  is  more  convenient  than  a  round  or  narrow  wick ; 
a  wick  of  this  sort,  though  hardly  so  wide,  is  used  in  some  of  the  open 
burning-fluid  (naptha)  lamps  now  in  common  use.  The  wick-holder 
should  be  filed  off'  on  its  longer  dimension  a  little  obliquely,  and  the 
wick  cut  parallel  to  the  holder,  in  order  that  the  blow-pipe  flame  may 
be  directed  downwards  when  necessary  (Fig.  64). 

The  cheapest  and  best  form  of  mouth  blow-pipe  for  chemical  pur- 
poses is  a  tube  of  tin-plate,  about  18  c.  m.  long,  2  c.  m.  broad  at  one 
end,  and  tapering  to  0.7  c.  m.  at  the  other  (Fig.  63)  ;  the  broad  end  is 
FlG-  63.  closed,  and  a  little  above  this  closed  end 

a  small  cylindrical  tube  of  brass  about  5 
c.  m.  long  is  soldered  in  at  right  angles ; 
this  brass-tube  is  slightly  conical  at  the  end 
and  carries  a  small  nozzle  or  tip,  which 
may  be  made  either  of  brass  or  platinum. 
The  tip  should  be  drilled  out  of  a  solid 
piece  of  metal,  and  should  not  be  fastened 
upon  the  brass-tube  with  a  screw.  A 
trumpet-shaped  mouth-piece  of  horn  or 
-u— -*  box-wood  is  a  convenient,  though  by  no 

means  essential,  addition  to  this  blow-pipe. 

Exp.  206.  —  To  use  the  mouth  blow-pipe,  place  the  open  end  of  the 
tin  tube  between  the  lips,  or,  if  the  pipe  is  provided  with  a  mouth-piece, 
press  the  trumpet-shaped  mouth-piece  against  the  lips ;  fill  the  mouth 
with  air  till  the  cheeks  are  widely  distended,  and  insert  the  tip  in  the 
flame  of  a  lamp  or  candle ;  close  the  communication  between  the  lungs 
and  the  mouth,  and  force  a  current  of  air  through  the  tube  by  squeezing 
the  air  in  the  mouth  with  the  muscles  of  the  cheeks,  breathing,  in  the 
meantime,  regularly  and  quietly  through  the  nostrils.  The  knack  of 
blowing  a  steady  stream  for  several  minutes  at  a  time,  is  readily 
quired  by  a  little  practice.  It  will  be  at  once  observed  that  tl 
appearance  of  the  flame  varies  considerably,  according  to  the  strenj 
of  the  blast  and  the  position  of  the  jet  with  reference  to  the  wick. 

When  the  je't  of  the  blow-pipe  is  inserted  into  the  middle  of  a  can- 
dle-flame, or  is  placed  in  the  lamp-flame  in  the  position  shown  in 
figure  64,  and  a  strong  blast  is  forced  through  the  tube,  a  long,  blue 
cone  of  flame,  a  6,  is  produced,  beyond  and  outside  of  which  stretches  a 
more. or  less  colored  outer  cone  towards  c.  The  point  of  greatest  heat 


XIDIZING    AND    REDUCING    FLAMES. 


349 


in  this  flame  is  at  the  point  of  the  inner  blue  cone,  because  the  com- 
bustible gases  are  there  sup-  [Fi«  64. 
plied  with  just  the  quantity 
of  oxygen  necessary  to  con- 
sume them,  but  between  this 
point  and  the  extremity  ol 
the  flame  the  combustion  is 
concentrated  and  intense. 
The  greater  part  of  the  flame 
thus  produced  is  oxidizing  in 
its  effect  and  this  flame  is  technically  called  the  oxidizing  flame.  From 
the  point  a  of  the  inner  blue  cone,  the  heat  of  the  flame  diminishes  in 
both  directions,  towards  6,  on  the  one  hand,  and  towards  c  on  the  other ; 
most  substances  require  the  temperature  to  be  found  between  a  and  c. 
Oxidation  takes  place  most  rapidly  at,  or  just  beyond,  the  point  c  of 
the  flame,  provided  that  the  temperature  at  this  point  is  high  enough 
for  the  special  substance  to  be  heated. 

A  flame  of  precisely  the  opposite  chemical  effect  may  be  produced 
with  the  blow-pipe.  To  obtain  a  good  reducing  flame,  it  is  necessary 
to  place  the  tip  of  the  blow-pipe,  not  within,  but  just  outside  of  the 
flame,  and  to  blow  rather  over  than  through  the  middle  of  the  flame 
(Fig.  65).  In  this  manner,  the  flame  is  less  altered  in  its  general 
character  than  in  the  former  FIG.  65. 

case,  the  chief  part  consisting 
of  a  large,  luminous  cone,  con- 
taining a  quantity  of  free  car- 
bon in  a  state  of  intense  ignition 
and  just  in  the  condition  for 
taking  up  oxygen.  This  flame 
is,  therefore,  reducing  in  its 
effect,  and  is  technically  called 
the  reducing  flame.  The  substance  which  is  to  be  reduced  by  exposure 
to  this  flame,  should  be  completely  covered  up  by  the  luminous  cone, 
so  that  contact  with  the  air  may  be  entirely  avoided.  It  is  to  be  ob- 
served that,  whereas  to  produce  an  effective  oxidizing  flame  a  strong 
blast  of  air  is  desirable,  to  get  a  good  reducing  flame,  the  operator 
should  blow  gently,  with  only  enough  force  to  divert  the  lamp-flame. 

Substances  to  be  heated  in  the  blow-pipe  flame,  are  supported,  some- 
times on  charcoal,  sometimes  on  platinum  foil  or  wire,  or  in  platinum 
spoons  or  forceps,  and  sometimes  on  little  capsules  made  of  clay  or 
bone-earth.  Charcoal  is  especially  suitable  for  a  support  in  experi- 
ments of  reduction. 


350  MELTING   PLATINUM. 

Exp.  2^7.  —  The  heat  of  the  oxidizing  flame  may  be  well  shown  by 
melting  the  extremity  of  a  very  fine  platinum  wire  into  a  little  ball. 
To  effect  this  fusion,  bend  the  wire  at  right  angles  at  5  to  6  m.  m.  from 
the  end,  and  hold  this  bent  end  precisely  in  the  axis  of  the  flame,  with 
the  angle  outward  and  the  extremity  of  the  wire  at  the  hottest  part  of 
the  flame.  If  the  wire  is  kept  steadily  in  position,  and  the  blast  is 
strong  and  the  flame  pure,  a  little  knob  will  soon  appear  on  the  end  of 
the  wire.  The  bend  in  the  wire  is  made  in  order  to  keep  a  certain 
length  of  the  wire  hot,  and  so  to  diminish  the  conduction  of  heat  from 
the  point. 

Exp.  208.  —  Place  a  kernel  of  metallic  tin,  as  large  as  this  o,  in  a 
little  hollow,  scooped  out  at  one  end  of  a  bit  of  charcoal  8  to  12  c.  m. 
long.  Melt  this  tin  in  the  reducing  flame  of  the  blow-pipe,  and  en- 
deavor to  preserve  the  metallic  lustre  of  the  fused  metal  untarnished. 
A  pure  reducing  flame  is  necessary  for  this  purpose.  A  touch  of  the 
oxidizing  flame  upon  the  metal  covers  its  surface  with  a  white,  infusi- 
ble, incandescent  ash. 

432.  Another  method  of  supplying  the  burning  fuel  with  air 
is  by  means  of  chimneys.  Chimneys,  whether  of  lamps  or  fur- 
naces, are  simply  devices  for  bringing  air  in  abundance,  and  there- 
fore oxygen,  into  the  fire ;  that  in  so  doing  they,  at  the  same 
time,  carry  off  the  waste  products  of  combustion,  is  an  inci- 
dental advantage. 

Exp,  209. —  Light  a  piece  of  a  candle  8  or  10  c.  m.  long  and  stand 
it  upon  a  smooth  table ;  over  the  candle  place  a  rather  tall,  narrow 
lamp-chimney  of  glass,  the  bottom  of  the  chimney  being  made  to  rest 
upon  the  table,  and  observe  that  the  candle-flame  will  soon  be  extin- 
guished. No  fresh  air  can  enter  the  chimney  from  below  to  maintain 
the  chemical  action,  and  the  small  quantity  of  air  which  can  creep 
down  the  chimney  from  above  is  altogether  insufficient  to  meet  the 
requirements  of  the  case. 

Exp.  210.  —  Relight  the  candle  of  Exp.  209,  and  again  place  over  it 
the  lamp-chimney ;  but  instead  of  allowing  the  chimney  to  rest  closely 
upon  the  surface  of  the  table,  prop  it  up  on  two  narrow  strips  of  wood, 
so  that  air  can  have  free  entrance  into  the  chimney  from  below.  The 
candle  will  now  continue  to  burn  freely,  for  the  heavy,  cold  air  outside 
will  continually  press  into  the  lower  part  of  the  chimney,  and  push 
out  the  warm,  light  products  of  combustion,  and  the  candle-flame  will 
all  the  while  be  supplied  with  fresh  air. 

Exp.  211. —  Prepare  several  strips  of  "nitre-paper"  by  soaking 
ordinary  brown  paper  in  a  strong  solution  of  nitrate  of  potassium  and 


CHIMNEYS    CREATE    DRAUGHTS. 


351 


drying   the   product.     On  being  lighted,   paper  thus   prepared   will 
burn  without  flame  while  emitting  clouds  of  FIG.  66. 

smoke.  Light. a  piece  of  the  nitre-paper 
and  place  it  at  the  foot  of  the  chimney  ar- 
ranged as  in  Fig.  66.  The  smoke  of  the 
burning  paper  will  instantly  pass  up  through 
the  chimney,  and  so  indicate  the  direction 
of  the  invisible  air  which  is  all  the  while 
entering  below  and  passing  out  above  at 
the  top  of  the  chimney,  as  fast  as  it  is  heated 
and  made  lighter  by  the  burning  of  the 
candle. 

Exp.  212.  —  Kepeat  Exp.  210,  and  when 
the  candle  is  burning  quietly,  cover  the  top  of  the  chimney  tightly 
with  a  piece  of  tin  or  sheet-iron,  or  with  a  strip  of  window-glass;  the 
candle  will  soon  cease  to  burn  precisely  as  if  the  chimney  were  closed 
at  the  bottom,  for,  the  escape  of  the  hot  products  of  combustion  being 
prevented,  no  air  can  pass  into  the  chimney  to  reach  the  candle-flame. 
It  is  by  inducing  the  current  of  fresh  air  (Exp.  210),  or  draught,  as 
it  is  ordinarily  termed,  that  chimneys  are  specially  useful.  They  give 
direction  and  precision  both  to  the  incoming  cold  air  and  the  outgoing 
hot  gas.  Where  there  is  no  chimney,  the  hot  air  from  a  lamp  goes  off 
at  a  comparatively  slow  rate,  and  vaguely ;  through  the  chimney,  on  the 
other  hand,  it  flows  straight  forward  and  rapidly,  and,  of  course,  a  cor- 
respondingly direct  and  rapid  current  of  fresh  air  presses  in  to  supply 
its  place.  Owing  to  this  power  of  rapidly  supplying  air,  chimneys  are 
employed  upon  lamps  burning  petroleum  and  other  highly  carbonized 
oils  which  are  liable  to  smoke ;  in  general  they  are  made  use  of  upon 
lamps  in  all  cases  where  intensity  flames  are  required. 

Exp.  213.  —  It  is  not  absolutely  necessary 
that  the  fresh  air  should  flow  into  a  chimney 
from  below.  Divide  the  upper  part  of  the 
chimney  of  Exp.  208,  into  two  channels,  by 
hanging  in  it  a  strip  of  sheet-iron  or  tin,  as  a 
partition  at  the  centre  of  the  chimney.  (See 
Fig.  67.)  Place  the  chimney  thus  divided 
over  a  burning  candle,  and  observe  that  the 
candle  will  continue  to  burn  as  if  in  a  strong 
draught  of  air,  although  no  air  can  enter  the 
chimney  from  below.  Hold  a  piece  of  burning 
nitre-paper  (Exp.  211)  at  the  top  of  the 
divided  chimney;  the  smoke  will  be  drawn 
down  into  the  chimney  on  one  side  of  the  partition  and  thrown  out 


Fro.  67. 


352  MATTER   INDESTRUCTIBLE. 

again  upon  the  other,  as  indicated  by  the  arrows  in  Fig.  67.  It  ap- 
pears from  this,  as  well  as  from  the  tremulous  motion  of  the  flame, 
that  a  current  of  cold  air  presses  down  upon  one  side  of  the  division 
wall  and  supplies  the  required  oxygen. 

433.  One  exceedingly  important  point  in  chemical  philosophy, 
which  we  have  hitherto  taken  for  granted,  may  now  be  readily 
illustrated.  As  the  result  of  long-continued  experimentation  and 
the  most  rigid  scrutiny  of  all  chemical  facts,  it  is  admitted,  as  a 
fundamental  truth,  that  matter  is  indestructible.  When  sub- 
stances undergo  chemical  change,  none  of  their  ingredients  are 
really  destroyed,  not  an  atom  of  them  is  annihilated,  nor,  upon 
the  other  hand,  is  any  new  matter  created  ;  it  is  the  form  only 
of  the  old  substances  which  is  changed ;  their  weight  remains, 
in  every  case,  unaltered. 

By  bringing  about  chemical  combination  between  two  or  more 
bodies,  we  can  entirely  change  their  appearance,  their  condition, 
and  their  properties,  but  in  every  case  it  will  be 'found  that  the 
weight  of  the  resulting  compounds  is  precisely  equal  to  the  sum 
of  the  weights  of  their  components.  Thus,  when  a  candle  burns 
in  the  air  and  gradually  disappears,  none  of  the  elements  which 
compose  it  are  either  lost  or  destroyed.  Though  by  uniting 
with  the  oxygen  of  the  air,  the  components  of  the  candle  have 
been  converted  into  compounds  which  are  invisible,  it  is,  never- 
theless, easy  to  satisfy  ourselves  of  the  existence  of  these  com- 
pounds. Already  in  Exps.  171,  172,  it  has  been  demonstrated 
that  carbonic  acid  and  water  are  products  of  the  combustion 
of  the  candle,  and  we  now  proceed  to  show  that  these  products 
weigh  more  than  the  candle. 

Exp.  214. —  Take  a  glass-tube  2  or  3  c.  m.  in  diameter  and  25  or  30 
c.  m.  long,  such,  for  example,  as  the  neck  of  a  broken  retort ;  fit  a  cork 
to  each  extremity  of  the  tube,  and  at  a  distance  of  6  or  8  c.  m.  from 
its  upper,  wider  end,  fix  across  the  tube  a  partition  of  wire-gauze. 
Through  the  upper  cork  insert  a  glass-tube,  No.  3  or  4,  bent  at  a  right 
angle,  and  in  the  lower  cork  bore  several  open  holes,  besides  a  centra 
orifice  into  which  a  small  wax  taper  is  fastened.  Fill  the  space  between 
the  upper  cork  and  the  shelf  of  wire  gauze  with  recently  slaked  lime 
in  not  too  fine  powder,  replace  the  cork,  hang  the  tube,  with  its  con- 
tents, at  the  end  of  one  arm  of  a  balance,  and  counterpoise  the  appa- 
ratus precisely  by  placing  shot  or  sand  in  the  pan  upon  the  other  side 


KINPLING    TEMPERATURE.  353 

of  the  scales.  Next  make  an  "  aspirating  flask,"  by  fitting  to  the  upper 
orifice  of  a  bottle  with  stop-cock,  such  as  is  depicted  in  the  upper  part 
of  Fig.  xvii.  in  the  Appendix,  a  sound  cork,  carrying  a  glass-tube,  No. 
3  or  4  ;  the  tube  should  reach  to  the  bottom  of  the  bottle  and  be  bent 
at  a  right  angle  a  short  distance  above  the  cork. 

By  means  of  flexible  tubing,  connect  the  upper  tube  of  the  aspirating 
flask  with  the  tube  issuing  from  the  upper  cork  of  the  apparatus  upon 
the  balance,  and  open  the  stop-cock  of  the  aspirator  to  such  an  extent 
that  water  shall  flow  out  from  it  slowly.  Remove  the  lower  cork  from 
the  apparatus  upon  the  balance,  in  order  to  light  the  taper,  then  quickly 
replace  it  and  regulate  the  flow  of  water  from  the.  aspirator,  so  that 
sufficient  air  shall  be  drawn  in  through  the  holes  in  the  cork  which 
supports  the  taper  to  maintain  the  latter  in  lively  combustion. 

After  the  taper  has  burned  during  4  or  5  minutes,  close  the  aspirator, 
remove  the  flexible  tube  from  the  candle  apparatus,  so  that  the  latter 
may  again  hang  freely  from-  the  arm  of  the  balance,  and  observe  that 
the  weight  of  the  apparatus  is  now  greater  than  it  was  at  the  begin- 
ning of  the  experiment,  for  the  lime  within  the  tube  has  absorbed  the 
carbonic  acid  and  water  which  were  produced  by  the  combination  of 
the  ingredients  of  the  candle  with  the  oxygen  of  the  air. 

434.  In  order  that  any  combustible  substance  shall  burn,  or, 
in  other  words,  in  order  that  brisk  chemical  action  shall  occur 
between  the  combustible  and  the  oxygen  of  the  air,  it  must  first 
be  heated  to  a  certain  temperature,  and  then  kept  at  that  heat. 
The  temperature  at  which  any  substance  takes  fire  is  known  as 
the  kindling  temperature  of  that  substance. 

Exp.  215.  —  Place  a  small  bit  of  phosphorus  and  another  of  sulphur, 
not  in  contact  with  the  first,  upon  a  fragment  of  porcelain  6  or  8  c.  m. 
across,  and  heat  them  slowly  over  the  gas-lamp  ;  the  phosphorus  will 
soon  take  fire  at  a  temperature  of  68°-70°,  but  the  sulphur' will  not 
inflame  until  the  temperature  of  the  porcelain  support  has  risen  to 
about  250°,  as  can  be  readily  ascertained  by  the  thermometer. 

As  was  just  now  said,  the  degree  of  heat  necessary  to  start 
any  fire  must  be  kept  up  continually,  or  the  fire  will  go  out. 
Whenever  burning  bodies  are  cooled  below  the  kindling  tem- 
perature they  are  extinguished,  —  the  chemical  action  which 
occasioned  the  appearance  of  heat  and  light  ceases. 

Exp.  216.  —  Pile  up  upon  an  iron  grate,  thick  in  metal,  and  sup- 
ported in  such  manner  that  air  may  enter  beneath  it,  several  pieces  of 
red-hot  charcoal ;  the  charcoal  will  go  on  burning  until  nearly  all  of  it 
23 


354  FLAMES    PUT    OUT    BY    GOOD    CONDUCTORS. 

has  been  consumed,  for  the  heat  generated  by  the  combustion  of  the 
portions  first  burned  keeps  up  the  temperature  necessary  to  kindle  the 
subsequent  portions. 

Upon  a  cold  grate,  similar  to  the  one  just  employed,  scatter  about 
several  small  pieces  of  red-hot  charcoal,  taking  care  that  no  two  pieces 
of  the  coal  shall  come  in  contact,  or  be  placed  so  as  to  heat  one  an- 
other. Each  of  the  pieces  of  charcoal  will  soon  cease  to  burn,  for 
the  metallic  grate  is  so  good  a  conductor  of  heat  that  it  removes  heat 
from  the  isolated  pieces  of  charcoal  more  rapidly  than  these  can  pro- 
duce it ;  the  temperature  of  the  charcoal  is,  consequently,  soon  reduced 
to  below  the  kindling  point. 

A  result  similar  to  the  foregoing  is  obtained  when  any  fire  is  broken 
up  and  scattered  about  to  such  an  extent  that  its  several  'portions  can- 
not assist  in  one  another's  combustion ;  though  if,  instead  of  being 
placed  upon  iron,  the  separate  glowing  coals  be  laid  upon  ashes  or  dry 
earth,  they  will  be  extinguished  only  after  a  much  longer  time,  for 
ashes  and  dry  earth  are  very  poor  conductors  of  heat  in  comparison 
with  iron. 

In  all  ordinary  fires  the  heat  evolved  by  the  combustion  of 
the  fuel  is  more  than  sufficient  to  maintain  a  temperature  higher 
than  the  kindling  point  of  the  fuel ;  though,  generally  speaking, 
the  fuel  becomes  at  last  so  clogged  with  ashes,  that  the  oxygen 
of  the  air  cannot  get  at  the  remaining  combustible  matter  in 
sufficient  quantity  to  maintain  lively  chemical  action. 

435.  Precisely  as  coals  can  be  extinguished  by  placing  them 
upon  cold  metal,  so  flames  may  be  put  out. 

Exp.  21 7.  —  Upon  a  ring  of  the  iron-stand  place  a  sheet  of  clean 
jvire-gauze  about  10  c.  m.  square;  lower  the  ring  so  that  the  gauze 
shall  be  pressed  down  upon  the  flame  of  a  lamp  or  candle  almost  to  the 
wick,  as  shown  in  Fig.  68.     No  flame  will  be  seen  above  the  gauze,  but 
FIG.  68.  instead  of  flame  a  cloud  of  smoke.     The  gauze  is 

a  mere  open  sieve ;  there  is  nothing  about  it  which 
can  prevent  the  gas,  which  was  just  now  burning 
with  flame  above   the  wick  of  the  candle,  from 
passing  through.     Indeed,  it  may  be  seen  from  the 
smoke  that  the  particles  of  carbon  which,  in  the 
original  undisturbed  flame,  were  becoming  incan- 
descent, and  so  affording  light,  do  now  actually  come  through  the  gauze. 
The  explanation  of  the  phenomenon  is  simply  that   the  metallic 
sieve  conducts  away  so  much  heat  that  the  temperature  of  the  candle- 
flame  is  reduced  to  below  the  kindling  point.     That  this  is  really  so,  is 


SAFETY    LAMPS.  355 

proved  by  the  fact,  that  after  the  gauze  has  become  sufficiently  heated 
by  long-continued  contact  with  the  flame  below,  —  after  it  has  attained 
the  kindling  point  of  the  candle-gas,  —  it  will  no  longer  extinguish  the 
Harne.  In  like  manner,  a  candle-flame  may  be  cooled  to  such  an  extent 
that  it  will  go  out  by  placing  over  it  a  small  coil  of  cold  copper  wire, 
while,  if  the  wire  be  previously  heated,  the  flame  will  continue  to  burn. 

If  the  smoke  and  unburned  gas  which  has  passed  through  the  cold 
wire-gauze  be  touched  with  a  lighted  match,  and  so  brought  to  the 
kindling  temperature,  it  will  burst  into  flame.  x^ 

The  power  of  wire-gauze  to  prevent  the  passage  of  flame  has  been 
usefully  applied  in  several  ways,  notably  for  the  prevention  of  explo- 
sions in  those  coal-mines  which  are  liable  to  accumulations  of  marsh- 
gas.  For  this  purpose  safety-lamps  are  constructed  by  enclosing  an 
ordinary  oil-lamp  completely  in  wire-gauze,  so  that  the  flame  within  the 
gauze  cannot  kindle  any  combustible  or  explosive  gas  into  which  it  may 
be  carried.  In  case  such  a  lamp  be  carried  into  a  place  filled  with 
explosive  gas,  the  latter  will,  of  course,  pass  into  the  lamp  through  the 
meshes  of  the  gauze  and  burn  within  the  cage.  This  combustion  gives 
warning  of  the  presence  of  the  dangerous  gas,  and  indicates  to  the 
workman  that  he  should  withdraw  from  the  locality;  the  gas  can  then 
be  expelled  by  appropriate  methods  of  ventilation. 

The  wire-gauze  lamps,  employed  in  chemical  laboratories  (for 
one  form  of  which  see  Appendix,  Fig.  viii.)  are. simply  applica- 
tions of  the  same  general  idea. 

Exp.  218.  —  Beneath  the  sheet  of  wire-gauze  of  Exp.  217,  place  an 
unlighted  ordinary  gas-lamp  (Bunsen:s  burner),  at  such  distance  that 
the  gauze  shall  be  3  or  4  c.  m.  above  the  top  of  the  lamp ;  turn  on 
the  gas  and  light  it  above  the  wire-gauze ;  it  will  continue  to  burn  on 
top  of  the  gauze  for  an  indefinite  period,  for  the  gauze  will,  in  this 
case,  always  be  kept  cool  by  the  cold  gas  which  is  continually  passing 
through  it.  Carefully  and  gradually  lift  the  ring  which  carries  the 
gauze,  and  determine  how  far  it  is  possible  to  lift  the  gauze  above  the 
gas  jet  without  extinguishing  the  flame. 

The  student  will  remember  that  other  experiments,  illustrating  the 
influence  of  cooling  agencies  in  extinguishing  combustion  (Exps.  140 
and  159),  have  already  been  performed.  Compare,  also,  Exp.  200 
and  §  200,  as  regards  kindling. 

436.    An  effect  somewhat  similar  to  that  produced  by  \vire-| 
gauze  is  often  seen  in  ordinary  fires.     When  a  mass  of  red-hot 
anthracite,  charcoal,  or  coke  is  burning  freely  upon  a  grate  in 
the  open  air,  there  is  always  a  blue  flame  of  carbonic  oxide 


356  FLAMING    FIRES. 

burning  above  the  coal.  This  gas  results  from  the  reduction  of 
carbonic  acid  by  means  of  hot  carbon,  precisely  as  in  Exp.  185. 
Air  enters  at  the  bottom  of  the  grate  and  combines  with  the  hot 
coal  which  it  finds  there  to  form  carbonic  acid  (CO2).  This 
carbonic  acid,  as  it  rises  through  the  hot  coal  in  the  middle  of 
the  fire,  is  deprived  by  the  heated  carbon  of  half  its  oxygen :  — 

C02  +  C  =  2CO, 

so  that  two  molecules  of  carbonic  oxide  gas  finally  emerge  at  the 
top  of  the  coal,  instead  of  the  single  molecule  of  carbonic  acid 
which  was  formed  at  first.  The  carbonic  oxide  being  combus- 
tible, will  at  once  take  fire  on  coming  in  contact  with  the  air, 
provided  the  temperature  at  -the  summit  of  the  fire  be  equal  to 
the  kindling  temperature  of  carbonic  oxide.  But  if  the  tem- 
perature of  the  fire  is  in  any  way  reduced  below  this  point,  as, 
for  example,  by  throwing  on  too  large  a  quantity  of  cold  fuel, 
which  is,  of  course,  equivalent  to  covering  the  fire  with  a  sheet 
of  wire-gauze,  then  the  carbonic  oxide  will  be  extinguished,  and, 
escaping  into  the  chimney,  will  produce  no  useful  effect. 

Were  it  not  for  the  formation  of  carbonic  oxide,  as  above 
mentioned,  neither  anthracite  coal  nor  coke  nor  charcoal  would 
burn  with  fiame  after  having  once  got  well  on  fire ;  they  would 
simply  glow,  as  a  single  live  coal,  or  a  bar  of  metal,  glows  when 
taken  from  the  fire. 

437.  In  heating  steam  boilers  and  other  large  vessels,  it  is 
often  a  point  of  great  importance  to  obtain  from  the  fuel  a  large 
flame,  in  order  that  the  heat  from  the  fuel  may  be  quickly  dis- 
tributed and  brought  into  contact  with  the  matter  to  be  heated. 
With  anthracite  and  coke  this  result  is  effected  by  placing  be- 
neath the  grate,  upon  which  the  fuel  is  burned,  a  quantity  of 
•water.  From  this  water  steam  gradually  rises,  as  hot  ashes  and 
cinders  fall  into  it,  and  as  heat  radiates  down  upon  it  from  the 
fire  above.  The  steam,  as  it  enters  the  fire,  is  decomposed  by 
the  hot  coal  (see  Exp.  161),  in  accordance  with  the  following 
reaction :  — 

C  -f  H20  =  CO  +  2H, 
and  the  combustible  gases  thus  obtained  are  superadded  to  the 


CARBONIC  OXIDE  SHOULD  BE  BURNED.        357 

carbonic  oxide  which  is  formed  in  due  course  from  the  action  of 
air  upon  the  coal.  All  these  gases  burn  again  to  carbonic  acid 
and  water  above  the  fire  where  air  is  thrown  in  to  meet  them 
through  appropriate  orifices.  In  this  use  of  water,  as  an  adjunct 
to  the  combustion  of  coal,  the  absolute  amount  of  heat  given  off 
by  the  fuel  is  in  nowise  increased ;  but  in  many  instances  much 
heat  may  undoubtedly  be  saved  by  thus  equally  distributing  and 
applying  it  by  means  of  flame. 

438.  On  the  other  hand,  furnaces  are  sometimes  seen  con- 
suming fuel  under  such  conditions  that  all  the  carbonic  oxide 
produced  within  them  escapes  unburned  into  the  chimney.  In 
such  cases,  more  than  two-thirds  of  the  amount  of  heat  which 
the  fuel  is  capable  of  yielding  must  necessarily  be  lost ;  for  while 
1  gramme  of  charcoal  gives  off  in  burning  to  carbonic  acid 
8080  units  of  heat  (§  55),  1  gramme  of  carbon  in  burning  to 
carbonic  oxide  gives  off  only  2473  units  of  heat.  The  number 
last  given  is  determined  as  follows.  It  has  been  found,  by  direct 
experiment,  that  1  gramme  of  carbonic  oxide,  on  being  burned 
to  carbonic  acid,  yields  2403  units  of  heat ;  carbonic  oxide  is 
composed  (§  422)  of  one  atom  of  carbon,  weighing  12,  and  one 
atom  of  oxygen  weighing  16,  —  the  weight  of  the.molecule  of  car- 
bonic oxide  being  consequently  28.  In  one  gramme  of  carbonic 
oxide,  therefore,  there  can  be  only  ^|  •=.  0.4286  of  a  gramme 
of  carbon;  but  0.4286  :  1  =  2403:  x  =  5607,  whence  it  ap- 
pears that  there  is  evolved  by  one  gramme  of  carbon  in  car- 
bonic oxide  5607  units  of  heat  when  this  carbon  unites  with  the 
additional  oxygen  to  form  carbonic  acid ;  and  the  difference  be- 
tween this  number,  5607,  and  the  number  (8080)  denoting  the 
amount  of  heat  given  off  by  one  gramme  of  charcoal  in  burning, 
to  carbonic  acid,  will  show  how  much  heat  is  evolved  by  one 
gramme  of  carbon  burned  to  carbonic  oxide :  8080  —  5607  = 
2473,  as  above  stated. 

In  order  to  thoroughly  burn  the  carbonic  oxide  in  any  case, 
the  stove  or  furnace  should  be  so  arranged  that  a  volume  of  air, 
as  large  as  that  which  has  already  passed  through  the  fire,  can 
be  constantly  supplied  to  the  carbonic  oxide  and  nitrogen  as  they 
emerge  from  the  coal,  and  be  intimately  mixed  with  these  gases 
while  they  are  still  hot. 


358  CHLORIDE    OF    CARBON. 

439.  The  amount  of  air  needed  for  the  complete  combustion 
of  coal  or  other  fuel  can  always  be  readily  calculated.     We  have 
only  to  determine  how  much  oxygen  will  be  needed  by  the  com- 
bustible, and  then  how  much  air  must  be  taken,  in  order  to  sup- 
ply this  oxygen.     Let  it  be  supposed,  for  example,  that  we  wish 
to  learn  how  much  air  is  needed,  in  order  to  burn  one  kilogramme 
of  charcoal.     Having  learned  the  full  significance  of  the  formula 
CO2 ,  a  moment's  consideration  of  this  formula  informs  us  that, 
for  every  12  parts  by  weight  of  carbon,  32  parts  by  weight  of 
oxygen  are  needed,  in  order  to  its  complete  combustion,  or,  for 
pne  part  of  carbon,  2.67  parts  of  oxygen.     Air  contains  23.1  per 
cent,  of  oxygen  by  weight;  hence  the  proportion  23.1 :  100  = 
2.67:  x=  11.558,  from  which  it  appears  that  to  burn  1  kilo, 
of  charcoal,  11.558  kilos,  of  air  are  needed.     Since  the  weight 
of  a  litre   of  air,  at  the  ordinary  temperature,  is  only  1.2258 
grms.,  these  11.558  kilos,  of  air  will  occupy  about  9429  litres,  or, 
in  other  words,  nearly  9^  cubic  metres.     In  round  numbers,  it 
may  be  said  that  about  12  kilos.,  or  9|  cubic  metres,  of  air  are 
required  to  burn  one  kilo,  of  charcoal.     For  coke  and  anthra- 
cite, corrections  must,  of  course,  be  made  for  the  ashes  which 
they  contain,  as  well  as  for  a  certain  portion  of  hydrogen  which 
may  also  be  present.     If  a  gramme  of  pure  carbon  will  disen- 
gage 8080  units  of  heat,  a  gramme  of  well-burned  coke,  contain- 
ing 15  per  cent,  of  ashes,  will  disengage  only  6868  units. 

440.  Chloride  of  Carbon   (CC14).     Chlorine  does  not  unite 
directly  with  carbon,  but  several  compounds  of  the  two  elements 
can  be  obtained  by  subjecting  compounds  of  carbon  and  hydro- 
gen to  the  action  of  chlorine.     Of  these  compounds,  only  the  so- 
called  bichloride  (CC14)  need  here  be  mentioned,  the  others  being 
usually  treated  of  in  works  upon  organic  chemistry.     Bichloride 
of  carbon  may  be  obtained  by  the  action  of  chlorine  on  marsh- 
gas,  by  subjecting  chloroform  or  wood-spirit  to  the  action  of  an 
excess  of  chlorine  in  sunlight,  by  passing  a  mixture  of  bisulphide 
of  carbon  vapor  and  chlorine  through  a  red-hot  porcelain  tube, 
or  by  the  action  of  quinquichloride  of  antimony  upon  bisulphide 
of  carbon :  — 

CS2  +  2SbCl5  =  CC14  4-  2SbCl3  +  2S . 
At  the  ordinary  temperature  of  the  air  it  is  a  transparent,  color- 


CARBON   AND    NITROGEN.  359 

less  liquid,  of  pungent  aromatic  odor,  boiling  at  77°,  and  having 
a  specific  gravity  of  1.56.  At  — 23°  it  solidifies  in  the  form  of 
crystals  of  pearly  lustre.  The  specific  gravity  of  its  vapor  has 
been  determined  to  be  76.96,  which  would  indicate  that  a  molecule 
of  the  vapor  is  composed  of  1  atom  of  carbon  and  4  volumes  of 
chlorine,  condensed  to  2  unit-volumes :  — 

For,  since  the  weight  of  one  atom  of  carbon  is  V  .  .  12 
And  the  weight  of  4  atoms  of  chlorine  is  ' :  '  .  «•  .  142 

The  weight  of  the  2  volumes  of  gas  produced  would  be     .        .       154 

And  the  weight  of  1  volume  would  be  77  ;  a  number  with  which  the 
experimental  determination  very  nearly  agrees. 

In  composition,  this  chloride  of  carbon  (CC14)  is  analogous  to 
the  hydride  CH4  which  we  have  already  studied  under  the  name 
light  carburetted  hydrogen,  or  marsh-gas  (§  392),  and  in  the 
same  way  that  the  hydride  may  be  converted  into  the  chloride 
by  acting  upon  it  with  chlorine,  so,  conversely,  the  chloride,  on 
being  brought  into  contact  with  water  and  sodium-amalgam 
(§  97),  may  be  deprived  of  chlorine  and  converted  back  into  the 
hydride,  —  the  hydrogen  of  the  water  being  substituted  for  the 
chlorine,  which,  like  the  oxygen  of  the  water,  unites  with  the 
sodium. 

441.  Compounds  of  Carbon  and  Nitrogen.  With  nitrogen, 
carbon  forms  a  number  of  highly  interesting  compounds,  which 
may  be  found  treated  of  in  works  upon  organic  chemistry. 
Prominent  among  these  compounds  are  cyanogen,  CN  (§  384), 
and  cyanhydric  acid,  HCN.  Cyanhydric  acid  corresponds  to 
chlorhydric  acid  and  the  other  hydrides  of  the  chlorine  group  of 
elements  ;  by  its  action  upon  metallic  oxides  there  may  be  formed 
a  series  of  cyanides  of  the  metals  corresponding  perfectly  with 
the  metallic  chlorides  :  — 

M2O  +  2H(CN)  —  2M(CN)  +  H2O. 

M3O  +  2HC1        =  2MC1        +  H,0  . 

The  group  of  atoms  (CN)  which  constitutes  cyanogen,  acts,  in 
fact,  as  if  it  were  a  single  element.  In  the  same  way  that  the 
group  NH4 ,  called  ammonium,  is  capable  of  replacing  a  metal 
like  sodium  (see  §  91),  so  the  group  CN  can  replace  chlorine  and 


360  BISULPHIDE    OF    CARBON. 

the  elements  allied  to  chlorine.  Groups  or  knots  of  atoms,  such 
as  these,  are  often  called  compound  radicals.  Cyanhydric  acid, 
or  prussic  acid  as  it  is  sometimes  called,  is  notorious  as  a  violent 
poison.  Many  of  the  cyanides  are  of  great  importance  in  the 
arts,  particularly  in  processes  of  gilding,  plating,  electrotyping, 
and  dyeing,  as  well  as  in  the  preparation  of  pigments.  Some  of 
them  will  be  described  hereafter  under  the  respective  metals. 

442.  Bisulphide  of  Carbon  (CS2),  or  sulpho-carbonic  acid,  as 
it  is  often  called,  is  especially  interesting  from  its  correspondence 
with  the  binoxide  of  carbon,  carbonic  acid,  in  accordance  with 
the  general  rule  that  sulphur  compounds  are  analogous  to  the 
compounds  of  oxygen.  With  the  metallic  sulphides  this  com- 
pound forms  a  series  of  salts,  of  the  general  formula  M2CS3 ,  or 
M2S,CS2 ,  analogous  to  those  formed  by  the  union  of  carbonic 
acid  'with  the  metallic  oxides,  the  general  formula  of  which,  as 
we  know,  is  M2C03  or  M2O,CO2. 

Bisulphide  of  carbon  may  be  obtained  by  bringing  the  vapor 
of  sulphur  in  contact  with\  red-hot  charcoal.  It  is  a  mobile, 
colorless  liquid  of  1.27  specific  gravity,  which  refracts  light 
powerfully.  It  boils  at  45°  and  evaporates  rapidly  at  the  ordi- 
nary temperature  of  the  air.  The  density  of  its  vapor  is  38.19. 
It  has  a  peculiarly  fetid  odor,  and  is  very  easily  inflammable, 
burning  with  a  blue  flame  to  carbonic  and  sulphurous  acids.  It  is 
not  soluble  in  water,  but  is  easily  soluble  in  alcohol,  and  is  itself 
a  powerful  solvent  of  fats  and  various  other  substances  ;  it  has 
been,  of  late  years,  somewhat  extensively  employed  as  a  solvent 
of  phosphorus  (§  274)  and  of  chloride  of  sulphur  (§  246)  in  the 
cold  process  of  vulcanizing  caoutchouc.  Mixtures  of  its  vapor 
with  oxygen,  air,  nitric  oxide,  and  other  gaseous  oxygen  com- 
pounds burn,  without  violent  explosion,  with  a  sudden  brilliant 
flash  of  intensely  blue  light.  This  flame  is  remarkable  for  its 
great  actinic  power ;  it  acts  very  energetically  upon  a  prepared 
daguerreotype  plate,  and  causes  a  mixture  of  hydrogen  and 
vchlorine  to  combine  almost  as  readily  as  sunlight  would  (Exp.  56). 

Exp.  219.  —  Fill  a  tall  bottle,  of  the  capacity  of  700  or  800  c.  c.,  with 
nitric  oxide  gas,  at  the  water-pan  ;  cover  the  bottle  with  a  plate  of  glass 
and  stand  it  upright  upon  the  table ;  draw  the  cover  aside  far  enough 
to  admit  of  the  introduction,  from  a  pipette,  of  8  or  10  c.  c.  of  liquid 


BORON.  361 

bisulphide  of  carbon  ;  replace  the  cover  and  leave  the  bottle  at  rest 
for  a  few  minutes,  in  order  that  the  vapor  of  the  bisulphide  may  have 
time  to  diffuse  through  the  nitric  oxide.  Finally,  touch  a  lighted  match 
to  the  opened  mouth  of  the  bottle  and  observe  the  brilliant  flame  which 
is  produced. 


CHAPTER    XXI. 

BORON. 

443.  Boron  is  an  element  of  the  same  natural  family  as  car- 
bon. It  is  found  in  nature,  in  combination  with  oxygen,  as 
boracic  ac  id,  and  in  combination  with  oxygen  and  some  of  the 
metals,  notably  as  a  biborate  of  sodium,  commonly  called  borax, 
and  as  a  double  borate  of  sodium  and  of  calcium. 

In  certain  volcanic  districts  in  Tuscany,  jets  of  steam  mixed 
with  other  vapors  escape  continually  from  cracks  in  the  soil,  and 
bring  to  the  surface  small  quantities  of  boracic  acid.  Since 
boracic  acid  is  not  volatile,  in  the  ordinary  sense  of  the  term,  at 
temperatures  so  low  as  100°,  it  appears  that  it  is  transported 
mechanically  by  the  steam,  much  in  the  same  way  that  dust  is 
carried  along  by  a  current  of  air.  The  jets  of  vapor,  laden  with 
boracic  acid,  are  made  to  bubble  through  water  as  they  escape 
from  the  earth ;  this  water  retains  the  acid,  and  so  concentrates 
it  to  a  very  considerable  extent.  After  the  water  has  become 
as  highly  charged  with  the  acid  as  has  been  found  in  practice  to 
be  desirable,  it  is  run  off  into  pans,  lower  down  upon  the  hill- 
side, beneath  which  hot  jets  of  vapor  from  the  earth  are  caused 
to  circulate.  The  excess  of  water  is  thus  evaporated  by  heat 
which  the  earth  supplies,  and  the  solution  becomes  so  concen- 
trated that,  on  cooling,  crystals  of  boracic  acid  separate  from 
it.  About  120  millions  of  kilogrammes  of  water  are  thus  evap- 
orated an  IIP  ally,  and  1,300,000  kilogrammes  of  boracic  acid 
produced  without  the  intervention  of  any  artificial  motor  or  the 
consumption  of  any  fuel.  After  having  been  purified,  the 
boracic  acid  is  sent  into  commerce  as  such,  or  it  is  treated 


362  ALLOTROPISM    OP   BORON. 

with  a  hot  solution  of  carbonate  of  sodium,  and  so  converted  into 
borax. 

Besides  this  principal  source  of  the  boron  compounds,  a  cer- 
tain quantity  of  native  borax  is  obtained  from  the  mud  and 
waters  of  certain  lakes  in  Tartary,  Ceylon,  Thibet,  and  Cali- 
fornia. In  Peru;  also,  a  mineral  composed  of  borate  of  sodium 
and  borate  of  calcium  is  found  associated  with  nitrate  of  sodium. 

444.  The  element  boron,  like  carbon,  has  been  obtained  in 
three  distinct  allotropic  conditions  (§§  162,  366).     It  can  be  had 
amorphous  like  charcoal,  crystallized  like  the  diamond,  and  in  a 
condition  like  that  of  graphite. 

To  prepare  the  diamond-like  modification,  fused  boracic  acid  is  in- 
tensely heated  with  metallic  aluminum ;  a  portion  of  the  aluminum 
takes  oxygen  away  from  the  boron,  while  another  portion  of  the  molten 
metal  dissolves  this  boron  as  fast  as  it  is  formed.  As  soon  as  the  solu- 
tion has  become  saturated,  that  is  to  say,  when  the  melted  aluminum 
has  dissolved  all  the  boron  that  it  is  capable  of  dissolving,  a  portion  of 
this  boron  is  deposited  in  diamond-like  crystals.  When  the  crucible 
and  its  contents  are  broken  up  after  cooling,  these  crystals  are  found 
lining  cavities  within  the  mass. 

Graphitoidal  boron  is  obtained  by  fusing  a  compound  known  as  fluo- 
borate  of  potassium  with  aluminum,  as  before.  When  the  crucible  is 
broken,  there  is  found  a  compact  metallic  mass  containing  both  aluminum 
and  boron ;  by  treating  this  mass  with  chlorhydric  acid,  the  aluminum 
is  dissolved,  while  boron  is  left  in  the  form  of  hexagonal  plates  or  scales. 

The  amorphous  modification  may  be  prepared  by  heating  together 
boracic  acid  and  metallic  sodium  or  potassium  beneath  a  layer  of  fused 
chloride  of  sodium.  After  having  been  allowed  to  cool,  the  products 
of  the  reaction  are  treated  with  water,  which  dissolves  away  the  chlo- 
ride and  the  borate  of  sodium,  and  leaves  the  boron  as  an  amorphous 
powder  which  may  be  collected  upon  a  filter. 

445.  Of  the  properties  of  these  varieties  of  boron,  little  need 
here  be  said  ;  they  are  all  analogous  to,  and  closely  resemble,  the 
corresponding  modifications  of  carbon,  which  have  already  been 
fully  described.     The  diamond  modification  is  transparent,  and 
sometimes  colorless,  though  usually  of  a  yellow  or  reddish  color. 
It  crystallizes  in  the  same  forms  as  the  real  diamond,  and  refracts 
light  very  powerfully.     Its  specific  gravity  is  2.68.     It  is  almost 
as  hard  as  the  real  diamond,  being  capable   of  scratching  the 
ruby,  and  even  of  polishing  the  diamond.     It  is  only  with  ex- 


COMPOSITION    OF   BORACIC    ACID.  363 

treme  difficulty  that  it  can  be  burned  in  oxygen,  since  a  coating 
of  boracic  acid  soon  forms  which  protects  it  from  further  action 
of  the  oxygen.  It  is  remarkable  that  at  the  moment  of  its  com- 
bustion it  swells  up  as  the  diamond  does  when  intensely  heated. 
The  second  variety  of  boron  crystallizes  in  soft,  hexagonal  plates, 
having  the  same  lustre  as  graphite,  but  differing  from  graphite 
in  that  they  are  of  a  reddish  tint.  It  is  acted  upon  by  oxygen 
only  at  very  high  temperatures.  Amorphous  boron  is  an  infusi- 
ble greenish  powder,  which  readily  takes  fire  on  being  heated  in 
air  or  oxygen.  It  is,  in  fact,  necessary,  in  preparing  it,  to  take 
care  that  the  filters  upon  which  it  has  been  collected  shall  be 
dried  at  low  temperatures,  lest  the  finely-divided  boron  take  fire 
spontaneously ;  if  exposed  to  the  sun's  rays  in  summer,  the  filters 
containing  boron  will  often  take  fire  as  soon  as  they  have  become 
dry.  Unlike  the  other  modifications  of  boron,  it  is  readily  at- 
tacked by  most  chemical  agents.  Like  the  corresponding  modi- 
fication of  carbon,  it  is  an  energetic  reducing  agent. 

No  compound  of  boron  and  hydrogen  has  yet  been  discovered, 
but  it  unites  readily  with  chlorine,  bromine,  and  iodine  ;  it  can 
be  made  to  combine  also  with  fluorine,  nitrogen,  and  sulphur. 

446.  The  best  known  of  the  compounds  of  boron  is  the  oxide 
B203 ,  called  boracic  acid.  This  is  found  ready  formed  in  nature, 
as  has  been  said,  and  is  the  sole  product  when  boron  is  burned 
in  oxygen.  The  composition  of  boracic  acid  may  be  determined 
synthetically  by  burning  amorphous  boron  in  pure  oxygen,  or  by 
treating  it  with  nitric  acid.  A  certain  definite  weight  of  boron 
being  taken  in  the  first  instance,  the  weight  of  the  dry  boracic 
acid  obtained  is  carefully  determined,  and  from  these  data  the 
percentage  composition  of  the  boracic  acid  is  calculated ;  100 
grammes  of  boracic  acid  consist  of 

Oxygen      .         .         .         .         .         ,         .68.78  grammes. 
Boron 31.32        " 

From  the  specific  gravities  of  the  vapors  of  chloride  of  boron 
and  of  fluoride  of  boron,  as  determined  by  experiment,  and  from 
some  other  rather  inconclusive  considerations  which  need  not  here 
be  dwelt  upon,  chemists  have  been  led  to  admit  that  the  atomic 
composition  of  boracic  acid  may  be  represented  by  the  formula 
B2O3 .  If  boracic  acid  be  really  composed  of  3  atoms  of  oxygen 


364  PROPERTIES    OF    BORACIC    ACID. 

and  2  atoms  of  boron,  the  weight  of  the  atom  of  boron  will  follow 
from  the  proportion  68.78  :  31.32  =  (16  X  3)  :  x,  in  which  16 
equals  the  weight  of  an  atom  of  oxygen  and  x  the  weight  of  two 
atoms  of  boron.  Upon  this  assumption,  the  weight  of  one  atom 
of  boron  will  be  10.93. 

447.  Boracic  acid  is  but  a  feeble  acid  at  the  ordinary  tem- 
perature, and  may  be  set  free  from  its  compounds  by  almost  any 
of  the  acids,  excepting  carbonic  acid. 

Exp.  221.  —  Dissolve  4  gratis,  of  borax  in  10  grms.  of  boiling  water, 
in  a  beaker-glass  or  porcelain  capsule  of  30  or  40  c.  c.  capacity, 
and  add  to  the  solution  2.5  grms. '  of  concentrated  chlorhydric  acid. 
After  the  lapse  of  some  time,  hydrated  boracic  acid  will  be  deposited 
from  the  solution  in  the  form  of  glistening,  colorless  plates  or  'scales. 
These  crystals  contain  as  much  as  43.6  per  cent,  of  water ;  their  for- 
mula is  H3BO3,  or,  dualistic,  3H2O,  B2O3. 

On  being  heated  in  a  clean  iron-spoon,  the  crystals  will  first  dissolve 
in  the  water  which  they  contain,  or,  as  the  fact  is  usually  stated,  they 
will  "  melt  in  their  water  of  crystallization";  if  the  heat  be  continued, 
the  mass  will  become  pasty,  and  will  swell  up  as  the  water  is  expelled. 
After  all  the  water  has  been  driven  off  by  strong  heat,  the  anhydrous 
acid  is  left  as  a  clear,  viscous  liquid,  from  which  long  threads  of  the 
solid  acid  may  be  drawn  out  by  touching  to  the  surface  of  the  liquid 
the  end  of  a  stick  or  glass  rod,  and  then  gently  pulling  away  the  stick 
with  the  matter  which  has  adhered  to  it. 

If  the  fused  acid  be  allowed  to  cool,  it  will  solidify  to  a  hard,  trans- 
parent glass,  which  soon  cracks  in  every  direction  and  splits  up  into 
fragments. 

Anhydrous  boracic  acid  is  of  about  1 .8  specific  gravity  ;  it  is 
odorless  and  destitute  of  corrosive  power ;  it  has  a  slightly  bit- 
ter, but  not  sour,  taste.  It  is  much  more  soluble  in  hot  than  in 
cold  water,  and  more  soluble  in  alcohol  than  in  water.  It  im- 
parts to  the  flame  of  burning  alcohol  a  peculiar  green  tint,  which 
is  quite  characteristic,  and  affords  a  valuable  test  by  which  the 
presence  of  the  acid  may  be  detected.  Upon  litmus  and  tur- 
meric, boracic  acid  acts  somewhat  differently  from  other  acids. 

Exp.  222.  —  Dissolve  a  little  of  the  crystallized  boracic  acid  of  Exp. 
221,  in  a  teaspoonful  of  alcohol  in  a  small  porcelain  capsule.  Set  fire 
to  the  alcohol  and  stir  the  burning  solution  with  a  rod,  or  agitate  it  by 
jarring  the  dish.  Or  moisten  a  tuft  of  cotton  with  alcohol,  strew  upon 
it  some  powdered  boracic  acid,  and  light  the  alcohol.  In  either  case 
the  flame  of  the  alcohol  will  be  of  a  fine  green  color. 


CHLORIDE    OF    BORON.  365 

Exp.  223. —  Pour  into  a  test-glass  20  or  30  c.  c.  of  a  solution  of 
blue  litmus ;  in  a  small  quantity  of  water,  contained  in  another  test- 
glass  or  tube,  dissolve  a  little  of  the  boracic  acid  of  Exp.  221 ;  add  the 
solution  of  boracic  acid  to  the  litmus,  and  observe  that  the  color  of 
the  latter  changes  to  a  brownish,  wine-red,  decidedly  different  from  the 
bright,  clear  red,  which  is  obtained  by  the  action  of  other  acids  upon 
litmus.  If  a  large  quantity  of  boracic  acid,  however,  be  added  to  a 
small  portion  of  the  litmus  solution,  the  latter  will  be  colored  strongly, 
as  if  by  a  powerful  acid. 

Exp.  224.  —  Dip  into  a  solution  of  boracic  acid  a  slip  of  yellow  tur- 
meric paper,  and  observe  that  the  yellow  color  is  changed  to  brown,  as 
it  would  be  by  ammonia-water,  or  by  any  other  alkaline  solution. 
None  of  the  other  acids  produce  a  like  effect. 

448.  When  an  aqueous  solution  of  boracic  acid  is  boiled,  an 
appreciable  quantity  of  the  acid  goes  off  with  the  vapor  of  water, 
but  the  dry  acid,  wnen  heated  by  itself,  is  nevertheless  one  of 
the  least  volatile  of  all  the  acids.     It  does  slowly  sublime,  how- 
ever, at  a  white  heat,  and  may  be  completely  evaporated,  if  left 
for  a  long  time,  in  the  hottest  part  of  a  porcelain  furnace.     As 
a  consequence  of  this  fixity,  or  lack  of  volatility,  it  follows  that 
boracic  acid  is  a  comparatively  powerful  acid  at  temperatures 
high  enough  to  volatilize  the  ordinary  acids.     On  being  heated 
with  nitrates,  or  sulphates,  for  example,  it  quickly  expels  nitric 
or  sulphuric  acid,  and  unites  with  the  other  ingredients  of  the 
salt;  though  either  of  these  acids  would  at  once  decompose  the 
borate  thus  formed,  if  they  were  collected  and  added  to  it  at  the 
ordinary  temperature.     Even  phosphoric  acid  is  expelled  by  it 
from  the  phosphates. 

449.  Chloride  of  Boron  (BC13),  is  a  colorless,  mobile  liquid, 
of  1.35  specific  gravity,  which  boils  at  17°.     The  specific  grav- 
ity of  its  vapor  has  been  found  to  be  58.78,  a  result  which  points 
directly  to  the  formula  BC13,  as  representing  the  true  composi- 
tion of  the  compound. 

From  the  weight  of  2  vols.  of  chloride  of  boron  (58.78  X  2)    .  11 7.56 
Subtract  the  weight  of  3  vols.  of  chlorine  (35.5  X  3)       .         .  106.50 

and  the  remainder  will  be         .......     11.06 

a  number  almost  precisely  equal  to  the  weight  of  one  atom  of  boron, 
as  previously  determined  (§  446). 


366  FLUORIDE    OF    BORON. 

Upon  being  mixed  with  water,  chloride  of  boron  decomposes 
with  formation  of  boracic  and  chlorhydric  acids  :  — 

2BC13  +  3H20  =  B2O3  +  6HC1. 

Chloride  of  boron  may  be  prepared  by  slightly  heating  amor- 
phous boron  in  an  atmosphere  of  chlorine,  or  more  readily,  by 
passing  a,  current  of  chlorine  over  a  mixture  of  anhydrous 
boracic  acid  and  charcoal,  heated  to  redness  in  a  porcelain  tube. 
In  presence  of  the  hot  charcoal  which  stands  ready  to  take  oxy- 
gen from  the  boracic  acid,  chlorine  can  take  boron  away  from 
boracic  acid ;  and,  conversely,  in  presence  of  the  chlorine,  ready 
to  combine  with  the  boron,  carbon  can  take  away  oxygen :  — 

BA  +  3C  +  6C1  =  2BC13  +  SCO. 

The  method  here  described  of  converting  an  oxide  into  a  chlo- 
ride, through  the  intervention  of  carbon,  is  a  method  of  very 
general  applicability,  and  is  often  employed  for  the  preparation 
of  chlorides  of  the  metals. 

450.  Fluoride  of  Boron  (BF13),  is  a  colorless  gas  of  34.19 
specific  gravity,  as  determined  by  experiment.     Upon  the  as- 
sumption that  an  atom  of  boron  weighs   11,  its  specific  gravity 
would  be  (11  +  3   X   19)   -5-  2  =  34.     jt  fumes  strongly  in 
damp  air,  and,  by  pressure,  may  be  readily  condensed  to  a  color- 
less and  very  mobile  liquid.     The  gas  is  exceedingly  caustic  and 
corrosive ;  it  carbonizes  and  destroys  wood  and  other  organic 
substances,  in  the  same  way  as  concentrated  sulphuric  acid.     As 
with  the  sulphuric  acid  (Exp.  108),  so  here  the  fluoride  of  boron 
unites  with  the  elements  of  water  which  are  contained  in  the 
organic  matter,  and  the  integrity  of  the  latter  is  destroyed. 

451.  Fluoride  of  boron  is  absorbed  by  water  rapidly  and  in 
large  quantity,  1  volume  of  water  being  capable  of  dissolving 
700  or  800  volumes  of  the  gas;  but,  in  the  act  of  solution,  de- 
composition occurs  as  well,  and  there  is  obtained,  not  a  simple 
solution  of  fluoride  of  boron  in  water,  but  a  mixture,  or,  rather 
a  compound,  of  fluorhydric  and  boracic  acids  :  — 

2BF13  +  3H20  =  B2O3,6HF1. 

The  reaction  is  interesting  in  all  its  stages,  inasmuch  as  it  well 
illustrates  the  vagueness  and  indefiniteness  of  a  considerable 


FLUOBORIO    ACID.  367 

class  of  chemical  reactions.  When  water  dissolves  fluoride  of 
boron,  it  increases  in  bulk  to  a  considerable  extent,  and  in 
density  also,  its  specific  gravity  rising  as  high  as  1.77.  Upon 
warming  the  saturated  solution,  some  fluoride  of  boron  is  a^ain 
disengaged,  perhaps  as  much  as  one-fifth  of  all  that  had  been 
absorbed,  but,  on  continuing  to  heat  the  solution,  it  distils  over 
unchanged,  and  the  condensed  liquid  presents  the  appearance  of 
oil  of  vitriol.  In  it  the  elements  of  boracic  and  fluorhydric  acids 
are  undoubtedly  held  together  in  a  loose  condition  of  chemical 
combination.  By  many  chemists,  the  compound  is  called  fluo- 
boric  acid,  though,  in  order  to  avoid  confusion,  it  would,  perhaps, 
be  better  if  the  name,  fluorhydrate  of  boracic  acid,  were  allotted 
to  it;  for  when  this  compound  is  largely  diluted  with  water, 
boracic  acid  is  deposited,  and  another  acid  compound  is  left  in 
solution,  the  composition  of  which  may  be  represented  by  the 
formula  HF1,BF13 .  This  new  acid  is  called  indifferently  fluo- 
boric  acid  or  fluorborhydric  acid,  and  the  salts  formed  by  its  union 
with  metals  are  called  fluoborates.  It  is  remarkable  that  the 
first  named  compound,  the  fluorhydrate  of  boracic  acid,  upon 
being  neutralized  with  alkalies,  yields,  not  mixtures  of  a  borate 
and  a  fluoride  of  the  alkali  employed,  but  true  chemical  com- 
pounds, double  salts,  of  the  general  formula  M/),B203 ;  6MF1, 
whence  the  name  fluoboric  acid  has  taken  rise.  The  best  way 
of  preparing  the  fluorhydrate  of  boracic  acid  is  to  dissolve  boracic 
acid  by  small  portions  in  fluorhydric  acid. 

Fluoride  of  boron  may  be  itself  prepared  by  heating  in  a 
glass  flask  a  mixture  of  1  part  of  fused  boracic  acid,  2  parts  of 
powdered  fluorspar,  and  10  or  12  parts  of  concentrated  sulphuric 
acid :  — 

3CaFl2  +  B2O3  +  3H2SO4  =  3CaS04  +  311,0  +  2BFJ,. 

On  account  of  its  easy  solubility  in  water,  the  gas  must  be  col- 
lected over  mercury.  Fluoride  of  boron  may  be  employed  as 
a  test  to  determine  whether  or  no  a  given  sample  of  any  gas  is 
completely  dry ;  if  a  few  bubbles  of  it  are  added  to  the  gas  to 
be  tested,  the  slightest  trace  of  moisture  in  the  gas  will  be  made 
manifest  by  the  appearance  of  white  fumes  of  the  fluorhydrate 
of  boracic  acid  above  described. 


368  BORON    LIKE    AND    UNLIKE    CARBON. 

452.  Sulphide  of  Boron  (B2SS)  is  a  white,  crystalline  solid, 
decomposable   by  water,  in  accordance  with   the  following  for- 
mula :  — 

B2S3  +  3H2O  =  B2O3  +  3H2S. 

It  may  be  prepared  by  passing  a  current  of  sulphide  of  carbon 
vapor  over  a  mixture  of  boracic  acid  and  charcoal  strongly 
heated  in  a  porcelain  tube. 

453.  Nitride  of  Boron  (BN)  is  a  soft,  white  amorphous  solid, 
tasteless,  odorless,  infusible,  and  non-volatile.     It  is,  in  general, 
but  little  acted  upon  by  chemical  agents. 

454.  It  will  be  remarked,  that  while  boron  is   closely  analo- 
gous to  carbon  in  many  respects,  it  differs  from  it  decidedly  in 
others.     Thus,  while  in  their  several  allotropic  modifications  the 
two  elements  are  almost  precisely  alike  in  appearance  and  prop- 
erties, and  while  many  of  the  salts  of  boracic  acid  are  strikingly 
similar  to  the  corresponding  carbonates,  the  compounds  of  boron 
are  not  comparable  as  regards  their  composition  with  the  com- 
pounds of  carbon.     While  one  atom  of  carbon  unites  by  prefer- 
ence with  four  atoms  of  any  member   of  the    chlorine  group 
(as  in  CC14),  or  with  two  atoms  of  any  member  of  the  sulphur 
group  (as  in  CO2),  an  atom  of  boron  unites  with  only  three 
atoms  of  chlorine  (BC13),  or  two  atoms  of  it  unite  with  three 
atoms   of  oxygen  or  sulphur   (as  in   B2O3).     The   exceptional 
character  of  the  composition  of  boron  compounds  will  appear 
still  more  clearly  in  the  next  chapter,  where  it  will  be  shown 
that  silicon,  the  third  member  of  the  carbon  group,  resembles  car- 
bon as  regards  the  atomic  composition  of  its  compounds,  as  well 
as  in  other  respects.     The  atomic  weight  of  boron  above  given 
(§  446)  cannot  be  accepted  as  established  beyond  a  doubt,  and  it 
may  happen  that  further  investigation  will  show  that  the  boron 
compounds  are  really  formed  upon  the  same  type  or  pattern  as 
those  of  carbon  and  silicon ;  but  in  face  of  the  experimental  evi- 
dence now  at  hand,  this   view  cannot  be  maintained.     In  the 
mean  time,  the  student  will  better  understand  the  physical  and 
chemical  properties  of  boron  and  its  compounds,  if  this  element 
is  studied  in  company  with  carbon  and  silicon,  which  it  so  closely 
resembles,  than  if  it  were  described  in  connection  with  arsenic 


SILICON.  369 

antimony,  and  the  other  elements  of  the  nitrogen  group,  which 
form  teroxides  and  terchlorides  indeed^  but  which  present  not 
the  least  other  analogy  to  boron,  either  in  the  simple  or  the 
compounded  condition. 


CHAPTER    XXII, 

SILICON. 

455.  Like  the  other  members  of  the  carbon  group  of  elements, 
silicon  may  be  obtained  in  the  three  distinct  allotropic  conditions 
which  have  been  designated  as  amorphous,  diamond-like,  and 
graphitoidal.  As  it  occurs  in  the  modification  last  named,  it  so 
closely  resembles  the  corresponding  modification  of  boron  that, 
were  it  not  for  a  difference  of  hardness,  it  would  be  difficult  to 
distinguish  the  one  element  from  the  other  without  resorting  to 
actual  analysis.  After  oxygen,  silicon  is  the  most  abundant  and 
widely  diffused  of  all  the  chemical  elements ;  at  least  one  quarter 
of  the  solid  crust  of  the  earth  is  composed  of  it.  In  combina- 
tion with  oxygen,  it  occurs  in  silicic  acid,  which  is  one  of  the 
commonest  substances  upon  the  surface  of  the  globe.  Common 
quartz  and  flint,  as  well  as  rock-crystal,  agate,  and  the  like,  are 
pure  silicic  acid.  The  yellow  sand  of  sea-beaches  and  of  many 
sterile  tracts  of  country  is  silicic  acid  contaminated  with  a  trace 
of  oxide  of  jron.  In  like  manner,  sandstones  and  a  great  variety 
of  other  rocks  are  mainly  composed  of  it. 

In  order  to  obtain  pure  silicon,  the  following  methods  may  be  em- 
ployed :  When  metallic  potassium  is  heated  with  a  double  compound 
of  fluoride  of  potassium  and  fluoride  of  silicon,  known  as  fluosilioate  of 
potassium,  a  violent  reaction  occurs,  fluoride  of  potassium  is  formed 
and  silicon  set  free  in  the  amorphous  state, 

KFl,SiFl,  +  4K  =  5KF1  +  Si ; 

by  washing  with  water,  the  silicon  may  then  be  readily  freed  from  the 
fluoride  of  potassium,  which  is  soluble.  The  graphitoidal  modification 
of  silicon  can  be  prepared  either  by  heating  the  amorphous  variety 

24 


370  MODIFICATIONS    OF    SILICON/ 

very  strongly,  in  which  event  the  powder  contracts  upon  itself  and 
becomes  much  more  dense,  or  by  melting  together  in  a  crucible  a  mix- 
ture of  metallic  aluminum,  in  excess,  and  fluosilicate  of  potassium ;  a 
fusible  double  fluoride  of  aluminum  and  potassium  is  formed,  and  the 
silicon  thus  set  free  dissolves  in  the  melted  aluminum.  If  the  aluminum 
be  dissolved  away,  after  the  mass  has  become  cold,  by  means  of 
chlorhydric  acid,  the  silicon  will  be  left  in  the  form  of  hexagonal  scales. 
Still  a  third  method  must  be  resorted  to,  in  order  to  obtain  the  dia- 
mond-like modification  of  silicon  ;  a  mixture  of  dry  fluosilicate  of 
potassium,  metallic  zinc  and  metallic  sodium  is  thrown  into  a  hot 
crucible,  the  mass  is  covered  with  a  layer  of  fluosilicate  of  potassium, 
and  the  crucible  covered.  A  lively  reaction  ensues,  and  the  mixture 
within  the  crucible  fuses ;  the  fused  mass  is  then  stirred  with  an  iron 
rod  until  the  zinc  begins  to  escape  as  vapor.  The  crucible  is  then 
removed  from  the  fire  and  allowed  to  cool ;  within  it  there  will  be  found 
a  button  of  metallic  zinc  filled  with  long  crystals  of  silicon,  which  can 
readily  be  isolated  by  dissolving  the  zinc  in  chlorhydric  acid. 

456.  The  amorphous  variety  of  silicon  is  a  brown  powder, 
which,  when  touched,  soils  the  fingers.  It  may  be  melted  at  a 
temperature  not  far  from  that  at  which  cast-iron  becomes  liquid. 
When  mixed  with  common  salt  and  exposed  to  a  degree  of  heat 
strong  enough  to  volatilize  the  salt,  amorphous  silicon  changes  to 
the  graphitoidal  variety,  as  has  been  already  remarked.  Amor- 
phous silicon  burns  readly  in  the  air  and  in  oxygen.  It  is  not 
acted  upon  by  acids,  excepting  fluorliydric  acid,  which  dissolves 
it  with  disengagement  of  hydrogen. 

Graphitoidal  silicon  is  very  similar  to  true  graphite ;  it  crys- 
tallizes in  lustrous  hexagonal  plates  of  a  leaden,  gray  color,  and 
is  an  excellent  conductor  of  electricity.  It  is,  however,  much 
harder  than  graphite.  By  exposing  it  to  an  exceedingly  high 
temperature,  it  can  be  transformed  to  the  diamond-like  condition. 
The  diamond-like  modification  of  silicon  occurs  in  the  form  of 
regular  octahedral  crystals,  exhibiting  a  decided  metallic  lustre. 
The  specific  gravity  of  these  crystals  is  2.49.  They  are  less 
hard  than  the  corresponding  crystals  of  carbon  and  boron,  and 
melt  at  the  same  temperature  at  which  cast-iron  melts. 

Either  graphitoidal  or  diamond-like  silicon  may  be  heated  to 
redness  in  oxygen  gas  without  burning  to  any  appreciable  extent, 
for  a  film  of  silicic  acid  is  formed  which  protects  the  remainder 


SILICON    AND    HYDROGEN.  371 

of  the  silicon  and  pravents  it  from  being  consumed  ;  but  if  the 
silicon  be  heated  together  with  a  substance  capable  of  furnishing 
oxygen  in  presence  of  a  base  competent  to  unite  with  silicic  acid 
and  form  a  fusible  silicate,  it  can  readily  be  oxidized  and  con- 
verted into  a  silicate.  Thus,  when  heated  to  intense  redness 
with  carbonate  of  sodium,  it  decomposes  the  latter  with  evolution 
of  light  and  heat,  and  there  is  produced  silicate  of  sodium,  while 
carbon  is  set  free  :  — 

Na20,C02  +  Si  =  Na20,Si02  +  C. 

In  a  similar  way,  if  silicon  be  heated  in  very  concentrated  solu- 
tions of  caustic  potash  or  soda,  water  will  be  decomposed  and  a 
silicate  of  potassium  or  of  sodium  formed,  while  hydrogen  is  set 
free :  — 

2NaHO  +  H2O  +  Si  =  Na2Si03  +  4HL 

Diamond-like  silicon  is  not  attacked  at  the  ordinary  temperature 
by  any  of  the  acids,  excepting  a  mixture  of  fluorhydric  and  nitric 
acids,  by  which  it  is  converted  into  fluoride  of  silicon.  Hot 
chlorhydric  acid  gas,  as  well  as  chlorine,  attacks  it  readily. 

457.  Silicon  and  Hydrogen  (SiH4?).     A  gaseous  compound 
of  these  elements,  called  siliciuretted  hydrogen,  may  be  obtained, 
mixed  with  free  hydrogen,  by  acting  upon  silicide  of  magnesium 
(Mg2Si)  with  chlorhydric  acid.     It  is  a  colorless  gas,  which  takes 
fire  spontaneously  on  coming  in  contact  with  the  air,  and  burns 
to  silicic  acid  and  water.     On  being  heated"  in  a  tube,  out  of  con- 
tact with  the  air,  it  is  decomposed  into  free  hydrogen  and  free 
silicon,  and  the  latter  is  deposited  upon  the  walls  of  the  tube  as 
a  shining  mirror,  similar  to  the  mirrors  of  arsenic  and  antimony 
obtained  in  Exps.  137,  140. 

Silicon  and  Oxygen.     Two  compounds  of  these  elements  have 
been  discovered,  though  but  one  of  them  is  as  yet  well  known. 

458.  Oxide  of  Silicon  is  described  as  a  white,  amorphous, 
hydrated  substance,  so  light  that  it  floats  upon  water ;  it  may  be 
obtained  by  treating  with  water   at  0°  the  compound  of  silicon,, 
hydrogen,  and  chlorine,  described  in  §  472.     It  is  scarcely  at  all, 
acted  upon  by  cold  water,  but  in  presence  of  alkalies  it  decom- 
poses water  rapidly  with  evolution  of  hydrogen  and  formation  of 
the  higher  oxide  of  silicon,  silicic  acid,  directly  to  be  described. 


372  OXIDES    OF    SILICON. 

At  temperatures  above  300°,  it  suffers  decomposition,  breaking 
up  into  silicic  acid  and  siliciuretted  hydrogen.  It  burns  bril- 
liantly in  the  air,  and  still  better  in  oxygen.  None  of  the  acids, 
excepting  fluorhydric  acid,  have  any  action  upon  it. 

459.  Silicic  Acid  (SiO2)  constitutes  at  least  one  half  of  the 
crust  of  the  earth.     Besides  being  thus  abundant,  it  is  one  of  the 
most  important   acids   known,  regarded  merely  as  a   chemical 
agent.     It  is  found  everywhere,  sometimes  free,  as  quartz  and 
sand,  sometimes  in  combination  with  metallic  oxides,  in  the  form 
of  salts  known  as  silicates.     It  occurs  more  or  less  abundantly 
in  all  soils,  plants,  and  waters,  while  rocks  like  granite  and 
earths  like  clay  are  largely  composed  of  it.     The  common  min- 
erals, feldspar  and  mica,  are  double  silicates  of  aluminum  and 
potassium,  and  of  aluminum,  iron,  and  potassium  respectively. 
In  plants,  the  silicic  acid,  or  silica,  as  it  is  often  called,  is  con- 
tained particularly  in  the  outer  covering  of  the  stalks  and  the 
husks  of  grain.     The  cuticle  of  rattan,  for  example,  contains  a 
large  proportion  of  silica,  and  the  same  remark  is  true  of  most 
of  the  grasses  and  grains.     The  value  of  the  plant  called  horse- 
tail (jEquisetum)  as  a  polishing  or  scouring  agent,  depends  upon 
the  large  quantity  of  silica  contained  in  it. 

460.  Silicic  acid  occurs  in  at  least  two  distinct  isomeric  modi- 
fications ;  in  one  of  which  it  is  completely  insoluble  in  water  and 
in  acids,  excepting  fluorhydric  acid,  and  is  only  slowly  soluble  in 
boiling  potash-lye ;  while,  when  in  the  other  modification,  it  dis- 
solves easily  in  a  solution  of  potash,  and  may  be  retained  in  solu- 
tion in  considerable  quantities  both  by  water  and  by  acids.    Rock 
crystal,  as  it  is  found  in  nature,  may  be  regarded  as  the  type  of 
the  first  or  insoluble  modification,  and  the  soluble  variety  may 
be  prepared  artificially  by  treating  the  solution  of  some  one  of 
the  soluble  silicates  with  chlorhydric  or  sulphuric  acid. 

Exp.  225.  —  In  a  block  of  charcoal  12  or  15  c.  m.  long  by  4  or  5 
c.  m.  wide  and  thick,  scoop  out  a  small  cup-shaped  cavity  large  enough 
to  hold  a  pea ;  place  in  this  cavity  a  fragment  of  amorphous  quartz  or 
flint,  and  heat  it  intensely  by  means  of  the  blow-pipe  (Exp.  205)  dur- 
ing several  minutes.  When  the  quartz  has  become  red-hot,  suddenly 
throw  it  from  the  cpal  into  a  dish  of  cold  water  ;  it  will  break  up  into 
numerous  small  fragments  or  become  filled  with  a  m  altitude  of  cracks, 


SOLUBLE    SILICIC    ACID.  373 

and  can  hence  be  readily  pulverized.  Grind  the  broken  quartz  to  fine 
powder  in  a  wedgewood,  or,  better,  in  an  iron  or  agate  mortar ;  weigh 
out  I  grm.  of  the  powder,  also  2  grms.  of  caustic  soda,  and  to  the  mix- 
ture of  these  ingredients  add  8  or  10  c.  c.  of  water.  Boil  the  mixture 
in  a  porcelain  dish  for  an  hour  or  two,  taking  care  to  add,  from  time  to 
time,  water  enough  to  supply  that  lost  by  evaporation  ;  then  pour  the 
solution  into  a  tall,  narrow  bottle,  and  leave  it  at  rest  until  the  undis- 
solved  portions  of  silica  have  settled  and  the  liquid  has  become  clear. 

The  product  thus  obtained  is  the  aqueous  solution  of  a  highly  alka- 
line silicate  of  sodium.  Unlike  the  results  obtained  by  dissolving  oxide 
of  lead  (Exp.  42)  and  ammonia-water  (Exp.  33)  in  nitric  acid,  the 
compound  produced  by  this  union  of  silicic  acid  with  an  alkali  is  not 
a  neutral  salt ;  not  only  one,  but  several  molecules  of  the  base,  oxide 
of  sodium,  have  united  with  one  molecule  of  the  silicic  acid.  Compounds 
such  as  this,  in  which  the  alkaline  constituent  or  base  predominates,  are 
called  basic  salts.  The  product  of  the  experiment  is  basic  silicate  of 
sodium. 

Exp.  226.  —  To  one-third  part  of  the  solution,  obtained  in  Exp.  225, 
add  10  or  12  times  its  bulk  of  water,  and  to  this  solution  add  dilute 
chlorhydric  acid,  drop  by  drop,  until  the  liquor  manifests  a  decided 
acid  reaction.  (Exp.  33.)  The  liquid  will  remain  clear,  and  no  silicic 
acid  will  be  deposited.  All  the  silicic  acid  which  has  been  set  free 
from  its  combination  with  the  alkali  remains  dissolved  in  the  acidulated 
water. 

If  the  clear  solution  be  placed  in  a  dialyser  (§327),  all  the  chloride 
of  sodium,  together  with  the  free  chlorhydric  acid,  will  pass  off  through 
the  parchment-paper  in  the  course  of  a  few  days,  and  there  will  be  left 
in  the  dialyser  nothing  but  pure  silicic  acid  and  water.  Aqueous  solu- 
tions of  silicic  acid,  containing  from  5  to  14  per  cent,  by  weight  of  the 
acid,  may  be  thus  obtained.  This  pure  aqueous  solution  of  silicic  acid 
exhibits  a  decided  acid  reaction.  It  has  little  or  no  taste,  though  when 
applied  to  the  tongue  it  occasions  an  unpleasant  sensation.  It  is  by  no 
means  so  permanent  as  the  solution,  above  mentioned,  acidulated  with 
chlorhydric  acid.  When  left  to  itself,  the  aqueous  solution  slowly  de- 
posits silicic  acid  as  an  insoluble  gelatinous  precipitate.  In  like  man- 
ner, it  has  been  observed  that  when  chlorhydric  or  another  acid  is 
added  to  the  aqueous  solution  of  basic  silicate  of  sodium  in  no  greater 
quantity  than  is  sufficient  to  exactly  neutralize  the  alkali,  the  liquor, 
though  it  remain  clear  for  a  considerable  space  of  time,  will  gradually 
become  cloudy  and  deposit  silicic  acid. 

Exp.  227.  —  To  one-third  part  of  the  solution  obtained  in  Exp.  225, 
'add  at  once  enough  concentrated  chlorhydric  acid  to  render  the  solu- 


374  SILICA    IN   NATURAL    WATERS. 

tion  acid.     All  the  silicic  acid  will  immediately  be  thrown  down  as  a 
thick  insoluble  jelly. 

Exp.  228. —  Place  a  portion  of  the  solution  of  silicic  acid  in  acidu- 
lated water,  obtained  in  Exp.  226,  in  a  small  porcelain  dish,  evaporate 
it  to  dryness  upon  a  water-bath  and  heat  the  residue,  over  the  gas-lamp, 
to  a  temperature  of  180°  or  190°.  Add  water  to  the  cold,  dry  residue 
and  observe  that  the  silicic  acid  does  not  redissolve  ;  it  remains  as  a 
fine  white  powder,  which  may  be  readily  collected  upon  a  filter  and 
there  washed  clean  by  pouring  upon  it  several  successive  portions  of 
water.  After  having  been  once  thoroughly  dried,  silicic  acid  is  com- 
pletely insoluble  in  water. 

461.  By  adding  chlorhydric  acid  to  a  sufficient  quantity  of  the 
water  of  almost  any  spring  or  river,  and  then  evaporating  the 
water  to  dryness,  a  small  quantity  of  silicic  acid  will  be  found  in 
the  residuum.     The  silica,  in  this  case,  may  be  combined  with 
an  alkali,  or  may  be  held  in  solution  by  the  action  of  an  alkaline 
carbonate..    Silicic  acid,  which  has  been  finely   powdered,  or, 
better,  that  whicb  has  recently  been  precipitated,  is  soluble,  to  a 
considerable  extent,  in  aqueous  solutions  of  the  alkaline  carbon- 
ates, or  even  of  the  bicarbonates,.  only  a  small  proportion  of  the 
carbonic  acid  being  expelled  by  it  as  it  dissolves.     It  is  not  im- 
probable, therefore,  that  the  silicic  acid  found  in  waters  may  be 
retained  in  solution  by  force  of  this  solvent  action  of  the  alkaline 
carbonates.     It  is,  at  all  events,  a  matter  of  fact,   that  waters 
highly  charged  with  carbonate  of  sodium,  such  as  flow  from  the 
geysers  or  boiling  springs  of  Iceland,  contain,  in  solution,  very 
large  quantities  of  silica,  much  of  which  is  deposited  as  the 
water  becomes  cold,  upon  the  rocks  and  other  objects  with  which 
it  comes  in  contact ;  silicious  petrifactions  are  thus  formed. 

462.  Besides   the  soluble  and  insoluble  modifications  above 
mentioned,  a  distinction  is  made  between  the  variety  of  silicic 
acid,  which  is  found  crystallized  in  nature,  and  that  which  occurs 
in  the  'amorphous  state.     Crystallized  silicic  acid  is  anhydrous, 
has  a  specific  gravity  of  from  2.6  to  2.66,  and  is  scarcely  at  all 
acted  upon  by  alkaline  lyes.     The  amorphous  silicic  acid  found 
in  nature  contains  a  certain  amount  of  water,  has  a  specific 
gravity  of  only  2.1  to  2.2,  and  is  soluble  in  alkaline  solutions. 

As  it  exists  in  the  insoluble  modification,  either  as  found  in  na- 
ture or  as  prepared  by  calcining  the  artificial  product,  silicic  add 


SILICIC   ACID.  375 

is  a  white,  tasteless  solid,  incapable  of  forming  a  cohesive  plastic 
mass  with  water.  It  is  infusible,  excepting  at  very  high  tem- 
peratures, such  as  may  be  obtained  by  means  of  the  oxyhydro- 
gen  blow-pipe,  in  the  flame  of  which  it  melts  to  a  colorless  glass. 
When  melted,  it  is  tough  and  viscous,  and,  like  glass,  can  be 
drawn  out  into  fine  threads  which  are  exceedingly  elastic,  par- 
ticularly if  they  be  dipped,  while  white-hot,  into  water.  Silicic 
acid  is  not  volatile  by  itself,  but  when  exposed  to  a  current  of 
gas  or  vapor  at  a  white  heat,  portions  of  it  are  carried  off  by  the 
vapor ;  like  boracic  acid,  it  can  be  transported  in  considerable 
quantities  by  superheated  steam. 

When  in  the  soluble  modification,  whether  in  the  gelatinous 
hydrated  condition  or  in  the  state  of  an  air-dried  powder  which 
has  never  been  exposed  to  heat,  it  dissolves  readily  in  solutions 
of  the  caustic  and  carbonated  alkalies,  particularly  if  these  be 
heated.  Some  varieties  of  native  silica,  such,  for  example,  as 
the  soft,  pulverulent,  infusorial  earth  found  at  the  bottoms  of  many 
pondsjand  swamps,  are  as  readily  soluble  in  the  alkalies  as  that 
which  has  been  prepared  artificially.  All  the  varieties  of  silica 
can  be  dissolved  by  heating  them  with  alkaline  lyes  in  close 
boilers  under  a  pressure  of  4  or  5  atmospheres,  —  in  other  words, 
even  crystallized  silica  is  soluble  in  the  alkalies  at  high  tempera- 
tures. 

k  463.  At  the  ordinary  temperature,  silicic  -acid  is  but  a  weak 
acid ;  almost  any  of  the  other  acids  are  capable  of  decomposing 
the  aqueous  solution  of  a  salt  of  silicic  acid  and  of  expelling  the 
latter  from  its  combination  with  the  metal.  But,  as  is  the  case 
with  boracic  acid  also  (§  448),  at  high  temperatures  the  reverse 
of  all  this  is  true,  silicic  acid  being  then  capable  of  expelling  all 
acids  which  are  more  volatile  than  itself.  Neither  variety  of 
silicic  acid  is  acted  upon  at  any  temperature  by  either  carbon, 
hydrogen,  phosphorus,  or  chlorine,  if  these  elements  be  taken 
separately ;  but  when  exposed  at  high  temperatures  to  the  com- 
bined action  of  carbon  and  chlorine,  chloride  of  silicon  and  car- 
bonic oxide  are  produced,  much  in  the  same  way  that  chloride 
of  boron  is  formed  under  similar  circumstances,  as  has  been 
shown  in  §  449.  When  silicic  acid  is  heated  together  with  car- 
bon, or  other  reducing  agents,  in  contact  with  metals  like  iron  or 


376  SILICATES. 

platinum,  some  of  it  is  decomposed,  and  a  silicide  of  the  metal  is 
formed.  Platinum  crucibles  are  often  injured  in  this  way  by  the 
nascent  silicon  which  forms  when  certain  minerals  are  fused  in 
them. 

464.  Silicic  acid  combines  with  many  of  the  metallic  oxides 
to  form  salts,  many  of  which  are  of  very  complex  composition. 
Hundreds  of  silicates  are  found  in  nature  as  crystallized  miner- 
als, and  they  may,  in  general,  be  formed  by  fusing  together 
silicic  acid  and  the  appropriate  metallic  oxide,  or  carbonate,  in 
suitable  proportions.  Most  of  the  silicates  are  fusible,  and  the 
greater  number  of  them,  when  in  the  molten  state,  have  the 
power  of  dissolving,  either  an  excess  of  base,  or  an  excess  of 
silicic  acid  over  and  above  the  quantities  corresponding  to  strict 
atomic  proportions ;  hence  it  happens  that  mixtures  of  silicic  acid 
and  of  bases  may  be  melted  together  in  the  most  varied  propor- 
tions. The  study  of  the  silicates  thus  becomes  difficult,  since  it 
is  often  impossible  to  determine  whether  a  given  silicate  be  really 
a  definite  chemical  compound,  or  a  chemical  compound  contam 
inated  with  an  excess  of  silicic  acid  or  of  the  base ;  in  very 
many  instances,  moreover,  it  is  probably  true  that  the  excess  of 
silica,  above  the  normal  atomic  proportion,  or  of  the  base,  is 
really  held  in  combination  by  virtue  of  the  chemical  force,  though 
it  be  held  feebly  and  indefinitely,  as  is  the  case  with  the  ingre- 
dients of  many  solutions  and  metallic  alloys.  (See  §§  49,  76.) 

As  a  general  rule,  the  simple  silicates  containing  but  a  single 
base,  and  no  excess  of  either  base  or  acid,  crystallize,  in  passing 
from  the  liquid  to  the  solid  c'ondition,  but  the  double  silicates — 
compounds  formed  by  the  union  of  two  or  more  simple  silicates, 
and,  therefore,  containing  at  least  two  different  metals  —  usually 
solidify  to  a  non-crystalline,  homogeneous  glass.  All  the  common 
varieties  of  glass  consist  essentially  of  such  double  salts  of  silicic 
acid.  In  general,  the  silicates  containing  an  excess  of  base  are 
more  readily  fusible  than  the  normal  or  acid  salts  ;  those  which 
contain  easily  fusible  oxides  melt  at  correspondingly  low  tem- 
peratures ;  and  many  mixtures  of  two  or  more  different  silicates 
fuse  at  temperatures  lower  than  the  melting  point  of  either  of 
the  simple  silicates  of  which  the  mixture  is  composed. 

The  silicates  of  potassium  and  of  sodium,  containing  1  or  2 


FOKMULJ2    OF    SILICATES.  377 

or  more  molecules  of  base  to  one  molecule  of  the  acid,  are  read- 
ily soluble  in  cold  water,  and  compounds  containing  as  many  as 
4  molecules  of  the  acid  to  one  molecule  of  the  alkaline  base,  may 
be  completely  dissolved  in  water  when  boiled  therewith  for  a 
considerable  time.  These  acid  compounds,  such,  for  example, 
as  the  sodium-salt  of  the  formula  Na2O,  2  to  4Si02 ,  are  much 
employed  in  the  arts  under  the  name  of  Waterglass  or  Soluble- 
glass.  When,  however,  the  proportion  of  acid  is  more  than  4J 
molecules  to  1  molecule  of  the  alkaline  base,  the  silicate  may, 
for  all  practical  purposes,  be  regarded  as  insoluble  in  water. 
The  silicates  of  all  the  other  non-alkaline  metals  are,  in  like 
manner,  insoluble  in  water,  in  the  ordinary  sense  of  the  word, 
though,  in  the  last  analysis,  it  would  be  hard  to  find  any  silicious 
minerals,  or  glasses,  which  are  not  susceptible  of  being  decom- 
posed and  dissolved,  to  a  greater  or  less  extent,  by  water.  Some 
metallic  silicates  may  be  prepared,  not  only  by  the  method  of 
fusion,  but  also  by  adding  the  solution  of  an  alkaline  silicate  to 
the  solution  of  a  salt  of  the  metal  whose  silicate  is  desired. 

465.  The  most  commonly  occurring  silicates  may  be  referred 
to  3  or  4  general  classes :  1st.  Normal  silicates  of  the  general 
formula  M20,Si02 ,  such  as  the  crystallizable  silicate  of  sodium 
Na2O,Si02 ,  or  the  normal  silicate  of  calcium,  CaO,Si02.     2d. 
Bisilicates,  of  the  formula  M2O,2SiO2,  such  as  the  bisilicate  of 
calcium,    CaO,2SiO2.     3d.    Disilicates   (basic   silicates)    of  the 
general  formula  2M2O,SiO2,  such,  for  example,  as  the  silicates 
of  iron,  2FeO,SiO2,  of  magnesium,  2MgO,SiO2,  of  manganese, 
2MnO,SiO2 ,  and  of  zinc,  2ZnO,SiO2 .     Also,  sesqui-silicates.  of 
the  formula  2M2O,3SiO2,  to  which  the  mineral  meershaum  may, 
perhaps,  be  referred,  its  formula  being  2MgO,3SiO2  -)-  2H2O.. 
The  student  will  do  well  to  notice  how  many  of  these  types  of 
salts  are  found  among  the  carbonates  also. 

466.  Many  of  the  natural  silicates  may  be  decomposed  by 
digestion  with  the  strong  mineral  acids,  such  as   concentrated 
chlorhydric  acid,  and  this  is  especially  true   of  those  silicates 
which  contain  a  large  proportion  of  base,  and  of  those  containing 
water  of  crystallization.     But  many  anhydrous  normal  or  acid 
silicates  are  not  decomposed  by  any  acid,  excepting  fluorhydric 
acid.     Fluorhydric   acid  attacks  and   dissolves,  not   only  pure 


378  DECOMPOSITION    OP    SILICATES. 

silicic  acid  in  all  its  varieties,  but  all  silicates  as  well,  and  other 
things  being  equal,  the  rapidity  of  its  action  is  in  proportion  to 
the  degree  of  its  concentration.  Crystallized  silica,  however,  is 
much  less  readily  acted  upon  by  fluorhydric  acid  than  the  amor- 
phous variety ;  it  dissolves  slowly  and  without  development  of 
heat,  while  amorphous  silica,  not  only  dissolves  rapidly,  but  the 
act  of  solution  is  accompanied  with  the  evolution  of  considerable 
heat.  From  those  silicates,  which  are  decomposed  with  difficulty 
by  chlorhydric  acid,  the  silicic  acid  separates  out  as  a  soft  pow- 
der; but  from  those  which  are  more  readily  decomposed,  the 
silicic  acid  separates  as  a  hydrated  gelatinous  mass,  when  the 
finely-powdered  mineral  is  digested  with  chlorhydric  acid,  —  in 
this  case,  the  mineral  is  said  to  gelatinize  with  acids.  Some 
silicious  minerals  may  even  be  completely  dissolved,  silica  and 
all,  in  dilute  chlorhydric  acid. 

467.  It  is  remarkable  that,  while  some  of  the  hydrated  sili- 
cates, such,  for  example,  as  the  class  of  minerals  called  zeolites, 
can  no  longer  be  readily  decomposed  by  chlorhydric  acid  after 
they  have  been  ignited,  there  are  other  silicates,  such  as  the 
minerals  garnet,  epidote,  and  idocrase,  which,  though  scarcely  at 
all  acted  upon  by  chlorhydric  acid  when  in  the  natural  condition, 
may  be  decomposed  thereby  with  gelatinization  after  they  have 
been  melted.  Facts  like  these  would  seem,  at  first  sight,  to 
indicate  that  silicic  acid  exists  in  combination,  sometimes  in  the 
insoluble,  and,  at  other  times,  in  the  soluble  modification,  and 
that  it  may  be  transformed  from  the  one  state  to  the  other  with- 
out being  first  set  free,  but  some  of  them  admit  of  being  explained 
in  another  way.  When  any  silicate,  no  matter  whether  it  be 
decomposable  by  chlorhydric  acid  or  unacted  upon  by  this  agent, 
is  mixed  with  an  excess  of  one  of  the  fixed  alkalies,  or  alkaline 
earths,  or  with  a  carbonate  or  nitrate  of  either  of  the  alkaline  or 
alkaline  earthy  metals,  and  the  mixture  then  heated  to  intense 
redness,  there  will  be  obtained  a  melted  or  agglutinated  mass, 
which  decomposes  readily  on  being  treated  with  chlorhydric  acid 
and  yields  gelatinous  silica.  At  the  high  temperature  to  which 
the  mixture  is  exposed,  the  strong  alkaline  or  alkaline  earthy 
bases  combine  with  the  silicic  acid,  and  there  are  formed  sili- 
cates of  potassium,  sodium,  calcium,  or  the  like,  easily  decom- 


COMPOSITION    OF    SILICIC    ACID.  379 

posable  by  acids.  Now,  in  the  case  of  the  minerals  garnet, 
epidote,  and  idocrase,  above  mentioned,  which  are  attacked  by 
chlorhydric  acid  after  they  have  been  melted,  it  may  be  con- 
ceived that  the  strong  bases,  such  as  lime,  magnesia,  and  protoxide 
of  iron,  which  are  contained  in  these  minerals,  have  acted  upon 
the  silicate  of  alumina,  also  contained  in  them,  in  much  the  same 
way  that  an  extraneous  alkali  would  act  if  it  were  fused  with  the 
powdered  mineral. 

468.  Sulphuric  acid,  when  concentrated  or  but  slightly  diluted, 
acts  much  more  energetically  than  chlorhydric  acid  upon  the  sili- 
cates, apparently  because  of  its  comparatively  high  boiling  point. 
Common  clay,  for  example  (silicate  of  aluminum),  which  is  but 
little  acted  upon  by  chlorhydric  acid,  may  be  completely  decom- 
posed when  digested  with  boiling  concentrated  sulphuric  acid, 
silicic  acid  being  set  free  and  sulphate  of  aluminum  produced. 
This  decomposition,  like  that  of  the  garnet  above  mentioned,  is, 
in  any  event,  very  much  more  readily  effected  if  the  clay  be 
first  gently  roasted  or  calcined.     The  mineral  feldspar  also,  when 
in  fine  powder,  may  be  decomposed  by  boiling  oil  of  vitriol. 

469.  Contrary  to  the  view  formerly  held  by  many  chemists, 
the  atomic  weight  of  silicon  is  now  thought  to  be  28,  instead  of 
21,  as  formerly.     This  change  makes  the  formula  of  silicic  acid 
SiO2 ,  in  strict  analogy  to  carbonic  acid,  instead  of  Si03   which 
was  for  a  long  time  the  accepted  formula. 

The  percentage  composition  of  silicic  acid  has  been  determined 
by  direct  experiment,  —  by  oxidizing  a  weighed  quantity  of 
silicon  to  silicic  acid,  —  to  be  51.92  per  cent,  of  oxygen  and 
48.08  per  cent,  of  silicon.  But  since  it  is  difficult  in  this  way  to 
oxidize  the  whole  of  the  silicon,  inasmuch  as  portions  of  it 
become  covered  with  silicic  acid,  and  are  so  protected  from  the 
further  action  of  oxygen,  —  the  above  result  can  be  regarded 
only  as  a  rough  approximation  to  the  truth.  These  numbers 
would  correspond  to  29.63,  as  the  atomic  weight  of  silicon.  For 

51.92  :  48.08  : :  (32  =  2  atoms  of  O)  :  (x  =  1  atom  of  Si  =  29.63). 

More  accurate  experiments,  however,  upon  another  plan,  have 
since  shown  that  the  real  atomic  weight  of  silicon  is  28,  or  very 
nearly  28,  as  will  be  fully  set  forth  in  the  next  section. 


380  CHLORIDE    OF    SILICON. 

470.  Chloride  of  Silicon  (SiCl4)  is  formed  when  silicon  is 
heated  in  an  atmosphere  of  dry  chlorine,  or  more   conveniently 
by   heating  together  finely   divided  silicic  acid,    charcoal,   and 
chlorine. 

The  experiment  may  be  conducted  as  follows :  Mix  together  inti- 
mately equal  weights  of  lamp-black  and  dry  pulverulent  silicic  acid, 
such  as  is  obtained  by  decomposing  an  alkaline  silicate  with  chlorhydric 
acid  and  evaporating  the  residue  to  dryness,  as  in  Exp.  228.  Knead 
into  the  mixture  enough  oil  to  render  it  plastic,  and  from  the*  thick 
paste  thus  formed  make  a  number  of  small  balls  or  pellets ;  roll  these 
pellets  in  powdered  charcoal,  place  them  in  a  Hessian  crucible,  cover 
the  crucible,  and  heat  in  a  strong  fire  until  all  the  oil  has  been  distilled 
off  or  decomposed.  The  mixture  of  silicic  acid  and  charcoal  is  thus 
left  in  an  open  porous  condition,  well  adapted  for  the  action  of  the 
chlorine.  Place  the  dry  pellets  in  a  porcelain  tube  about  2  c.  m.  in 
diameter,  connect  one  end  of  the  porcelain  tube  with  a  U-tube  sur- 
rounded with  a  freezing  mixture  of  ice  and  salt,  and  to  the  other  end 
attach  a  flask  in  which  to  generate  chlorine,  taking  care  to  interpose 
between  this  flask  and  the  porcelain  tube  one  U-tube  filled  with  pumice- 
stone,  soaked  in  oil  of  vitriol,  and  another  filled  with  chloride  of  cal- 
cium, in  order  that  the  gas  may  be  thoroughly  dried.  Place  the 
porcelain  tube  in  a  suitable  stove  or  furnace ;  pass  into  it  a  slow  current 
of  chlorine,  in  order  to  expel  the  air,  and  then  build  around  it  a  hot 
fire  of  charcoal  or  coke ;  finally,  when  the  tube  has  become  heated  to 
intense  redness,  pass  in  the  chlorine  as  fast  as  it  is  absorbed.  The 
chloride  of  silicon  will  be  condensed  in  the  U-tube  as  a  liquid  which 
has  usually  a  yellow  color,  owing  to  the  presence  of  dissolved  chlorine. 
In  order  to  purify  it,  the  liquid  may  be  shaken  in  a  dry  flask  with  a 
quantity  of  quicksilver,  in  which  a  little  potassium  has  been  dissolved ; 
after  having  been  decanted  from  the  mercury  and  subjected  to  redis- 
tillation in  dry  vessels,  it  will  be  found  to  be  pure. 

It  will  be  noticed  that  the  metbod  of  preparing  chloride  of  silicon, 
here  described,  is  similar  to  that  recommended  for  preparing  chloride 
of  boron  (§449).  Chlorine  alone  is  not  capable  of  decomposing  either 
silicic  or  boracic  acids,  but  in  presenc?  of  carbon,  combination  of  chlo- 
rine and  silicon  is  effected  at  the  san:e  time  that  carbon  and  oxygen 
unite  to  form  carbonic  oxide  :  — 

SiO2  +  2C  +  4C1  —  S1C1,  +  2CO. 

471.  Chloride  of  silicon  is  a  transparent,  colorless,  very  mo- 
bile, volatile  liquid,  of  pungent,  acid  and  irritating  odor.     It 


COMPOSITION    OF    CHLORIDE    OF    SILICON.  381 

fumes  strongly  in  the  air,  boils  at  59°,  and  is  of  1.52  specific 
gravity.  The  specific  gravity  of  its  vapor  has  been  found  to  be 
85.74,  which  accords  well  with  the  supposition  that  a  molecule 
of  chloride  of  silicon  contains  4  volumes  of  chlorine  and  1  vol- 
ume of  silicon-vapor  condensed  to  2  volumes  :  — 
For  if  to  the  weight  of  4  volumes  of  chlorine  (4  X  35.5)  =  .  142 
There  be  added  the  weight  of  1  volume  of  silicon-vapor  =  .  28 

The  two  volumes  of  gas  produced  will  weigh      .        .        •        ,     170 
The  weight  of  one  volume  of  the  gas  should  consequently  be  equal  to 

170  -f-  2  =  85. 

Chloride  of  silicon  is  at  once  decomposed  by  water  with  for- 
mation of  chlorhydric  acid  and  deposition  of  gelatinous  silicic 
acid,  — 

SiCl4  +  2H20  =  Si02  +  4HC1, 

no  gas  of  any  kind  being  set>  free,  and  no  other  products  being 
formed.  The  fact  is  important,  since  it  shows  at  once  that  the 
composition  of  chloride  of  silicon  must  correspond  with  that  of 
silicic  acid ;  that  in  it  four  atoms  of  chlorine  simply  replace  the 
two  atoms  of  oxygen  contained  in  the  silicic  acid.  For  knowing, 
as  we  do,  the  composition  of  chlorhydric  acid  (§  98)  and  of  water 
(§  36),  we  are  sure  that  for  every  atom  of  chlorine  eliminated 
from  the  chloride  of  silicon,  one  atom  of  hydrogen  is  required, 
in  order  that  chlorhydric  acid  may  be  formed,  and  that  for  every 
two  atoms  of  hydrogen  thus  taken  from  the  water,  one  atom  of 
oxygen  will  be  detached.  But,  as  no  oxygen  escapes  from  the 
solution,  all  of  that  derived  from  the  water  must  evidently  have 
^combined  with  the  silicon  to  form  silicic  acid.  Now,  it  is  a  com- 
paratively easy  matter  to  determine  the  amount  of  chlorine  in 
any  solution  by  adding  to  the  solution  nitrate  of  silver,  and  then 
collecting  and  weighing  the  insoluble  chloride  of  silver  which  is 
formed.  Hence,  if  a  weighed  quantity  of  chloride  of  silicon  be 
decomposed  with  water,  and  the  amount  of  chlorine  contained  in 
the  solution  be  determined,  the  difference  between  the  weight  of 
chlorine  found  and  the  weight  of  chloride  of  silicon  taken  will 
give  us  the  weight  of  the  silicon  with  which  the  chlorine  was 
combined.  By  careful  experiments,  it  has  been  proved  in  this 
way  that  chloride  of  silicon  contains  in  100  parts  by  weight 


382  COMPOSITION    OF    CHLORIDE    OF    SILICON. 

16.52  parts  of  silicon  and  83.48  parts  of  chlorine.  But  83.48 
parts  of  chlorine  are  equivalent  to  18.81  parts  of  oxygen,  for: 

(35.5   X   2  =  71)  :  16  ::  83.48  :  (x  =  18.81). 

Weight  of  two  Weight  of 

atoms  of  Cl.  one  atom  of  O. 

Hence  18.81  parts  of  oxygen  must  have  combined  with  every 
16.52  parts  of  silicon  contained  in  the  solution;  or,  reduced  to 
per  cent.,  every  46.75  parts  of  silicon  must  have  combined  with 
53.25  parts  of  oxygen ;  and  if  this  proportion  of  oxygen  lead 
to  the  formula  SiO2,  the  proportion  of  chlorine  will,  in  like 
planner,  require  the  formula  SiCl4.  From  the  composition  of 
chloride  of  silicon,  as  thus  determined,  the  atomic  weight  of 
silicon  may  be  accurately  derived  (compare  §  469)  :  — 

53.25    :    46.75       =       32    :    (x  =  28.1) 

Per  cent,  of       Per  cent,  of  Weight  of  two  Weight  of  one 
Oxygen              Silicon                  atoms  of  atom  of 

" v '  Oxygen.  Silicon. 

in  Silicic  Acid. 

Or,  more  directly  by  the  proportion 

83.48    :     16.52       =       142    :     (a;  =  28.1) 

Per  cent,  of       Per  cent,  of  Weight  of  Weight  of 

Chlorine             Silicon  four  atoms  one  atom 

* 7^ '  of  Chlorine.  of  Silicon. 

in  Chloride  of  Silicon. 

At  the  ordinary  temperature,  liquid  chloride  of  silicon  does 
not  act  upon  potassium,  but  if  this  metal  be  heated  in  its  vapor, 
chloride  of  potassium  is  formed  and  silicon  set  free  ;  the  reaction 
affords,  in  fact,  a  good  method  of  obtaining  silicon  in  the  amor- 
phous state.  There  is  a  bromide  of  silicon  (SiBr4)  analogous  to 
the  chloride  in  composition  and  properties ;  it  may  be  prepare^ 
in  a  similar  way.  No  compound  of  iodine  and  silicon  has  yet 
been  discovered. 

472.  Compound  of  Silicon,  Hydrogen,  and  Chlorine.  By 
passing  a  current  of  dry  chlorhydric  acid  gas  over  crystallized 
silicon,  heated  nearly  to  redness  in  a  glass-tube,  and  condensing 
the  product  in  a  receiver  immersed  in  a  freezing  mixture,  there 
is  obtained  a  colorless,  fuming  liquid,  of  1.65  specific  gravity, 
which  boils  at  42°.  At  a  red  heat  it  decomposes,  yielding  chlo- 
ride of  silicon,  chlorhydric  acid,  and  amorphous  silicon.  Its 
vapor  is  very  inflammable,  and,  when  mixed  with  air  or  oxygen, 


FLUORIDE    OF    SILICON.  383 

explodes  violently  on  being  lighted ;  chloride  of  silicon,  together 
with  chlorhydric  and  silicic  acids,  are  the  products  of  this  reac- 
tion. Water  decomposes  it  instantly,  with  production  of  oxide 
of  silicon,  as  has  been  already  set  forth  (§  458). 

473.  Fluoride  of  Silicon  (SiFl4)  is  formed  whenever  dry 
fluorhydric  acid  comes  in  contact  with  silicic  acid,  either  free  or 
combined.  It  may  be  readily  prepared  by  treating  a  mixture  of 
silicious  sand  and  fluorspar  with  oil  of  vitriol. 

In  a  flask  of  about  250  c.  c.  capacity,  place  an  intimate  mixture  of 
9  grins,  of  finely-powdered  quartz-sand,  9  grms.  of  powdered  fluorspar, 
and  53  grms.  of  concentrated  sulphuric  acid.  Heat  the  flask  and  col- 
lect the  gas,  which  is  evolved  in  tall  bottles  of  150  c.  c.  capacity,  at  the 
mercury  trough.  The  sand  and  fluorspar  should  both  be  heated  before 
being  used,  in  order  that  they  may  be  perfectly  dry,  the  flask  also  and 
the  mercury  in  the  trough  should  be  thoroughly  dried,  for  fluoride  of 
silicon  is  decomposed  by  moisture,  and  the  bulky  precipitate  of  silicic 
acid  which  is  formed  might  clog  the  delivery-tube  of  the  apparatus  or 
coat  the  glass  vessels  and  render  them  opaque. 

The  decomposition  of  the  gas  by  water  can  readily  be  shown  by 
causing  some  of  it  to  pass  out  from  the  delivery-tube  into  a  bottle  in- 
verted upon  the  mercury -trough,  one  half  of  which  has  been  filled  with 
water  and  the  other  half  with  quicksilver.  The  water  in  the  Bottle 
will  soon  become  filled  with  gelatinous  silica.  With  the  quantities  of 
material  above  given,  there  will  be  obtained  first  a  bottle  of  150  c.  c. 
capacity  of  air  from  the  flask,  which  should  be  thrown  away ;  a  bottle  of 
the  same  size  can  then  be  filled  with  sufficiently  pure  fluoride  of  sili- 
con, and  in  a  third  bottle  the  decomposition  with  water  may  be  sKpwn. 

Instead  of  sand,  coarsely-powdered  glass  may  be  placed  in  the  flask 
as  the  source  of  silicon.  In  any  event  the  glass  of  the  flask  will  be 
somewhat  corroded  ;  it  is  better  to  conduct  the  operation  in  a  platinum 
retort,  if  such  a  vessel  can  be  had. 

The  reactions  which  occur  among  the  materials  in  the  flask  may  be 
represented  by  the  following  equations :  In  the  first  place,  fluorhydric 
acid  is  formed  by  the  action  of  sulphuric  acid  upon  the  fluoride  of 
calcium, 

CaFl2  -f  H2SO4  =  CaSO4  +  2HF1, 
and  the  fluorhydric  acid  then  attacks  the  silica, 

SiO2  +  4HF1  =  2H2O  +  SiFI*. 

The  water  here  formed  unites  with  the  sulphuric  acid  which  has  been 
employed  in  excess,  and  is  thus  prevented  from  acting  upon  the  fluoride 


384  FLUORIDE    OP   SILICON. 

of  silicon.  The  reaction  last  named  is  essentially  the  same  as  that 
which  occurs  when  glass  is  etched  with  fluorhydric  acid  gas  (§  158). 
When  the  fluoride  of  silicon  is  brought  in  contact  with  water,  there  is 
produced,  together  with  the  silicic  acid,  fluosilicic  acid  (2HFl,SiFl4),  a 
compound  to  be  described  directly. 

474.  Fluoride  of  silicon  is  a  colorless  gas  which  fumes  strongly 
in  the  air,  especially  if  the  air  be  moist.  It  has  an  acid  odor 
and  taste  ;  extinguishes  combustion,  though  itself  uninflammable  ; 
has  no  action  upon  glass ;  can  support  a  high  degree  of  heat 
without  being  decomposed,  and  may  be  condensed  by  pressure 
and  cold  to  a  colorless,  very  mobile  liquid.  Its  specific  gravity 
has  been  determined  to  be  51.98,  or  3.6  as  compared  with  air. 
It  can  consequently  be  readily  collected  by  displacement,  if  pains 
be  taken  to  protect  it  from  contact  with  moist  air  by  means  of 
drying  tubes. 

The  specific  gravity  of  this  gas  points  very  decidedly  to  the 
formula  SiFl4,  and  consequently  to  the  number  28  as  the  atomic 
weight  of  silicon,  and  to  the  formula  Si02  for  silicic  acid.  For, 
if  a  molecule  of  gas  be  composed  of 

One  volume  of  silicon-vapor  =  .         ....        .         .        .28 

And  four  volumes  of  fluorine  (19  X  4).==      ....  76 

Condensed  to  two  volumes  =  .  .  •• ,. ,  •;....•  ,,•'•  .•  •  104 
One  volume  of  the  gas  will  weigh  52,  or  almost  precisely  as  much  as 
has  been  indicated  by  experiment. 

The  behavior  of  the  gas  with  water  is  peculiarly  interesting. 
If  water  be  sprinkled  into  a  bottle  filled  with  fluoride  of  silicon, 
or  if  the  gas  be  conducted  into  water,  decomposition  occurs 
instantly,  and  silicic  acid  is  deposited  in  the  gelatinous  condition. 
At  first  sight  it  might  seem  as  if  the  reaction  were  strictly  an- 
alogous to  that  which  occurs  when  chloride  of  silicon  is  mixed 
with  water  (see  §  471),  and  that  it  could  be  represented  by  the 
formula, 

SiFl4  +  2H2O  =  SiO2  +  4HF1, 

but  if  it  be  remembered  that  fluorhydric  acid,  unlike  chlorhydric 
acid,  is  capable  of  dissolving  silica,  it  will  be  at  once  evident  that 
the  reaction  between  water  and  fluoride  of  silicon  cannot  be 
directly  comparable  with  the  other  reaction  in  which  chloride  of 


FLUOSILICIC    ACID.  385 

silicon  is  concerned.  The  reaction  which  really  occurs  between 
water  and  fluoride  of  silicon  may  be  represented  by  the  follow- 
ing equation :  — 

3  SiFl4  -f  2  H.O  =  Si02  +  2(2  HF1,  SiFl4), 
a  double   compound    of  fluorhydric   acid   and   fluoride   of  sili- 
con, known  as  fluosilicic  acid,  being  the  other  product  besides 
silica. 

Fluosilicic  acid  may  be  prepared  either  in  the  manner  indicated  in 
§  473,  or  more  conveniently  by  conducting  fluoride  of  silicon  gas  into 
an  upright  bottle  full  of  water ;  in  this  case,  however,  the  orifice  of  the 
tube  which  delivers  the  gas  should  be  immersed  in  a  layer  of  mercury, 
3  or  4  c.  m.  deep,  beneath  the  water,  so  that  the  gas  may  bubble  up 
through  the  mercury,  and  the  water  never  come  in  contact  with 
the  mouth  of  the  delivery-tube.  In  this  way  the  tube  may  be  kept  free 
from  the  silicic  acid  which  would  quickly  clog  it  if  it  dipped  directly 
into  the  water.  As  the  bubbles  of  fluoride  of  silicon  escape  from  tire 
mercury,  each  of  them  becomes  covered  with  a  thick  crust  of  gelatinous 
silica,  which  it  carries  with  it  to  the  surface  of  the  water.  Sometimes 
one  of  these  crusts  will  remain  adhering  to  the  mercury  and  will  be 
gradually  prolonged  into  the  form  of  a  tube  extending  from  the  mer- 
cury to  the  surface  of  the  water.  Such  tubes  should,  however,  be 
broken  up,  by  stirring  the  liquor,  lest  fluoride  of  silicon  escape  through 
them  into  the  air  without  coming  in  contact  with  water. 

Another  modification  of  the  method  of  preparing  fluosilicic  acid  is  to 
conduct  the  gas  into  a  lar<*e  flask  containing  but  little  water,  and  to 
agitate  this  flask  so  that  its  sides  may  be  continually  moistened  with 
•vyater,  although  no  water  can  come  in  contact  with  the  opening  of  the 
gas  delivery-tube.  The  following  quantities  of  materials  have  been 
found  convenient  in  practice.  In  a  flask  of  600  or  700  c.  c.  capacity 
place  a  mixture  of  35  grms.  of  powdered  sand,  35  grms.  of  powdered 
fluorspar,  and  210  grins,  of  concentrated  sulphuric  acid.  Place  the  flask 
upon  a  sand-bath  over  the  gas-lamp,  and  by  means  of  a  delivery-tube 
connect  it  either  with  the  bottom  of  a  tall  bottle  containing  150  c.  c.  of 
water  and  enough  mercury  to  form  the  layer  above  described,  or  with  the 
middle  of  a  flask  of  700  or  800  c.  c.  capacity,  and  cpntaining  150  c.  c.  of 
water.  In  the  latter  case  the  receiving-flask  must  be  agitated,  as  afore- 
said, so  that  its  walls  may  be  kept  moist.  When  the  evolution  of  gas 
has  ceased  the  gelatinous  mass  in  the  bottle  should  be  thrown  upon  a 
piece  of  cotton  cloth,  and  the  liquid  contained  in  it  separated  from  the 
solid  matter  by  pressure.  The  cloudy  liquor  thus  obtained  may  be 
rendered  clear  by  filtering  it  through  paper.  The  clear  liquid  is  a 
25 


386  FLUOSILICATES. 

strong  aqueous  solution  of  fluosilicic  acid,  and  the  solid  matter  is  hy- 
drated  silicic  acid. 

475.  Fluosilicic  acid  is  known  only  in  aqueous  solution.  The 
saturated  solution  is  a  transparent,  colorless,  fuming,  and  very 
acid  liquid,  which  has  no  action  upon  glass,  and  may  consequent- 
ly be  kept  in  glass  bottles.  It  cannot  be  distilled  without  suf- 
fering decomposition,  nor  can  it  be  evaporated  beyond  a  certain 
degree  of  concentration  without  breaking  up  into  fluoride  of  sili- 
con and  fluorhydric  acid.  If  the  evaporation  is  conducted  in  a 
glass  vessel,  the  silica  of  the  glass  will  retain  the  fluorhydric  acid, 
and  only  fluoride  of  silicon  and  water  will  be  set  free.  The  re- 
action which  occurs  in  this  case, — 

SiO2  +  2  (2  HF1,  SiFl4)  =  2  H2O  +  3  SiFI4, 
is  just  the  reverse  of  that  which  takes  place  when  fluoride  of  sili- 
.con  is  decomposed  by  water : 

2  H,O  +  3  SiFl4  =  SiO2  +  2  (2HF1,  SiFl4). 
"With  bases  fluosilicic  acid  unites  to  form  compounds  known  as 
fluosilicates,  such  as  the  fluosilicate  of  potassium,  K2SiFl6  = 
2KF1,  SiFl4,  and  fluosilicate  of  barium,  BaSiFl6  =  BaFl2, 
SiFl4,  if  no  excess  of  the  base  be  present ;  but  if  an  excess  of  the 
base  be  added,  silicic  acid  will  be  precipitated,  and  the  whole  of 
the  fluorine  will  unite  with  the  metal  contained  in  the  base  to 
form  a  fluoride.  In  the  first  case,  where  no  excess  of  base  is 
employed,  the  reaction  may  be  thus  represented :  — 

2KHO  +  H2SiFl6  —  2H2O  +  K2SiFl6. 
In  the  second  case,  where  an  excess  of  the  base  is  present, 
6KHO  +  H2SiFl6  =  4H2O  +  Si02  +  6KF16. 
Most  of  the  fluosilicates  are  easily  soluble  in  water,  but  some  of 
them  are  so  nearly  insoluble  that  the  acid  is  sometimes  employed 
as  a  precipitant  of  various  oxides.     It  is  remarkable,  that  pre- 
cisely those  bases,  namely,  the  alkalies,  which  form  soluble  salts 
with   almost   all   the  other   acids,  should   here   yield   insoluble 
compounds.     Fluosilicic  acid  is  in  fact  a  good  reagent  for  the 
detection  of  potassium  ;  and  it  is  valuable  as  a  means  of  remov- 
ing potassium  from  many  of  its  salts,  when  we  desire  to  obtain 


THE    CARBON    GROUP.  387 

in  a  free  state  the  acids  which  these  salts  contain.  (Compare 
§  124.) 

476.  Sulphide  of  Silicon  (SiS2),  occurs  in  white,  needle-like 
crystals,  unalterable  in  dry  air.     It  is  volatile  at  the  tempera- 
ture of  redness,  and  is  decomposed  at  once  by  water,  with  depo- 
sition of  gelatinous  silica  and  evolution  of  sulphydric  acid  :  — 

SiS2  +  2H2O  =  SiO2  +  2H2S. 

Like  sulphide  of  boron,  it  may  be  obtained  by  passing  the  vapor 
of  bisulphide  of  carbon  over  a  mixture  of  silicic  acid  and  carbon 
heated  to  redness. 

477.  As  has  appeared  abundantly  from  the  foregoing,  the  three 
elements,  carbon,  boron,  and   silicon  constitute  a   distinct  nat- 
ural family.     This  family  differs  in  character   from   each  and 
every  one  of  the  other  natural  groups  of  elements  hitherto  de- 
scribed (§§  152,  257,  364).     The  occurrence  of  each  of  its  mem- 
bers in  the  three  distinct  and  extraordinary  modifications,  —  as 
diamond,  graphite,  and  charcoal,  —  to  which  allusion  has  been  so 
often  made,  distinguishes  this  group  from  all  others.     The  emi- 
nently refractory  nature  of  the  several  members,  and  their  fixity 
as  regards  heat,  are  marked  characteristics  of  the  three  elements. 
The  verifiable  character  of  the  oxides  of  boron  and  silicon  (bora- 
cic  and  silicic  acids),  and  of  the  borates  and  silicates  of  many  of 
the  metals,  is  a  noteworthy  resemblance  between  these  elements. 
Remarkable  resemblances  between  the  hydrated  carbonate,  bo- 
rate,  and  silicate  of  sodium  often  manifest  themselves  to  the  chem- 
ical manipulator.     The  corresponding  compounds  of  carbon,  bo- 
ron, and  silicon,  with  the  members  of  the  chlorine  group,  and 
with  the  members  of  the  sulphur  group,  have  similar  properties, 
result  from  like  reactions,  and  suffer,  for  the  most  part,  analogous 
decompositions ;  but,  in  the  present  state  of  the  science,  only  the 
compounds  of  carbon  and  silicon  can  be  said  to  have  also  the 
same  atomic  constitution.     The  three  members  of  this  group  are 
not  arranged  in  the  order  of  their  atomic  weights,  as  has  been  the 
case  with  all    the  precedinggroups  ;  but  if,  in  the  future,  boracic 
acid  comes  to  be  written  BO2,  the  atomic  weight  of  boron  will  be 
14,  and  therefore  higher  than  that  of  carbon.   The  existing  anom- 
aly in  the  arrangement  of  the  group  will  then  disappear.     The 


388  SODIUM. 

collocation  of  these  three  elements,  and  the  order  in  which  they 
now  stand,  is  based  upon  too  many  natural  resemblances  to  be 
lightly  set  aside. 


CHAPTER    XXIII. 

SODIUM. 

1 

478.  This  abundant  element  is  chiefly  found  in  nature  in  the 
state  of  chloride,  nitrate,  carbonate,  borate,  and  silicate.  The 
most  abundant  of  its  compounds  is  common  salt,  which  is  the 
combination  of  sodium  with  chlorine  (NaCIV  Sea-water  con- 
tains two  and  a  half  per  cent,  of  salt,  and  enormous  deposits  of 
the  same  substance  are  found  in  the  solid  crust  of  the  earth. 
There  are  also  many  natural  salt-springs,  whose  waters  yield  on 
evaporation  the  chloride  of  sodium  which  they  hold  in  solution. 
Sodium  also  occurs,  in  the  condition  of  silicate,  in  very  many 
common  minerals  and  rocks.  From  the  soil  which  has  resulted 
from  the  disintegration  and  decomposition  of  these  minerals  and 
rocks,  and  from  its  soluble  compounds,  like  common  salt,  sodium 
enters  into  plants,  and  thence  into  animals.  Its  chloride  is  one 
of  the  essential  mineral  constituents  of  the  food  of  man  and  other 
animals.  On  account  of  the  inexhaustible  abundance  of  common 
salt,  this  substance  constitutes  the  chief  source  from  which  all 
manufactured  compounds  of  sodium  are  more  or  less  directly  de- 
rived; one  other  natural  sodium-containing  mineral,  however, 
deserves  mention  as  a  source  of  sodium-compounds,  —  the  mineral 
Cryolite,  —  a  double  fluoride  of  sodium  and  aluminum.  Nitrate 
of  sodium  (NaNO3),  a  somewhat  deliquescent  and  very  soluble 
salt,  occurs  abundantly  on  the  surface  of  the  soil  in  certain  desert 
districts  of  Peru.  When  heated,  this  salt  first  fuses  and  then 
undergoes  decomposition.  It  is  employed  in  the  manufacture  of 
nitric  and  sulphuric  acids  and  as  a  manure ;  but  as  a  source  of 
sodium-compounds  it  is  comparatively  insignificant. 


CHLORIDE    OF    SODIUM.  389 

479.  Chloride  of  Sodium  (NaCl).  There  is  but  one  chloride 
of  sodium,  —  common  salt.  This  natural  mineral  is,  when  pure, 
a  colorless,  transparent,  anhydrous  stone,  which  crystallizes  in 
cubes,  dissolves  readily  in  about  three  times  its  weight  of  cold 
water,  and  possesses  a  specific  gravity  of  2.15,  and  an  agreeable 
taste,  which,  because  familiar,  is  the  representative  or  type  of 
that  peculiar  savor  called  saline.  A  saline  taste  means  a  taste 
suggestive  .of  that  of  common  salt,  just  as  the  phrase,  "saline 
substance,"  characterizes  a  very  large  class  of  bodies  which  re- 
semble more  or  less  in  appearance  and  properties  the  longest  and 
best  known  of  all  such  substances,  —  common  salt. 

There  are  three  sources  of  salt,  — the  beds  of  the  native  mineral,  sa- 
line springs,  and  sea-water.  In  all  cases  in  which  the  salt  is  obtained 
from  its  solution  in  water,  evaporation  by  fire,  or  by  the  heat  of  the 
sun  in  warm,  sunny  climates,  is  necessary.  When  pure  enough,  the 
rock-salt  is  mined  like  any  other  ore,  but  when  it  is  mixed  with  earth 
or  other  impurities,  as  it  lies  in  its  natural  bed,  the  solubility  of  the 
chloride  of  sodium  in  water  is  availed  of  to  free  the  salt  from  its  insolu- 
ble impurities,  and  to  facilitate  the  lifting  of  it  to  the  surface  of  the 
earth.  Water  is  let  in  to  the  bed  of  salt,  and  allowed  to  remain  there 
till  it  has  become  saturated ;  the  brine  is  then  pumped  out  and  evapo- 
rated. Some  natural  brine-springs  contain  so  small  a  proportion  of 
salt  that  some  cheaper  mode  of  evaporation  than  by  fire  is  essential  to 
their  profitable  working.  Such  waters  are  concentrated  by  a  process 
termed  graduation.  The  brine  is  pumped  up  to  a  sufficient  height,  and 
then  allowed  to  trickle  slowly  over  large  stacks  of  fagots,  which  are 
sheltered  by  a  roof  from  rain,  but  are  freely  exposed  to  the  prevailing 
wind.  The  brine,  thus  diffused  over  a  very  large  surface,  is  rapidly 
concentrated  by  the  draft  of  air.  By  repeating  the  process  a  moderate 
number  of  times,  a  weak  brine  may  be  brought  to  a  degree  of  concen- 
tration at  which  evaporation  by  fire  may  be  employed.  Since  almost 
all  brines  contain  sulphate  of  calcium  (CaSO4)  in  solution,  the  surfaces 
of  the  fagots  employed  in  the  graduation  process  becomes  covered  with 
a  stony  coating  of  this  comparatively  insoluble  salt.  If  the  strong  brine 
is  boiled  down  rapidly,  a  fine-grained  table-salt  is  obtained ;  if  it  is 
slowly  evaporated,  a  hard,  coarsely  crystallized  salt  is  the  product. 
During  the  earlier  stages  of  the  evaporation  a  deposit  is  formed,  con- 
sisting principally  of  sulphate  of  sodium  and  sulphate  of  calcium. 
Finally,  there  remains  a  thick  mother-liquor,  from  which  no  more  chlo- 
ride of  sodium  will  crystallize,  but  which  contains  the  more  soluble  salts 
of  the  original  brine,  such  as  chloride  of  calcium,  and  chloride  and 


390       j  SOLUBILITY    OF    SALT. 

bromide  of  magnesium,  besides  a  large  proportion  of  common  salt  which 
cannot  be  separated  from  the  liquor.  Such  mother-liquors  are  some- 
times so  rich  in  magnesium-salts  as  to  be  advantageously  worked  for 
these  substances,  and  they  are  also  sometimes  profitable  sources  of  bro- 
mine. Considerable  quantities  of  magnesium-salts  and  of  bromine  have 
also  been  extracted  from  concentrated  sea-water,  after  all  the  available 
chloride  of  sodium  has  been  withdrawn.  The  salt  of  commerce  gener- 
ally contains  a  small  proportion  of  chloride  of  magnesium,  which  makes 
it  slightly  deliquescent  and  bitter. 

Exp.  229.  —  Heat  a  few  crystals  of  coarse  salt  on  a  piece  of  sheet-iron 
over  the  gas-lamp.  The  crystals  will  decrepitate  forcibly,  and  the 
greater  part  of  the  salt  will  be  thrown  off  the  plate  ;  what  remains  will 
melt  as  the  temperature  rises,  and  if  the  heat  be  strong  enough,  it  will 
finally  volatilize.  The  decrepitation  is  due  to  little  particles  of  water, 
mechanically  inclosed  in  the  crystals,  which,  when  expanded  by  heat, 
burst  the  crystals  asunder. 

Exp.  230. —  Dissolve  9  grms.  of  fine  salt  in  25  c.  c.  of  water  at  about 
20°.  Add  to  the  solution  another  gramme  of  salt ;  it  will  not  dissolve. 
Bring  the  solution  to  boiling ;  the  added  gramme  of  salt  will  barely  dis- 
solve. Chloride  of  sodium  is  scarcely  more  soluble  in  hot  than  in  cold 
water,  wherein  it  differs  from  the  great  majority  of  soluble  salts.  Evap- 
orated brines  deposit  their  salt  with  almost  equal  facility  when  hot  and 
when  cold,  but  the  hot  liquors  will  hold  in  solution  a  much  greater 
proportion  of  the  salts  with  which  the  chloride  of  sodium  is  associated, 
than  the  cold  brines  could  retain.  In  the  process  of  evaporation  by 
fire,  the  associated  magnesium,  calcium,  and  sodium-salts  do  not,  there- 
fore, crystallize  with  the  common  salt,  but  remain  in  the  hot  mother- 
liquor. 

Exp.  231.  —  Expose  a  saturated  solution  of  salt  in  winter  weather 
to  a  temperature  of — 10°.  Large,  transparent,  six-sided  tables,  which 
contain  a  considerable  proportion  of  water,  chemically  combined,  will 
crystallize  from  the  solution.  The  warmth  of  the  hand  is  sufficient  to 
destroy  this  crystalline  compound ;  the  water  separates,  and  the  crys- 
tals are  resolved  into  a  mass  of  minute  cubes. 

480.  The  uses  of  common  salt  are  manifold  ;  since  it  is  a  con- 
stituent of  almost  all  kinds  of  food,  and  essential  to  the  life  of 
animals,  it  is  not  surprising  that  salt  exists  in  small  quantities  in 
almost  every  spring,  soil,  plant,  and  animal.  The  antiseptic  qual- 
ity of  salt  is  applied  to  the  preservation  of  fish,  meat,  and  wood. 
Salt  is  extensively  employed  in  glazing  earthen-ware,  its  volatility 
at  furnace-heat  (Exp.  229)  combining  with  other  qualities  to  fit  it 


SULPHATE    OF    SODIUM.  391 

for  this  use.  Immense  quantities  of  salt  are  consumed  in  preparing 
sulphate  of  sodium,  from  which  in  turn  common  "  soda  "  (carbon- 
ate of  sodium)  is  made.  From  the  carbonate  of  sodium  thus  ob- 
tained the  greater  number  of  other  sodium  compounds  are  pre- 
pared. Salt  is  also  the  source  from  which  chlorhydric  acid  and 
chlorine  are  derived  (§§  101,  105). 

481.  Bromide  and  Iodide  of  Sodium  (NaBr  and  Nal).    These 
salts  bear  a  close  resemblance  to  the  chloride  of  sodium ;  they 
both  crystallize  in  anhydrous  cubes,  and  both  occur  native  in  sea- 
water,  though  in  minute  proportion.     Many  marine  plants  appro- 
priate iodide  of  sodium  from  sea-water;  sea-weeds  are,  therefore, 
the  commercial  source  of  iodine  (§  135.) 

482.  Sulphate  of  Sodium  (Na2S04).    This  compound  is  made 
in  great  quantities  from  common  salt  and  sulphuric  acid  as  a  pre- 
liminary step  in  the  manufacture  of  carbonate  of  sodium. 

The  process  has  two  stages.  The  first  operation  is  performed  in. 
large,  covered,  cast-iron  pans,  capable  of  holding  250  kilos,  of  salt,  and 
an  equal  weight  of  sulphuric  acid  of  the  density  of  1.7.  A  very  gentle 
heat  suffices  to  disengage  from  such  a  mixture  enormous  volumes  of 
chlorhydric  acid  gas;  this  gas,  which  would  be  injurious  to  vegetation 
if  suffered  to  escape  into  the  air,  is  all  absorbed  by  being  passed  through 
vertical  stone  towers,  filled  with  lumps  of  coke,  over  which  water  is  kept 
trickling.  The  reaction  in  the  iron  pan  is  by  no  means  complete,  much 
chloride  of  sodium  remaining  undecomposed.  The  reaction  at  this  first 
stage  may  be  represented  as  follows :  — 

2  NaCl  +  H2S04  =  NaCl  +  NaHSO4  +  HC1. 
The  pasty  mass  is  then  pushed  into  an  adjoining  fire-brick  chamber, 
which  is  strongly  heated  by  Hues  from  a  furnace.  The  acid  sulphate 
of  sodium,  of  the  •last  reaction,  decomposes  the  remainder  of  the  salt, 
and  a  further  quantity  of  chlorhydric  acid  is  disengaged  to  be  con- 
densed by  the  water  in  the  coke-towers,  while  sulphate  of  sodium  re- 
mains :  — 

NaCl  -f  NaHSO,  =  Na2SO4  -f  HC1. 

The  sulphuric  acid  used  in  this  process  is  not  stronger  than  can  readily 
be  made  by  evaporation  in  leaden  pans  (compare  §  230)  ;  it  is  always 
made  on  the  spot.  The  crude,  weak,  chlorhydric  acid,  which  is  the  in- 
cidental product  of  the  manufacture  of  sulphate  of  sodium,  has  of  course 
some  value  in  the  arts ;  a  portion  of  the  product  is  often  immediately 
consumed  in  the  same  factory  in  the  manufacture  of  "  bleaching  pow- 


392  GLAUBER'S  SALT. 

der"  (§§  105,  120.)  In  some  works  the  smoke  and  gases  from -the  fires 
pass  through  the  coke-towers  in  which  the  chlorhydric  acid  is  absorbed ; 
in  others,  the  products  of  combustion  are  not  suffered  to  mix  with  the 
liberated  chlorhydric  acid,  but  are  conducted  by  separate  flues  around 
the  pans  and  chambers  to  be  heated,  and  thence  into  the  main  chim- 
ney. The  latter  mode  of  construction  is  the  best. 

The  sulphate  of  sodium,  resulting  from  this  process,  is  a  white, 
anhydrous  salt,  which  dissolves  easily  in  water  at  30°.  When  a 
strong  solution  of  the  anhydrous  salt,  made  at  this  temperature, 
is  cooled,  there  separate  large,  colorless  crystals  of  a  transparent 
salt,  bitter  and  cooling  to  the  taste.  This  salt,  long  known  as 
Glauber's  salt,  contains,  besides  the  elements  of  sulphate  of  so- 
dium, ten  molecules  of  water;  it  therefore  answers  to  the  formula, 
Na2S04,10H;iO.  There  is  another  hydrated  sulphate  of  sodium 
which  contains  only  seven  molecules  of  water.  These  hydrated 
salts  effloresce  in  dry  air,  and  crumble  into  an  opaque  powder  of 
the  anhydrous  salt. 

Exp.  232. —  Place  10  grms.  of  crystallized  Glauber's  salt  in  a  warm, 
dry  place.  When  it  is  completely  converted  into  a  white  powder, 
weigh  the  residue.  Since  Glauber's  salt  is  more  than  half  water,  the 
dry  residue  will  not  weigh  more  than  4.5  grms. 

Exp.  233.  —  In  a  flask  holding  about  250  c.  c.,  heat  50  c.  c.  of  water 
to  a  temperature  of  33°,  and  keep  the  water  at  this  temperature,  as  de- 
termined by  a  thermometer  immersed  in  it.  Add  to  the  warm  water 
161  grms.  of  crystallized  Glauber's  salt.  If  this  saturated  solution  be 
made  hotter,  the  anhydrous  sulphate  of  sodium  crystallizes  out  in  octa- 
hedrons of  rhombic  base ;  if,  on  the  other  hand,  the  solution  be  suffered 
to  cool,  crystals  of  the  common  hydrated  Glauber's  salt  appear.  This 
example  forcibly  illustrates  the  general  fact  that  the  relations  of  water 
to  other  bodies  are  greatly  affected  by  temperature.  • 

Exp.  234.  —  Dissolve  10  grms.  of  crystallized  Glauber's  salt  in  water, 
of  which  the  temperature  has  been  previously  observed ;  during  solu- 
tion, the  temperature  falls,  —  cold  is  produced  in  consequence  of  the 
expenditure  of  some  of  the  heat  of  the  mixture  in  overcoming  the  co- 
hesion of  the  crystallized  salt.  Dissolve  a  like  quantity  of  effloresced 
Glauber's  salt  (anhydrous  sulphate  of  sodium)  in  a  small  bulk  of  water ; 
heat  will  be  developed.  A  part  of  the  water  is  solidified  by  combining 
with  the  anhydrous  sulphate  to  form  the  hydrated  sulphate,  and  the 
heat,  which  before  kept  that  quantity  of  water  fluid,  being  set  free  to 


SUPERSATURATED  SOLUTIONS.  393 

do  other  work,  raises  by  a  certain  amount  the  temperature  of  the  mix- 
ture. 

Exp.  235.  —  Dissolve  50  grms.  of  Glauber's  salt  in  25  c.  c.  of  water 
in  a  small  flask,  by  heating  the  contents  of  the  flask  until  the  liquid 
boils.  Cover  the  mouth  of  the  flask  loosely  with  a  card  or  piece  of 
glass,  and  allow  the  liquid  to  cool,  at  perfect  rest,  to  the  ordinary  tem- 
perature. No  crystals  will  be  deposited  from  the  fluid,  although  the 
water  holds  in  solution  a  much  larger  quantity  of  salt  than  it  could  dis- 
solve at  the  atmospheric  temperature.  Such  a  solution  is  said  to  be 
supersaturated.  The  crystallization  of  such  a  solution  may  generally 
be  brought  about,  almost  instantaneously,  by  jarring  the  vessel  which 
contains  it,  or  by  permitting  some  foreign  body,  like  a  glass  rod,  a  wire, 
a  crystal  of  the  salt,  or  a  grain  of  dust,  to  come  in  contact  with  the 
fluid.  By  touching  with  a  stick  or  glass  rod  the  clear  solution  prepared 
as  above  described,  this  sudden  crystallization  will  be  strikingly  illus- 
trated, the  whole  mass  becoming  solid. 

Crystallized  sulphate  of  sodium  is  rapidly  soluble  in  chlorhydric  acid 
with  great  depression  of  temperature.  A  convenient  refrigerating  mix- 
ture may  be  prepared  in  climates  where  ice  is  dear,  by  pouring  5  parts 
of  the  commercial  acid  upon  8  of  the  crystallized  sulphate.  The  effects 
of  solution  on  temperature  and  the  phenomena  of  supersaturated  solu- 
tions, though  well  exhibited  by  sulphate  of  sodium,  are  by  no  means 
peculiar  to  this  substance ;  they  are  manifested  to  a  greater  or  less  de- 
gree by  a  large  number  of  salts. 

483.  The  Double  Sulphate  of  Sodium  and  Hydrogen  (NaHSO4) 
is  a  very  acid  salt,  to  which  the  name  of  bisulphate  is  commonly 
applied.     When  heated  it  first  gives  up  a  molecule  of  water,  and 
subsequently,  at  a  higher  temperature,  a  molecule  of  anhydrous 
sulphuric  acid;  the  anhydrous  salt  is  therefore  employed  as  a 
convenient  source  of  the  teroxide  of  sulphur. 

2  (NaHSO4)  =  H2O  +  S03  +  Na2S04. 

The  formation  of  this  double  sulphate  marks  an  intermediate 
stage  in  the  making  of  sulphate  of  sodium  and  chlorhydric  acid 
from  common  salt,  and  it  is  the  residue  of  the  nitric  acid  manu- 
facture, whenever  nitrate  of  sodium  is  employed. 

484.  Carbonate  of  Sodium  (Na2C03).     The  manufacture  of 
this  substance  constitutes  one  of  the  most  important  branches  of 
chemical  industry.     Immense  quantities  of  it  are  consumed  in 
the  fabrication  of  glass  and  soap,  in  the  preparation  of  the  vari- 


394  CARBONATE    OF    SODIUM. 

ous  compounds  of  sodium,  and  in  washing,  both  by  the  manufac- 
turer of  cloth  and  in  the  household.  The  ashes  of  sea  and  sea-shore 
plants  were  formerly  the  source  of  the  carbonate  of  sodium,  but 
it  is  now  chiefly  made  from  common  salt  by  a  process  called, 
from  the  name  of  its  French  inventor,  the  process  of  Leblanc. 

The  first  stage  of  this  process  we  have  already  studied ;  it  consists  in 
the  preparation  of  the  sulphate  of  sodium  from  common  salt.  The 
second  stage  consists  in  the  reduction  of  this  sulphate  to  the  condition 
of  sulphide  of  sodium  in  presence  of  the  carbonate  of  calcium  ;  by  in- 
terchange of  metals  there  results  sulphide  of  calcium  and  carbonate  of 
sodium.  The  sulphate  of  sodium  is  ground  up  with  an  equal  weight  of 
chalk  and  rather  more  than  half  its  weight  of  coal,  and  the  mixture  is 
thoroughly  melted  by  the  flame  of  a  reverberatory  furnace.  The  black 
mass,  which  is  called  "  black  ball  "  or  "  black  asli,"  is  cast  into  blocks 
in  iron  wheelbarrows,  cooled,  broken  up,  and  systematically  washed 
with  warm  water  until  all  the  soluble  portions  are  extracted.  The 
black  solution  is  evaporated  in  large  iron  pans  by  the  waste-heat  of  the 
reverberatory  furnaces.  The  residue  contains  some  caustic  soda, 
mixed  with  the  carbonate  ;  the  residue  is  therefore  mixed  with  about 
iUh  of  its  weight  of  sawdust,  or  like  material,  and  roasted  in  a  reverber- 
atory furnace.  The  product  of  this  heating  is  the  soda-ash  of  com- 
merce; it  is  almost  white,  and  generally  contains  about  80  per  cent,  of 
pure  anhydrous  carbonate  of  sodium. 

The  so-called  crystals  of  soda  are  obtained  by  dissolving  the  crude 
soda-ash  in  hot  water,  and  suffering  the  hot  solution  to  cool  in  large 
pans.  In  the  course  of  five  or  six  days,  large  transparent  crystals  are 
formed  which  contain  62.93  per  cent,  of  water,  and  correspond  to  the 
formula  Na.2CO3,10H2O.  The  impure  mother-liquor,  drained  from  the 
crystals,  is  used  in  the  manufacture  of  caustic-soda.  These  crystals 
effloresce  in  the  air ;  they  have  a  disagreeable  taste,  called  alkaline, 
are  soluble  in  very  large  proportion  both  in  hot  and  cold  water,  and 
even  melt  at  a  moderate  temperature  in  their  own  water  of  crystalliza- 
tion. The  crystals  readily  part  with  all  their  water,  and  the  dry 
residue  melts  at  a  bright  red  heat ;  this  residue  is  anhydrous  car- 
bonate of  sodium,  purified  by  the  process  of  crystallization  which  it 
has  undergone.  In  this  case,  as  in  all  others,  the  process  of  crystalliza- 
tion consists  essentially  in  the  aggregation  of  like  particles ;  the  strong 
tendency  is  to  exclude  heterogeneous  particles,  or,  in  other  words,  im- 
purities, from  the  crystallizing  structure.  There  is  no  more  universally 
applicable  and  valuable  means  of  purification  than  the  process  of  crys- 
tallization. 


BICARBONATE    OF    SODIUMc  395 

The  purchaser  of  carbonate  of  sodium  for  the  sake  of  the  alkali  which 
it  contains  will  prefer  soda  ash  to  soda  crystals,  unless  the  purity  of  the 
material  be  an  important  consideration.  The  crystals  are  purer  than 
the  ash,  but  more  than  half  their  weight  is  water,  which  must  be  trans- 
ported at  the  cost  of  the  consumer.  Effloresced  crystals  are  more  ad- 
vantageous to  buy  by  weight  than  crystals  which  have  not  lost  their 
water  by  exposure  to  the  air.  There  are  several  hydrates  of  carbonate 
of  sodium  of  different  solubilities. 

485.  Double  Carbonate  of  Sodium  and  Hydrogen  (NaHC03). 
When  masses  of  crystals  of  hydrated  carbonate  of  sodium  (soda 
crystals)  are  exposed  to  an  atmosphere  of  carbonic  acid  gas,  they 
absorb  carbonic  acid  with  an  evolution  of  heat  sufficient  to  expel  the 
greater  part  of  their  water  of  crystallization.  A  white  powder  re- 
mains, wliose  dualistic  formula  is  Na2O,H2O,2C02;  whence  its 
most  familiar  name  —  bicarbonate  of  soda.  This  substance  is 
one  of  the  ingredients  in  most  of  the  artificial  yeasts  used  for  rais- 
ing bread,  cake,  and  puddings,  and  is  known  to  grocers  and  cooks 
as  "  soda,"  although  the  constituent  which  is  really  utilized  is  its 
carbonic  acid.  This  carbonate  of  sodium  and  hydrogen  is  much 
less  soluble  than  the  carbonate  of  sodium  ;  it  may  be  washed  with 
cold  water  until  it  is  freed  from  the  sulphates  and  chlorides  which 
generally  contaminate  the  carbonate.  The  chemist  resorts  to  this 
process,  in  order  to  prepare  from  the  purified  bicarbonate  pure 
sodium-salts.  At  a  low  red  heat  the  double  carbonate  loses  its 
water  and  half  its  carbonic  acid,  and  is  converted  into  the  normal 
carbonate  (Na2C03).  If  the  aqueous  solution  of  the  double  car- 
bonate be  heated  it  loses  one  quarter  of  its  carbonic  acid. 

Weigh  out  two  separate  and  equal  portions  of  bicarbonate  of  sodium; 
Heat  one  of  these  portions  for  a  few  minutes  in  an  iron  spoon,  or  por- 
celain capsule,  to  a  low  red  heat.  Then  wrap  each  portion  in  a  piece 
of  paper  and  introduce  each  little  roll  into  a  cylindrical  jar,  filled  with 
mercury  and  standing  inverted  on  the  mercury  trough.  By  means  of 
a  curved  pipette  (see  Appendix,  §  22)  pass  equal  quantities  of  dilute 
sulphuric  acid  into  each  of  the  jars.  The  moment  the  sulphuric  acid 
comes  in  contact  with  the  carbonates  of  sodium,  carbonic  acid  is 
evolved,  and  upon  comparing  the  volumes  of  gas  yielded  by  the  two 
samples,  both  of  which  contain  the  same  quantity  of  sodium,  it  will  be 
found  that  the  carbonic  acid  evolved  from  the  unheated  bicarbonate  is 
twice  as  great  as  that  yielded  by  the  portion  which  was  ignited. 


396  YEAST    POWDERS. 

Bicarbonate  of  sodium  may  be  deprived  of  its  carbonic  acid  by 
almost  any  acid  or  acid  salt.  In  the  experiment  just  described 
sulphuric  acid  displaced  the  gaseous  carbonic  acid;  tartaric  acid, 
or  an  acid  tartrate  like  cream  of  tartar  (tartrate  of  potassium), 
will  effect  the  same  displacement,  as  will  also  the  acid  sulphate  of 
sodium,  NaHSO4,  the  acid  phosphate  of  calcium,  CaO,2H2O,p2O5, 
or  common  alum.  "  Soda  powders"  should  be  made  of  bicarbon- 
ate of  sodium  and  tartaric  acid.  "  Rochelle  powders  "  consist  of 
bicarbonate  of  sodium  in  one  paper  and  cream  of  tartar  in  another; 
when  these  two  materials  are  mixed  in  water,  carbonic  acid  is 
set  free,  and  a  double  tartrate  of  sodium  and  potassium,  called 
Rochelle  salt,  and  used  as  a  purgative,  remains  in  the  liquid. 
When  bread  or  cake  is  "raised"  with  "soda"  and  cream  of 
tartar,  the  escaping  carbonic  acid  is  the  agent  in  puffing  up  the 
dough,  and  the  same  Rochelle  salt  remains  in  the  bread.  Tar- 
taric acid  and  cream  of  tartar  having  been  dear  in  late  years,  a 
cheaper  chemical  yeast  powder  has  been  made  from  acid  phos- 
phate of  calcium  ;  when  this  substance  reacts  within  the  dough 
with  bicarbonate  of  sodium,  there  remains  in  the  bread  a  mixture 
of  the  phosphates  of  sodium  and  calcium.  Alum  is  sometimes 
used  for  the  same  purpose.  It  is  necessary  to  employ  for  such 
purposes,  in  connection  with  the  bicarbonate,  acids  or  acid  salts 
which  are  solid,  and  not  so  corrosive  as  to  be  obviously  dangerous 
and  harmful. 

There  exists  a  native  sesqui-carbonate  of  sodium  called  trona 
or  natron  ;  it  is  a  saline  efflorescence,  always  contaminated  with 
the  sulphate  and  chloride  of  sodium,  and  less  soluble  in  water 
than  the  carbonate,  but  more  soluble  than  the  bicarbonate  of 
sodium.  All  the  carbonates  of  sodium  have  an  alkaline  reaction 
on  vegetable  colors ;  carbonic  acid  is  too  weak  an  acid  to  over- 
come the  intensely  alkaline  reaction  of  caustic  soda. 

486.  Sulphides  of  Sodium.  In  the  conversion  of  sulphate  of 
sodium  into  carbonate  of  sodium  (§  483)  one  step  is  the  reduction 
of  the  sulphate  to  the  sulphide  of  sodium  by  coal.  The  oxygen 
of  the  sulphuric  acid  used  in  making  the  sulphate  is  thus  com- 
bined with  carbon  and  lost ;  the  sulphur  of  the  acid  goes  to  the 
waste-heap  in  a  useless  combination  with  calcium.  Herein  lies 
the  wastefulness  of  the  Leblanc  process,  which  the  experience  of 


SULPHIDES    OF    SODIUM.  397 

Exp.  236.  —  Mix  a  little  powdered  anhydrous  sulphate  of  sodium, 
obtained  by  drying  Glauber's  salt,  with  as  much  powdered  charcoal, 
and  make  the  mixture  into  a  paste  with  a  drop  of  water.  Place  a  little 
ball  of  this  paste,  as  large  as  a  small  pea,  in  a  depression  in  a  piece  of 
charcoal,  and  heat  it  strongly  with  the  blow-pipe.  The  mixture  effer- 
vesces and  finally  melts  into  a  brownish  mass.  The  glowing  coal  takes 
all  the  oxygen  out  of  the  sulphate,  and  the  carbonic  oxide,  which  results 
from  the  union  of  the  carbon  and  oxygen,  in  escaping  causes  the  effer- 
vescence which  occurs  ;  the  sodium  and  sulphur  alone  remain  united. 
The  reaction  may  be  thus  formulated  :  — 


+  40  =  4CO  +  Na,S. 

If  the  brownish  solid  be  removed  from  the  coal,  placed  in  a  watch-glass, 
and  moistened  with  dilute  chlorhydric  or  sulphuric  acid,  sulphydric 
acid  gas  will  be  evolved,  as  may  be  recognized  by  the  smell,  or  by  the 
use  of  lead-paper  :  — 

Na2S  +  2HC1  =  2NaCl  -f  H2S. 

Sulphur  unites  with  sodium  in  more  than  one  proportion,  and 
it  may  indeed  be  doubted  whether  the  actual  reaction  in  the 
above  experiment  is  so  simple  as  the  formula  represents  it  to  be. 
There  probably  exist  five  distinct  sulphides  of  sodium,  Na2S, 
Na2S2,  Na2S8,  Na2S4  and  Na2S5.  All  these  sulphides  have  an  al- 
kaline reaction  to  test-paper,  and  evolve  a  more  or  less  distinct 
odor  of  sulphuretted  hydrogen.  When  they  are  brought  in  con- 
tact with  an  acid  they  are  decomposed,  sulphydric  acid  escapes, 
and  a  white  precipitate  of  finely  divided  sulphur  falls  in  every 
case,  except  that  of  the  first  sulphide,  Na2S.  Besides  these  sul- 
phides, a  compound  of  sodium,  hydrogen,  and  sulphur  (NaHS) 
is  known,  which  is  perfectly  analogous  in  composition  to  the  com- 
bination of  sodium,  hydrogen,  and  oxygen,  with  which  we  are  al- 
ready familiar  under  the  name  of  caustic  soda  (NaliO). 

487.  Sodium  (Na).  The  element  sodium  is  never  found  un- 
combined  in  nature,  for  the  reason  that  in  its  elementary  condi- 
tion it  cannot  exist  in  contact  with  either  air  or  water.  It  is, 
however,  artificially  prepared  from  the  carbonate  of  sodium  with- 
out serious  difficulty,  and  it  might  be  produced  in  considerable 
quantities,  if  there  were  any  large  use  for  the  element. 

A  mixture  of  20  parts  of  carbonate  of  sodium,  9  parts  of  coal,  and 
3  parts  of  chalk,  placed  in  an  iron  bottle  or  cylinder,  is  heated  to  a  very 


398  METALLIC    SODIUM. 

high  temperature  in  a  suitable  furnace ;  a  narrow  iron  tube  connects 
the  bottle,  or  cylinder,  in  the  furnace,  with  a  flat  sheet-iron  box  outside 
the  furnace ;  this  box  receives  the  sodium  which  distils  from  the  hot 
mixture,  and  a  hole  in  the  front  lower  corner  of  the  receptacle  permits 
the  melted  metal  to  fall  into  a  vessel  containing  petroleum-naphtha,  be- 
neath which  the  sodium  can  be  preserved.  The  reaction  is  theoreti- 
cally a  conversion  of  all  the  oxygen  in  the  carbonate  of  sodium  into 
carbonic  oxide,  partly  by  a  new  combination  with  the  carbon  in  the 
carbonate,  and  partly  by  union  with  the  carbon  which  the  coal  supplies, 

Na^CO,  +  2C  =  3CO  +  2Na, 

but  the  amount  of  sodium  practically  obtained  from  a  given  weight  of 
the  materials  is  by  no  means  as  large  as  this  simple  formula  would  indi- 
cate. The  chalk  has  no  chemical  effect,  but  is  practically  essential  to 
the  success  of  the  operation.  It  prevents  the  mass  from  melting,  and 
the  gas  which  it  gives  off  when  heated  assists  in  sweeping  the  sodium 
out  of  the  bottle  into  the  receiver.  To  purify  .the  crude  sodium  thus 
obtained,  it  is  melted  under  naphtha  and  cast  into  ingots  in  iron  moulds. 
It  must  be  kept  under  naphtha  in  tightly  closed  bottles. 

The  properties  of  the  element,  sodium,  are  very  curious.  The 
substance,  when  freshly  cut,  or  when  melted  under  naphtha  or  in 
an  atmosphere  artificially  deprived  of  oxygen,  has  the  brilliant, 
white  metallic  lustre  of  silver.  Though  possessing  so  eminent- 
ly this  characteristic  property  of  the  class  of  bodies  called 
metals,  and  being  like  them  a  good  conductor  of  heat  and  elec- 
tricity, sodium  is  far  from  resembling  the  ordinary  metals  in 
other  respects ;  thus  it  is  lighter  than  water,  having  a  specific 
gravity  of  only  0.972,  whereas  the  common  metals  are  dense  and 
heavy ;  again,  it  is  as  soft  as  wax  at  common  temperatures,  and 
melts  at  a  temperature  below  that  of  boiling  water,  while  it  has 
none  of  the  comparative  permanence  which  characterizes  lead, 
tin,  copper,  silver,  gold  and  other  familiar  metals.  If  exposed  to 
the  air,  even  for  a  few  seconds  only,  it  tarnishes,  and  soon  be- 
comes covered  with  a  coating  of  oxide.  Instead  of  being  quenched 
by  water,  it  takes  fire  when  thrown  into  warm  water.  We  have 
already  seen  that  it  decomposes  cold  water  (Exp.  14),  setting 
free  its  hydrogen,  and  combining  with  its  oxygen. 

Exp.  23  7.  —  Cover  the  bottom  of  a  large  bottle  (at  least  a  litre-bot- 
tle) with  hot  water,  drop  in  a  piece  of  sodium  as  large  as  a  small  pea, 
and  immediately  cover  the  mouth  of  the  bottle  with  a  card  or  glass 


HYDRATE    OF    SODIUM.  399 

plate.  The  heat  of  the  chemical  combination  between  the  sodium  and 
the  oxygen  of  the  water  is  sufficient  to  inflame  the  hydrogen ;  the 
escaping  hydrogen  carries  with  it  a  small  portion  of  the  volatilized  so- 
dium, and  therefore  burns  with  an  intensely  yellow  flame  which  is  very- 
characteristic  of  sodium-compounds.  The  metal  swims  rapidly  about 
on  the  surface  of  the  water,  and  is  completely  converted  into  caustic 
soda ;  at  a  little  interval,  after  the  flame  has  ceased  to  burn,  a  globule 
of  caustic  soda,  which  has  escaped  solution,  bursts  and  scatters  in  all 
directions  ;  the  mouth  of  the  bottle  should  always  be  covered  to  avoid 
the  possible  projection  of  particles  of  hot  soda  out  of  the  bottle.  The 
water  in  the  bottle,  tested  with  litmus  paper,  will  be  found  to  possess  a 
strong  alkaline  reaction.  If  the  bit  of  sodium  be  previously  wrapped 
up  in  muslin,  it  will  take  fire  in  cold  water  or  even  on  ice.  The  muslin 
prevents  the  sodium  from  moving  about,  and  the  heat  of  combination  is 
therefore  concentrated  upon  one  spot.  The  same  effect  may  be  pro- 
duced without  the  muslin  on  cold  water  made  viscid  with  gum. 

At  a  high  temperature  sodium  will  remove  oxygen  from  almost 
all  bodies  which  contain  it,  whether  solid,  liquid,  or  gaseous. 
Hence  the  necessity  of  preserving  the  metal  under  some  liquid 
which,  like  naphtha,  contains  no  oxygen.  Sodium  enters  directly 
into  combination  with  all  the  elements  of  the  chlorine  and  sulphur 
groups,  and  is  capable  of  withdrawing  these  elements  from  nearly 
all  the  compounds  into  which  they  enter.  In  this  substance, 
therefore,  the  chemist  possesses  a  very  potent  agent  for  effecting 
chemical  transformations.  Its  atomic  weight  is  23. 

488.  Hydrate  of  Sodium  (NaHO).  When  sodium  is  burnt 
upon  water,  a  solution  of  hydrate  of  sodium,  possessing  an  in- 
tensely alkaline  reaction,  remains  behind  ;  but  the  hydrate  is,  in 
practice,  made  from  the  carbonate. 

Exp.  238. —  Dissolve  100  grms.  of  crystallized  carbonate  of  sodium 
(§  484)  in  400  c.  c.  of  water.  Slake  20  grms.  of  quick-lime  with  water 
enough  to  make  the  slaked  lime  into  a  cream.  Boil  the  solution  of 
carbonate  of  sodium  in  an  iron  pan,  and  add  to  it,  little  by  little,  the 
cream  of  lime,  until  a  small  portion  of  the  liquor,  filtered  off,  produces 
no  effervescence  when  poured  into  dilute  chlorhydric  or  sulphuric  acid. 
The  calcium  of  the  lime  replaces  the  sodium  in  the  carbonate  of  so- 
dium ;  a  white  insoluble  precipitate  of  carbonate  of  calcium  is  formed, 
and  hydrate  of  sodium  remains  in  the  solution  :  — 

-f  CaH2O2  =  2NaHO  -f  CaCO3. 


400  CAUSTIC    SODA. 

When  this  reaction  is  complete,  no  carbonic  acid  remains  in  the  clear 
solution,  which,  therefore,  causes  no  effervescence  when  mixed  with  an 
acid.  When  this  test  shows  the  reaction,to  be  accomplished,  extinguish 
the  lamp ;  cover  the  pan,  and  let  the  carbonate  of  calcium  settle  to  the 
bottom  of  the  vessel.  After  several  hours,  draw  off  the  clear  super- 
natant liquid  into  a  bottle  by  means  of  a  siphon.  A  weaker  lye  may 
be  obtained  by  boiling  up  the  residue  in  the  iron  pan  once  more  with 
water,  and  again  decanting  the  clear  liquid  from  the  deposited  pre- 
cipitate. 

The  lye  thus  prepared  contains  a  very  little  lime,  inasmuch  as  the 
hydrate  of  calcium  is  not  completely  insoluble  in  a  dilute  solution  of 
hydrate  of  sodium.  If  the  lye  is  needed  stronger,  it  is  only  necessary 
to  concentrate  it  by  evaporation  in  a  clean  iron  pan  to  the  requisite 
strength.  Since  soda-lye  attacks  ground-glass  with  facility,  the  stoppers 
of  bottles  in  which  the  lye  is  kept  are  apt  to  get  stuck  so  fast  that  they 
cannot  be  removed.  Such  a  bottle  may  be  best  closed  with  a  caout- 
chouc-stopper, or  a  cork  soaked  in  melted  wax  or  paraffine. 

In  order  to  obtain  the  solid  hydrate  of  sodium,  its  solution  must  be 
evaporated  in  a  silver  dish,  since  iron  would  color  the  concentrated 
lye,  until  the  liquid  in  the  dish  flows  smoothly  like  oil  at  a  temperature 
near  to  a  red  heat.  This  thick,  oily  liquid  solidifies  when  turned  out 
upon  a  cold  plate  of  metal  or  poured  into  metal  moulds. 

Fused  caustic  soda  is  a  white,  somewhat  translucent  mass, 
whose  composition  corresponds  to  the  formula  NaHO.  Heat 
will  separate  from  it  no  more  water ;  if  the  attempt  be  made,  it 
volatilizes  in  caustic  vapors  without  change  of  constitution.  The 
commercial  caustic  soda  always  contains  much  more  water  than 
the  formula  indicates.  The  solid  hydrate  is  very  soluble  in  water, 
and  greedily  absorbs  both  water  and  carbonic  acid  from  the  air, 
until  the  formation  of  a  coating  of  the  non-deliquescent  carbonate 
of  sodium  arrests  the  process  by  protecting  the  enclosed  hydrate. 
It  is  the  prototype  of  the  class  of  bodies  called  bases.  It  colors 
litmus  blue  and  turmeric  brown,  and  when  mixed  in  due  propor- 
tion with  oxides  of  the  opposite  quality,  called  acid,  a  saline  com- 
pound is  formed  which  is  neither  acid  nor  alkaline,  and  which 
may  bear  no  more  resemblance  to  its  proximate  constituents  than 
bread  bears  to  flour  and  water,  or  rust  to  iron  and  oxygen. 

From  such  reactions  between  acids  and  hydrate  of  sodium,  water 
is  always  disengaged  simultaneously  with  the  saline  product,  and 
the  reaction  may  almost  always  be  as  well  considered  an  inter- 


REPLACEMENT DIRECT    COMBINATION.  401 

change  of  place  between  hydrogen  and  some  other  element,  as  an 
act  of  combination  between  one  oxide  and  another  oxide,  both  of 
which,  or  one  of  which,  contain  also  hydrogen.  The  following 
formulae  will  illustrate  the  meaning  of  this  statement :  — 

NaHO  +  NHO3  =  NaN03  +  H20,  or 

gf   !'}o  +  .^fo  =  g?.}o  +  !}o, 

2  NaHO  +  H2SO4  =  Na2S04  +  2  H20,  or 


NaHO  +  C2H4O2  =  C2H3NaO2  +  H20 . 
Acetic  Acid.     Acetate  of  Sodium. 

While  recognizing  the  frequent  occurrence  of  such  reactions 
as  are  above  represented  between  hydrated  oxides,  it  must  not 
be  forgotten  that  many  anhydrous  saline  compounds  can  be  made 
by  the  direct  combination,  under  appropriate  conditions,  of  two 
oxides  which  contain  no  hydrogen.  By  heating  one  molecule 
of  hydrate  of  sodium,  or  40  parts  by  weight,  with  one  molecule, 
or  23  parts  by  weight,  of  sodium,  an  oxide  of  sodium  is  obtained 
which  contains  no  hydrogen,  but  this  body  has  none  of  the  prop- 
erties described  by  the  adjective  alkaline,  any  more  than  the 
anhydrous  teroxide  of  sulphur  possesses  the  properties  suggested 
to  the  mind  by  the  term  "  acid  "  :  — 

NaHO  +  Na  =  Na2O  +  H. 

Now,  the  very  same  sulphate  of  sodium  which  results  from  the 
second  of  the  above  reactions,  may  be  prepared  by  bringing  to- 
gether this  anhydrous  oxide  of  sodium  and  anhydrous  sul- 
phuric acid :  — 

Na2O  +  SO3  =  Na2SO4. 

There  exists  another  anhydrous  oxjde  of  sodium,  corresponding 
in  composition  to  the  formula  Na2O2,  and  the  same  sulphate  of 
sodium  can  be  made  by  heating  this  oxide  with  sulphurous  acid 
gas:  — 

Na2O2  +  SO2  =  Na2S04. 

These  facts  show  that  a  knowledge  of  the  constituents  from 
which  a  salt  may  be  made  is  not  sufficient  to  establish  any  pre- 
sumption concerning  the  molecular  constitution  of  the  salt  itself. 

26 


402  PHOSPHATE    OF    SODIUM. 

The  preparation  of  solid  caustic  soda,  for  household  and  other 
uses,  has  lately  become  a  considerable  industry.  Soap  is  made 
by  boiling  together  grease  or  oil  with  caustic  soda  or  potash  ; 
soda-lye  yields  a  hard  soap,  potash-lye  a  soft  soap.  If  the 
maker  of  soap  starts  from  the  carbonate  of  sodium  or  potassium, 
he  must  first  make  a  solution  of  caustic  lye  by  the  method  of 
Exp.  238,  and  then  boil  the  lye,  so  obtained,  with  the  grease 
which  is  the  other  essential  ingredient  of  soap.  The  soap-maker 
is  saved  the  trouble  of  converting  the  carbonate  into  the  hydrate 
of  sodium  or  potassium  by  the  maker  of  the  solid  caustic  alkalies, 
which  need  only  to  be  dissolved  in  water  to  yield  the  requisite 
lye.  The  solid  alkali  is  commonly  put  up  for  transportation  in 
sheet-iron  canisters,  of  all  sizes.  The  manufacturer  of  caustic 
soda  directly  from  the  sulphate  of  soda  has  an  advantage,  in  that 
he  can  avail  himself  of  the  caustic  soda  which  the  "  black  ash  " 
always  contains.  He  is  not  obliged,  first  to  convert  this  caustic 
alkali  into  carbonate,  and  then  to  remove  all  the  carbonic  acid 
by  lime ;  he  can,  therefore,  dispense  with  the  heating  with  saw- 
dust which  is  necessary  in  the  manufacture  of  soda-ash. 

489.  Phosphates  of  Sodium.  There  are  three  sets  of  phos- 
phates of  sodium :  1.  Common  phosphates,  which  contain  three 
atoms  of  hydrogen  or  an  equivalent  metal ;  2.  Pyrophosphates, 
which  occupy  an  intermediate  place  between  common  phosphates 
and  metaphosphates ;  3.  Metaphosphates,  which  contain  only 
one  equivalent  of  hydrogen  or  an  equivalent  metal.  (See  §  293.) 

3H20,P205  =  2H3P04  ;      3Na20,P2O6  =  2Na3PO4 

2H2O,P2O5  =  H4P207    ;      2Na2O,P2O5  =  Na4P207 

H20,P205  =  HP03     ;      Na20,P206    =  NaPO3. 

The  most  familiar  of  the  ordinary  phosphates  is  the  rhombic 
phosphate  of  sodium,  of  the  formula  Na2HPO4,12H2O,  which  is 
the  salt  commonly  called  phosphate  of  sodium. 

Exp.  239. —  Digest  8  grms.  of  powdered,  white,  burnt  bones  with 
32  c.  c.  of  water  and  6  grms.  of  sulphuric  acid,  until  a  uniform  paste  is 
produced ;  strain  the  mass  through  a  piece  of  muslin,  stir  up  the  resi- 
due with  water,  and  squeeze  the  liquor  through  the  cloth  filter.  Evapo- 
rate the  filtered  solution  considerably,  again  filter  off  the  separated 
sulphate  of  calcium,  dilute  the  filtrate  with  water  until  it  measures  48 


PYROPHOSPHATE    OF    SODIUM.  403 

c.  c.,  and  gradually  neutralize  the  acid  solution  with  solid  carbonate  of 
sodium.  A  slight  excess  of  carbonate  may  be  added,  and  the  solution 
evaporated  until  it  crystallizes  ;  the  crystals  may  be  almost  freed  from 
adhering  sulphate  of  sodium  by  washing  them.  By  recrystallizing  the 
salt  it  may  be  obtained  in  large,  transparent  rhombic  prisms,  which  are 
decidedly  efflorescent.  They  have  a  cooling,  saline  taste,  are  soluble 
in  four  parts  of  cold  water,  and  readily  melt  in  their  water  of  crystal- 
lization ;  their  solution  has  a  faint  alkaline  reaction. 

Exp.  240.  —  Add  to  a  solution  of  the  purified  crystals  of  the  last  ex- 
periment a  few  drops  of  a  solution  of  nitrate  of  silver  ;  a  yellow  precipi- 
tate appears,  and  the  liquid  becomes  distinctly  acid  in  its  reaction  with 
litmus.  The  precipitate  is  phosphate  of  silver,  and  the  acidity  is  due 
to  the  simultaneous  liberation  of  nitric  acid  in  the  solution  :  — 


3AgN03  =  2NaNO3  +  HNO3  -f  Ag3PO4. 

From  the  rhombic  phosphate  two  other  terbasic  phosphates  may  be 
prepared  ;  by  adding  caustic  soda,  a  terbasic,  or  termetallic,  phosphate, 
Na3PO412H2O;  by  adding  phosphoric  acid,  a  so-called  "acid"  phos- 
phate NaH2PO4,H2O. 

Exp.  241.  —  Heat  4  or  5  grms.  of  rhombic  phosphate  of  sodium  in  a 
porcelain  crucible  to  bright  redness.  The  water  of  crystallization  first 
escapes,  then  another  portion  of  water  is  driven  off  at  a  higher  heat, 
the  residue  melts,  and  on  cooling  solidifies  again  to  an  opaque,  white, 
substance,  —  the  pyrophosphate  of  sodium,  Na4P2OT  :  — 

2(Na2HPO4,12H2O)  =  24H2O  -f  H2O  -f  Na4P207. 

Exp.  242.  —  Dissolve  the  new  salt  of  the  last  experiment  in  water 
and  add  a  few  drops  of  a  solution  of  nitrate  of  silver  ;  a  chalky-white 
precipitate  of  pyrophosphate  of  silver  will  be  produced,  while  the 
supernatant  liquid  is  neutral  :  — 

Na4P2O7  -f  4AgN03  =  4NaNO3  +  Ag4P2O7. 
The  student  will  observe,  in  passing,  that  the  pyrophosphate 
of  sodium  dissolves  in  water  with  more  difficulty  than  the  ordi- 
nary phosphate.  From  its  aqueous  solution  the  pyrophosphate 
crystallizes  in  prisms  which  contain  ten  molecules  of  water  of  crys- 
tallization, Na4P2O7,  10H2O.  These  crystals  are  not  efflorescent, 
but  permanent  in  the  air.  Two  of  the  atoms  of  sodium  in  neu^ 
tral  pyrophosphate  of  sodium  can  fre  replaced  by  hydrogen,  and 
we  thus  obtain  a  salt  of  the  formula  Na2H2P207  which,  like  the 
neutral  pyrophosphate,  throws  down  the  white  pyrophosphate 
of  silver  from  a  solution  of  nitrate  of  silver,  but,  unlike  the 


406  BORAX    AS    A    TEST. 

slightly  efflorescent,  they  are  generally  covered  with  a  white 
dust.  Borax  has  a  feebly  alkaline  taste  and  reaction.  When 
heated  it  bubbles  up,  loses  its  water,  and  melts  below  redness 
into  a  transparent  glass  ;  this  glass  dissolves  many  oxides  of  the 
metals,  acquiring  thereby  various  colors  characteristic  of  these 
oxides.  Hence  borax  is  much  used  as  a  blow-pipe  test  for  deter- 
mining the  presence  of  certain  oxides  of  the  metals. 

Exp.  246.  —  Make  a  little  loop,  about  as  large  as  this  O,  on  the  end 
of  a  bit  of  fine  platinum-wire  6  or  8  c.  m.  long.  Make  the  loop  white-hot 
in  the  blow-pipe  flame,  and  thrust  it  while  hot  into  some  powdered 
borax ;  a  quantity  of  borax  will  adhere  to  the  hot  wire ;  reheat  the  loop 
in  the  oxidizing  flame ;  the  borax  will  puff  up  at  first,  and  then  fuse 
to  a  transparent  glass.  If  enough  borax  to  form  a  solid,  transparent 
bead  within  the  loop  does  not  adhere  to  the  hot  wire  the  first  time,  the 
hot  loop  may  be  dipped  a  second  time  into  the  powdered  borax. 
When  a  transparent  glass  has  been  formed  within  the  loop  of  the  pla- 
tinum-wire, touch  the  bead  of  glass,  while  it  is  hot  and  soft,  to  a  speck 
of  black  oxide  of  manganese  no  bigger  than  the  period  of  this  type ; 
reheat  the  bead  with  the  adhering  particle  of  oxide  in  the  oxidizing 
flame ;  the  black  speck  will  gradually  dissolve,  and  on  looking  through 
the  bead  towards  the  light,  or  a  white  wall,  when  the  black  oxide  has 
disappeared,  the  glass  will  be  seen  to  have  assumed  a  purplish-red 
color. 

The  same  experiment  may  be  performed  with  oxide  of  iron,  which 
imparts  to  the  glass  a  yellow  color,  or  with  oxide  of  copper,  which  im- 
parts a  bluish-green  color.  The  oxidizing  flame  must  be  used  in  both 
these  cases,  as  with  the  oxide  of  manganese. 

The  power  which  borax  possesses  of  dissolving  metallic  ox- 
ides suggests  an  explanation  of  its  use  in  brazing,  and  in  sol- 
dering the  precious  metals.  The  solder  will  only  adhere  to  a 
bright  and  clean  metallic  surface,  and  the  borax  which  melts  with 
the  solder  removes  from  the  pieces  of  metal  the  film  of  oxide 
which  would  otherwise  prevent  the  adhesion  of  the  solder. 
Borax  is  also  used  by  the  assayer  and  refiner  as  a  flux.  In 
making  enamels  and  glazes,  it  is  frequently  added  for  the  pur- 
poses of  rendering  the  compound  more  fusible,  and  it  is  largely 
employed  in  fixing  colors  on  porcelain. 

There  is  a  normal,  or  neutral,  borate  of  sodium,  NaB02  or 
Na,,O,  B2O3,  which  crystallizes  with  various  quantities  of  water; 


SILICATES    OF    SODIUM.  407 

and  other  borates  of  sodium  are  known,  but  the  "  biborate  "  is  the 
only  one  of  any  practical  importance. 

491.  Silicates  of  Sodium  may  be  prepared  by  dissolving  silicic 
acid  in  caustic  soda,  as  in  Exp.  225,  or  by  fusing  together  silicic 
acid  and  carbonate  of  sodium,  or  a  mixture  of  silicic  acid,  sul- 
phate of  sodium,  and  carbon.  From  alkaline  solutions,  the  single 
crystallizable  silicate  of  sodium,  Na2SiO3,  can  readily  be  ob- 
tained in  hydrated  crystals.  The  silicate  of  sodium  of  commerce, 
called  water-glass  or  soluble  glass,  is,  however,  a  much  more 
silicious  silicate  of  varying  composition.  Some  samples  of  it 
have  very  nearly  the  composition  expressed  by  the  formula 
Na.2O,  2Si02,  while  other  specimens  approximate  to  the  formula 
Na2O,  4SiO2 .  Between  these  extremes,  the  acid  and  the  alkali 
unite  in  all  possible  proportions  to  form  compounds,  all  of  which 
are  soluble  with  more  or  less  difficulty  in  boiling  water. 

The  normal  silicate  of  sodium  (Na2SiO3)  is  readily  soluble  in 
cold  water,  and,  like  carbonate  of  sodium,  which  it  closely  re- 
sembles, may  even  be  melted  in  its  own  water  of  crystallization  ; 
but  the  acid-salt  of  commerce  is  as  good  as  insoluble  in  cold 
water.  Water-glass  may,  however,  be  completely  dissolved  by 
long  continued  boiling  in  water,  and  the  solution  thus  obtained  is 
largely  employed  by  calico-printers  and  by  soapmakers.  Water- 
glass  is  also  used  for  hardening  porous  stones,  or  even  for  bind- 
ing sand  into  artificial  stone,  for  painting  rough  wood-work  to 
protect  it  from  the  weather,  and  for  diminishing  the  combustibility 
of  wood,  canvas  and  other  coarse  stuffs,  such  as  are  used  for  the 
decorations  of  theatres.  It  has  been  suggested  that  the  interior 
surfaces  of  wooden-roofed  railway  bridges  might  be  protected 
from  the  sparks  of  the  locomotive  by  washing  them  with  a  solu- 
tion of  water-glass.  When  employed  as  paint,  the  coating  of 
water-glass  may  be  washed  over  with  a  solution  of  chloride  of 
ammonium,  which  decomposes  the  silicate,  with  deposition  of  free 
silicic  acid ;  or  the  silicate  may  simply  be  left  to  the  decomposing 
action  of  atmospheric  carbonic  acid.  The  chloride  of  sodium 
resulting  from  the  decomposition  in  the  one  case,  and  the  carbo- 
nate of  sodium  in  the  other,  are  subsequently  washed  away  by 
the  rain.  Combustible  matters,  when  covered  with  a  coating  of 
silica,  or  of  silicate  of  sodium,  as  above,  are  prevented  from  burn- 


406  BORAX    AS    A    TEST. 

slightly  efflorescent,  they  are  generally  covered  with  a  white 
dust.  Borax  has  a  feebly  alkaline  taste  and  reaction.  When 
heated  it  bubbles  up,  loses  its  water,  and  melts  below  redness 
into  a  transparent  glass  ;  this  glass  dissolves  many  oxides  of  the 
metals,  acquiring  thereby  various  colors  characteristic  of  these 
oxides.  Hence  borax  is  much  used  as  a  blow-pipe  test  for  deter- 
mining the  presence  of  certain  oxides  of  the  metals. 

Exp.  246.  —  Make  a  little  loop,  about  as  large  as  this  O,  on  the  end 
of  a  bit  of  fine  platinum-wire  6  or  8  c.  m.  long.  Make  the  loop  white-hot 
in  the  blow-pipe  flame,  and  thrust  it  while  hot  into  some  powdered 
borax ;  a  quantity  of  borax  will  adhere  to  the  hot  wire ;  reheat  the  loop 
in  the  oxidizing  flame ;  the  borax  will  puff  up  at  first,  and  then  fuse 
to  a  transparent  glass.  If  enough  borax  to  form  a  solid,  transparent 
bead  within  the  loop  does  not  adhere  to  the  hot  wire  the  first  time,  the 
hot  loop  may  be  dipped  a  second  time  into  the  powdered  borax. 
When  a  transparent  glass  has  been  formed  within  the  loop  of  the  pla- 
tinum-wire, touch  the  bead  of  glass,  while  it  is  hot  and  soft,  to  a  speck 
of  black  oxide  of  manganese  no  bigger  than  the  period  of  this  type  ; 
reheat  the  bead  with  the  adhering  particle  of  oxide  in  the  oxidizing 
flame ;  the  black  speck  will  gradually  dissolve,  and  on  looking  through 
the  bead  towards  the  light,  or  a  white  wall,  when  the  black  oxide  has 
disappeared,  the  glass  will  be  seen  to  have  assumed  a  purplish-red 
color. 

The  same  experiment  may  be  performed  with  oxide  of  iron,  which 
imparts  to  the  glass  a  yellow  color,  or  with  oxide  of  copper,  which  im- 
parts a  bluish-green  color.  The  oxidizing  flame  must  be  used  in  both 
these  cases,  as  with  the  oxide  of  manganese. 

The  power  which  borax  possesses  of  dissolving  metallic  ox- 
ides suggests  an  explanation  of  its  use  in  brazing,  and  in  sol- 
dering the  precious  metals.  The  solder  will  only  adhere  to  a 
bright  and  clean  metallic  surface,  and  the  borax  which  melts  with 
the  solder  removes  from  the  pieces  of  metal  the  film  of  oxide 
which  would  otherwise  prevent  the  adhesion  of  the  solder. 
Borax  is  also  used  by  the  assayer  and  refiner  as  a  flux.  In 
making  enamels  and  glazes,  it  is  frequently  added  for  the  pur- 
poses of  rendering  the  compound  more  fusible,  and  it  is  largely 
employed  in  fixing  colors  on  porcelain. 

There  is  a  normal,  or  neutral,  borate  of  sodium,  NaB02  or 
N%O,  B2O3,  which  crystallizes  with  various  quantities  of  water; 


SILICATES    OF    SODIUM.  407 

and  other  borates  of  sodium  are  known,  but  the  "  biborate  "  is  the 
only  one  of  any  practical  importance. 

491.  Silicates  of  Sodium  may  be  prepared  by  dissolving  silicic 
acid  in  caustic  soda,  as  in  Exp.  225,  or  by  fusing  together  silicic 
acid  and  carbonate  of  sodium,  or  a  mixture  of  silicic  acid,  sul- 
phate of  sodium,  and  carbon.  From  alkaline  solutions,  the  single 
crystallizable  silicate  of  sodium,  Na2Si03,  can  readily  be  ob- 
tained in  hydrated  crystals.  The  silicate  of  sodium  of  commerce, 
called  water-glass  or  soluble  glass,  is,  however,  a  much  more 
silicious  silicate  of  varying  composition.  Some  samples  of  it 
have  very  nearly  the  composition  expressed  by  the  formula 
Na,2O,  2SiO2,  while  other  specimens  approximate  to  the  formula 
Na2O,  4SiO2 .  Between  these  extremes,  the  acid  and  the  alkali 
unite  in  all  possible  proportions  to  form  compounds,  all  of  which 
are  soluble  with  more  or  less  difficulty  in  boiling  water. 

The  normal  silicate  of  sodium  (Na2SiO3)  is  readily  soluble  in 
cold  water,  and,  like  carbonate  of  sodium,  which  it  closely  re- 
sembles, may  even  be  melted  in  its  own  water  of  crystallization  ; 
but  the  acid-salt  of  commerce  is  as  good  as  insoluble  in  cold 
water.  Water-glass  may,  however,  be  completely  dissolved  by 
long  continued  boiling  in  water,  and  the  solution  thus  obtained  is 
largely  employed  by  calico-printers  and  by  soapmakers.  Water- 
glass  is  also  used  for  hardening  porous  stones,  or  even  for  bind- 
ing sand  into  artificial  stone,  for  painting  rough  wood-work  to 
protect  it  from  the  weather,  and  for  diminishing  the  combustibility 
of  wood,  canvas  and  other  coarse  stuffs,  such  as  are  used  for  the 
decorations  of  theatres.  It  has  been  suggested  that  the  interior 
surfaces  of  wooden-roofed  railway  bridges  might  be  protected 
from  the  sparks  of  the  locomotive  by  washing  them  with  a  solu- 
tion of  water-glass.  When  employed  as  paint,  the  coating  of 
water-glass  may  be  washed  over  with  a  solution  of  chloride  of 
ammonium,  which  decomposes  the  silicate,  with  deposition  of  free 
silicic  acid ;  or  the  silicate  may  simply  be  left  to  the  decomposing 
action  of  atmospheric  carbonic  acid.  The  chloride  of  sodium 
resulting  from  the  decomposition  in  the  one  case,  and  the  carbo- 
nate of  sodium  in  the  other,  are  subsequently  washed  away  by 
the  rain.  Combustible  matters,  when  covered  with  a  coating  of 
silica,  or  of  silicate  of  sodium,  as  above,  are  prevented  from  burn- 


408  WATER-GLASS. 

ing  freely,  in  the  same  way  that  the  carbon  of  the  paper  in  Exp. 
114  was  kept  from  burning  by  the  coating  of  phosphoric  acid. 

492.  The  chief  use  of  silicate  of  sodium,  however,  is  as  a  com- 
ponent of  common  glass.  The  various  glasses  of  commerce  are 
mixtures  of  a  highly  silicious  silicate  of  sodium,  or  of  potassium, 
or  of  both  these  substances,  with  silicates  of  other  metals,  such  as 
calcium,  aluminum,  and  lead.  In  green  bottle-glass  the  easily 
fusible  silicate  of  iron  replaces  in  part  the  silicate  of  sodium  or  of 
potassium. 

It  is  a  peculiarity  of  the  alkaline  silicates  that,  in  changing 
from  the  liquid  to  the  solid  state,  they  pass  through  an  interme- 
diate pasty  or  viscous  stage,  and  finally  solidify  in  transparent 
amorphous  masses,  totally  devoid  of  crystalline  structure.  While 
in  this  pasty,  ductile  state,  these  silicates  may  readily  be  moulded 
into  almost  any  form,  and  transparent  vessels  might  doubtless  be 
made  from  the  acid  alkaline  silicates  without  admixture  of  other 
materials.  The  alkaline  silicates,  however,  are,  by  themselves, 
far  too  easily  acted  upon  by  air  and  moisture  to  admit  of  being 
used  as  substitutes  for  ordinary  glass.  But  it  has  been  found 
that,  by  combining  the  alkaline  silicates  with  the  silicates  of  cer- 
tain other  metals,  such  as  calcium,  there  may  be  obtained 
compound  glasses  which,  while  they  retain  the  plasticity  of  the 
alkaline  silicates  as  well  as  their  amorphous  character  and  trans- 
parency, are  capable  of  resisting  the  action  not  only  of  air  and 
water,  but  even  of  acids  and  alkalies,  to  a  very  great  extent. 
Though  the  ordinary  glasses  are  so  difficultly  attacked  by  water 
that  they  may,  for  most  practical  purposes,  be  regarded  as  alto- 
gether insoluble,  it  is  nevertheless  true,  as  has  been  stated  in 
§  4G4,  that  glass  may  be  partially  dissolved  by  long-continued 
contact  with  water,  particularly  if  the  water  be  hot  and  the  glass 
in  the  condition  of  powder.  After  the  smooth  surface  of  glass,  as 
it  comes  from  the  n're,  has  once  been  removed,  the  corrosion  of 
the  glass  goes  on  more  rapidly. 

It  is  remarkable  that  mixtures  composed  of  several  different 
silicates  melt  at  temperatures  considerably  lower  than  the  mean 
of  the  melting-points  of  their  several  ingredients. 

The  different  varieties  of  glass  vary  in  composition,  according 
to  the  purposes  for  which  they  are  prepared ;  they  cannot  be  re- 


GLASS.  409 

garded  as  chemical  compounds,  being  really  indefinite  mixtures  of 
various  acid  silicates.  The  composition  of  window-glass  may  be 
represented  approximately  by  the  formula  Na2O,  2SiO2:  Ca02, 
2Si02;  and  that  of  the  hard,  Bohemian  glass,  suitable  for  ignition- 
tubes,  by  the  formula  2(K2O,  3SiO2);  3(CaO,  3Si02).  The  lus- 
trous "flint  "-glass,  employed  for  the  nicer  kinds  of  household  ware, 
may,  on  the  other  hand,  be  represented  by  the  formula  2(K2O, 
2Si02)  ;  3(PbO,  2Si02)  ;  it  is  prepared  from  the  purest  mate- 
rials attainable.  Bottle-glass  is  composed  of  the  silicates  of  lime 
and  of  alumina,  together  with  a  small  proportion  of  the  silicates 
of  iron,  of  potassium,  and  of  sodium  ;  in  this  glass,  as  in  the  other 
varieties  above  formulated,  small  quantities  of  various  other  sili- 
cates, such  as  the  silicates  of  magnesium  and  of  manganese,  almost 
always  occur. 

In  the  preparation  of  the  cheaper  kinds  of  glass  the  materials 
are  melted  in  large  open  crucibles  of  refractory  clay,  but  the  bet- 
ter sorts  of  glass,  such  as  flint-glass,  are  made  in  pots  so  covered 
that  no  smoke  or  dust  from  the  fire  can  come  in  contact  with 
their  contents.  In  both  cases  the  thoroughly  melted  mixture  is 
kept  in  the  liquid  state  until  it  has  become  perfectly  homogenous 
and  until  all  bubbles  of  air  have  escaped  from  it;  it  is  then 
allowed  to  cool  to  the  temperature  at  which  it  possesses  the  pe- 
culiar, pasty,  ductile,  condition  in  which  it  admits  of  being  blown, 
pressed,  and  moulded. 

493.  After  glass  has  been  moulded  into  the  shape  desired,  it 
must  still  be  subjected  to  a  process  of  annealing  before  it  can  be 
used.  Glass  which  has  been  suddenly  cooled  after  fusion  is  ex- 
tremely brittle,  and  in  general  all  glass  which  has  been  quickly 
cooled  after  heating  is  far  more  fragile  than  that  which  has  been 
allowed  to  cool  gradually.  The  operation  of  annealing  is  nothing 
but  a  process  of  slow  baking,  at  a  temperature  which,  though  not 
so  high  as  the  melting-point  of  glass,  is  nevertheless  high  enough 
to  allow  the  particles  of  softened  glass  to  move  among  themselves, 
and  to  come  into  easy  and  natural  positions  as  regards  one 
another;  after  the  baking  follows  a  process  of  slow  cooling, 
during  which  the  heated  material  contracts  uniformly  in  all  di- 
rections, as  it  assumes  the  dimensions  proper  to  it  when  cold. 

Unlike  the   silicates  of  the  alkali-metals,  most  metallic  sili- 


410  HYPOSULPHITE    OF    SODIUM. 

cates  have  a  tendency  to  assume  crystalline  form  on  cooling,  and 
it  is  not  difficult  to  bring  about  crystallization  of  silicate  of  cal- 
cium, or  silicate  of  aluminum,  from  ordinary  glass,  particularly 
from  the  coarser  kinds,  such  as  bottle-glass.  Glass,  in  which 
some  of  the  constituents  have  thus  crystallized,  has  the  appear- 
ance of  porcelain  ;  it  is  said  to  be  devitrified.  The  devitrification 
may  readily  be  shown  by  imbedding  bottle-glass  in  sand,  heating 
the  glass  almost  to  the  melting-point,  and  then  allowing  it  to  cool 
slowly.  In  annealing  some  kinds  of  glass  care  must  be  taken 
not  to  heat  the  ware  too  strongly,  lest  it  be  devitrified  during  the 
process. 

494.  Melted  glass,  like  melted  borax,  (Exp.  24f>)  is  capable  of 
dissolving  small  quantities  of  many  of  the  metallic  oxides,  a  trans- 
parent and  often  colored  silicate  of  the  oxide  being  formed,  which 
imparts  its  hue  to  the  entire  mass  of  glass.     In  this  way,  glass 
may  be  obtained  of  almost  any  desired  color.     The  green  color 
of  bottle-glass  is  due  to  silicate  of  protoxide  of  iron,  but  richer 
shades  of  green  may  be  obtained  by  using  protoxide  of  copper 
or  oxide  of  chromium.     Dinoxide  of  copper  gives  a  ruby  red 
color,  and  oxide  of  gold  various  shades  of  red,  inclining  to  purple. 
The  oxides  of  uranium,  of  antimony,  and  of  silver  yield  yellow 
glasses ;  oxide  of  cobalt  affords  a  beautiful  blue,  and  the  binoxide 
of  manganese  a  violet  glass,  while  mixtures  of  the  oxides  of  cobalt 
and  of  manganese  impart  to  the  glass  a  black  color. 

495.  Hyposulphite  of  Sodium  (Na2S2O3,5H2O).     This  easily 
crystallized  and  tolerably  permanent  salt  is  of  great  use  to  the 
photographer,  because  its  aqueous  solution  is  capable  of  render- 
ing soluble  the  chloride,  bromide,  and  iodide  of  silver,  compounds 
much  employed  by  the  photographer,  and  very  insoluble  in  water. 
The  photographic  paper  or  glass,  uniformly  coated  with  some 
silver  compound,  is  exposed  to  light  in  the  camera  or  press,  and 
then  immersed  in  a  strong  solution  of  the  hyposulphite,  which 
forms   with  the  silver  compound,  in  those  parts  of  the  picture 
which  have  not  been  acted  upon  by  the  light,  a  double  salt  which 
is  soluble  in  water.     This  double  salt  and  the  superfluous  hypo- 
sulphite must  be  washed  away  by  soaking  the  picture  several 
hours  in  water  which  is  constantly  renewed.     Hyposulphite  of 
sodium  is  also  used  as  an  "  antichlore,"  or  agent  for  removing 


POTASSIUM.  411 

the  last  traces  of  chlorine,  or  hypoehlorous  acid,  from  substances 
which  have  been  bleached  therewith.  The  salt  may  be  best  pre- 
pared by  digesting  sulphur  with  a  solution  of  sulphite  of  sodium. 

Exp.  247.  —  Dissolve  20  grms.  of  crystallized  carbonate  of  sodium  in 
80-40  c.  c.  of  water.  Place  the  solution  in  a  small  Woulfe-bottle,  and 
pass  sulphurous  acid  gas  (Exp.  100)  through  it  until  all  the  carbonic 
acid  is  expelled  from  the  carbonate,  and  effervescence  ceases.  The 
liquid  then  holds  in  solution  sulphite  of  sodium  (Na2SO3).  Pour  this 
solution  into  a  bottle  which  can  be  tightly  closed,  and  add  to  it  3  or  4 
grms.  of  finely  powdered  sulphur ;  let  this  mixture  stand  corked  up  for 
several  days  in  a  warm  place  ;  the  sulphur  will  gradually  dissolve,  and 
form  a  colorless  solution,  which  on  evaporation  will  yield  crystals  of 
hyposulphite  of  sodium.  Time  may  be  saved  by  keeping  the  solution 
of  sulphite  of  sodium  hot,  but  not  boiling,  during  the  digestion  of  the 
sulphur.  The  reaction  has  been  already  formulated  (§  243). 


CHAPTER    XXIV. 

POTASSIUM. 

496.  We  have  seen  that  the  proximate  source  of  sodium-com- 
pounds is  the  sea.  Potassium-compounds,  on  the  other  hand, 
are  derived  indirectly  from  the  soil.  Arable  soils  are  produced 
by  the  weathering  and  gradual  decomposition  of  the  common 
granitic  rocks.  Into  the  composition  of  these  rocks  there  enter 
largely  two  minerals,  called  feldspar  and  mica,  which  are  mixed 
silicates  of  potassium  or  sodium,  and  aluminum  or  iron.  The 
element  potassium  thus  becomes  a  normal  constituent  of  the 
earthy  food  of  plants.  The  soil  itself  is  not  directly  available  as 
a  source  of  potassium-salts,  because  no  cheap  and  easy  method 
has  yet  been  devised  for  separating  the  potassium-compounds 
from  the  other  ingredients  of  the  soil.  Plants,  however,  are  able 
to  pick  out  and  assimilate  the  potassium-salts  from  these  rocks 
and  soils,  so  that  by  burning  the  plants  and  extracting  the  ashes 
with  water  a  soluble  potassium-salt  is  obtained.  Plants  thus  con- 
centrate the  potassium  from  out  great  masses  of  earth,  and  make 


412  CARBONATE    OF    POTASSIUM. 

it  accessible  to  us.  The  salt  which  is  obtained  from  the  ashes  of 
plants  by  washing  and  evaporation,  is  called  pot-ash,  or,  if  refined, 
pearl-ash,  and  it  is  from  this  substance  that  the  bulk  of  potas- 
sium-compounds are  obtained. 

Exp.  248.  —  Place  a  handful  of  wood-ashes  on  a  filter,  and  pour  hot 
water  over  them,  collecting  the  filtrate  in  a  bottle  and  returning  it  upon 
the  ashes  two  or  three  times,  in  order  to  obtain  the  stronger  solution. 
To  exhaust  the  ashes  of  their  potash,  they  must,  of  course,  be  treated 
with  successive  portions  of  hot  water.  This  solution  has  a  strong  alka- 
line reaction  upon  test-paper.  A  few  drops  of  it,  poured  into  a  test- 
tube  containing  a  little  dilute  acid,  occasion  a  brisk  effervescence,  a  re- 
action from  which  we  readily  surmise  the  truth,  that  the  potassium-salt 
contained  in  the  solution  is  the  carbonate  of  potassium.  Proof  that  the 
gas  evolved  is  carbonic  acid  can  readily  be  obtained  by  conducting  the 
gas  into  lime-water,  as  in  Exp.  1 74.  By  evaporating  the  rest  of  the 
solution  to  dry  ness  in  a  porcelain  dish,  we  obtain  a  small  sample  of 
crude  potash.  By  treating  this  potash  with  a  quantity  of  cold  water, 
insufficient  to  dissolve  any  but  the  most  soluble  portions  of  the  mass, 
letting  the  mixture  stand  some  time,  and  evaporating  the  partial  solu- 
tion to  dryness,  a  whiter,  purer  carbonate  is  obtained,  the  pearl-ash. 

497.  Carbonate  of  Potassium  (K2CO3),  is  a  hygroscopic  and 
very  soluble  salt.  When  exposed  to  damp  air  it  becomes  moist, 
and  finally  deliquesces.  In  this  respect  it  does  not  resemble 
soda-ash,  which  is  not  hygroscopic,  and  is,  for  this  reason  among 
others,  better  adapted  than  potash  for  transportation,  storing,  and 
most  commercial  uses.  Carbonate  of  potassium  fuses  at  a  red 
heat,  but  cannot  be  decomposed  by  heat  alone.  At  a  red  heat  it 
is  decomposed  by  silica,  as  is  also  the  carbonate  of  sodium,  car- 
bonic acid  being  expelled  with  effervescence,  whilst  the  silica 
unites  with  the  alkali.  Advantage  is  taken  of  this  property  in 
the  analysis  of  minerals  which  contain  a  large  quantity  of  silica, 
and  are  not  easily  decomposed  by  acids.  The  finely  powdered 
mineral  is  fused  with  about  three  times  its  weight  of  carbonate  of 
sodium  or  of  potassium  ;  or.  better,  with  thrice  its  weight  of  a  mix- 
ture of  5  J  parts  of  carbonate  of  sodium  with  7  parts  of  carbonate 
of  potassium.  The  mixed  carbonates  produce  a  more  fusible 
mixture  than  either  alone  (§  492).  The  fused  mass  is  then  treated 
with  dilute  chlorhydric  acid,  which  decomposes  the  alkaline  sili- 
cates, and  dissolves  all  the  bases  of  the  mineral  which  were  before 
combined  with  the  silica. 


HYDRATE    OF    POTASSIUM.  413 

Carbonate  of  potassium  was  the  most  important  source  of  al- 
kali, until  Leblanc's  process  made  soda  cheaper  than  potash.  It 
is  still  largely  consumed  in  the  manufacture  of  soap,  glass,  caustic 
potash,  and  other  compounds  of  potassium,  but  sodium-salts  have, 
to  a  great  extent,  displaced  potassium-salts  in  commerce  and  the 
arts. 

498.  Carbonate  of  Potassium  and  Hydrogen  (KHC03).    This 
salt,  commonly  called  the   "bicarbonate"    of  potassium  (K2O, 
H20,  2CO2),  is  prepared  by  passing  a  current  of  carbonic  acid 
through  a  strong  solution  of  carbonate  of  potassium ;  crystals  of 
the  bicarbonate  will  be  deposited,  which  are  permanent  in  the 
air,  and  require  about  4  parts  of  cold  water  for  solution.     When 
the  solution  of  this  salt  is  long  exposed  to  the  air,  or  boiled,  it 
loses  one-fourth  of  its  carbonic  acid ;  when  the  dry  salt  is  fused, 
it  loses  half  its  carbonic  acid,  and  is  converted  into  the  carbonate. 
It  is  a  valuable  salt  to  the  chemist  and  the  apothecary,  because  it 
can  be  readily  obtained  in  a  state  of  purity ;  when  itself  made 
from  a  refined  carbonate  of  potassium,  it  may  be  advantageously 
used  as  the  material  from  which  to  prepare  other  pure  com- 
pounds of  this  important  element. 

499.  .Hydrate   of  Potassium  (KHO).     The   manufacture  of 
hydrate  of  potassium,  from  carbonate  of  potassium,  resembles,  in 
every  detail,  the  preparation  of  caustic  soda  from  carbonate  of 
sodium  (Exp.  238).     The  carbonate  of  potassium  is  dissolved  in 
10  or  12  times  its  weight  of  water,  and  decomposed  by  a  milk  of 
lime ;  carbonate  of  calcium  is  precipitated,  and  hydrate  of  potas- 
sium remains  in  solution.     All  that  has  been  said  of  the  concen- 
tration of  the  solution  of  hydrate  of  sodium  (§  487)  is  true,  also, 
of  hydrate  of  potassium. 

Hydrate  of  potassium  is  a  hard,  whitish  substance,  possessing 
a  peculiar  odor  and  a  very  acrid  taste.  Like  the  hydrate  of  so- 
dium, it  rapidly  absorbs  moisture  and  carbonic  acid  from  the  air, 
and  since  the  carbonate  of  potassium  thus  formed  is  a  deliques- 
cent salt,  this  change  will  go  on  until  the  entire  mass  of  hydrate 
is  converted  into  a  syrup  of  the  carbonate  ;  whereas,  in  the  case 
of  hydrate  of  sodium,  the  absorption  of  water  and  carbonic  acid 
is  soon  arrested  by  the  formation  of  a  coating  of  non-deliquescent 
carbonate  of  sodium  upon  the  surface  of  the  lump  of  hydrate.  In 


414  CAUSTIC    POTASH. 

chemical  industries  and  speculations,  the  question  of  success  or 
failure  often  turns  on  such  points  as  this ;  the  advantage  of  a  new 
material,  for  example,  often  depends  upon  just  such  differences 
as  this  between  caustic  soda  and  caustic  potash. 

500.  The  hydrate  of  potassium,  cast  into  small  sticks,  is  em- 
Ployed  by  physicians  as  a  cautery, — a  use  which  illustrates  forcibly 
its  destructive  effect  upon  animal  and  vegetable  matters.  Like 
hydrate  of  sodium,  its  solution  destroys  ordinary  paper,  and 
cannot  be  filtered  except  through  asbestos,  or  gun-cotton.  A 
clear  solution  is  best  obtained  by  decantation  from  off  the 
subsided  impurities.  All  vessels  made  of  materials  which  con- 
tain silica  are  attacked  by  this  caustic  solution,  and  even  plati- 
num is  slowly  oxidized  in  its  presence ;  vessels  of  gold  and  silver 
resist  it  best.  This  hydrate,  like  that  of  sodium,  forms  soaps  with 
oils  or  fats ;  the  sodium-soaps  are  hard,  the  potassium  soft.  At  a 
high  temperature  hydrate  of  potassium  volatilizes  without  change 
heat  alone  cannot  decompose  the  caustic  alkalies.  In  the  chemi- 
cal laboratory,  solutions  of  caustic  potash  and  caustic  soda  are  in 
frequent  use  for  absorbing  acid  gases,  such  as  carbonic  acid,  and 
especially  for  separating  the  hydrates  of  other  metals  from  solu- 
tions of  their  salts. 

Exp.  249. —  Dissolve  a  crystal  of  blue  vitriol  (sulphate  of  copper)  in 
a  few  centimetres  of  cold  water,  and  add  to  the  solution  several  drops 
of  a  solution  of  caustic  soda  (or  potash).  The  hydrate  of  copper  is 
precipitated  as  a  delicate,  blue,  insoluble  powder,  while  colorless  sul- 
phate of  sodium  (or  potassium),  remains  in  solution. 

CuSO4    +     2NaHO    =     CuH2O2    +    NasSO,. 
Sulphate  of  Copper.  Hydrate  of  Copper. 

Exp.  250.  —  Place  in  a  small  flask  4  or  5  grms.  of  chalk  or  marble 
(carbonate  of  calcium),  and  7  or  8  c.  c.  of  water;  then  cautiously 
add  chlorhydric  acid,  little  by  little,  until  effervescence  ceases  and  the 
chalk  is  dissolved.  When  the  effervescence  is  not  violent,  the  flask 
may  be  warmed  to  facilitate  the  process  of  solution.  A  rather  concen- 
trated solution  of  chloride  of  calcium  will  be  thus  obtained. 

CaC03    +     2HC1    =     CaCl2    +    H20    +     CO2. 
Carbonate  of  Calcium.  Chloride  of  Calcium. 

Add  to  this  solution  a  few  drops  of  a  solution  of  caustic  soda,  which  is 


STRONG   BASES.  415 

free  from  carbonic  acid.     A  white  precipitate  of  hydrate  of  calcium 
will  immediately  appear,  since  this  hydrate  is  insoluble  in  the  men- 
struum, while  chloride  of  sodium  will  be  found  in  the  clear  solution. 
CaCl2  -f  2NaHO  =  CaH3O2  -f  2NaCl. 
Hydrate  of  Calcium. 

501.  On  account  of  this  power  of  precipitating  other  hydrates 
from  solutions  of  their  salts,  the  caustic  alkalies  are  often  called 
strong  bases,  as  he  is  the  strongest  wrestler  who  throws  his  ad- 
versary ;  but  this  term  "  strong  "  is  applied  to  bases  and  acids  so 
confusedly  as  to  be  frequently  a  hindrance  rather  than  a  help  in 
classification.  Thus,  if  the  reaction  which  occurs  in  the  prepara- 
tion of  caustic  soda  (Exp.  238),  between  carbonate  of  sodium  and 
hydrate  of  calcium,  be  compared  with  the  reaction  last  given  be- 
tween chloride  of  calcium  and  hydrate  of  sodium,  it  will  be  seen 
that  in  the  first,  calcium  displaces  sodium,  while  in  the  second, 
sodium  displaces  calcium  ;  in  the  one  case,  hydrate  of  sodium  is 
eliminated  from  the  reaction,  and  in  the  other,  hydrate  of  calcium. 
So  of  acids,  —  we  have  before  had  occasion  to  remark  that  of  two 
acids,  now  the  one  and  now  the  other  will  be  stronger,  accord- 
ing to  the  temperature  at  which  the  contest  between  them  takes 
place,  or  other  extrinsic  conditions.  Thus  sulphuric  acid  is  at 
certain  temperatures  capable  of  displacing  phosphoric  acid  or 
boracic  acid,  but  at  high  temperatures  both  these  acids  displace  it 
(§§  294,  448).  It  is  obvious  that  the  definition  of  the  term 
"  strength  "  must  be  very  vague  and  unsatisfactory,  when  applied 
to  relations  thus  capable  of  actual  reversal.  Two  general  princi- 
ples, however,  have  been  arrived  at,  through  the  comparative 
study  of  such  reactions  ;  these  are :  1st.  When  from  all  or  part 
of  the  elements  of  any  mixture  of  liquefied  materials  a  substance 
can  be  formed  which  is  insoluble  in  the  existing  menstruum,  that 
substance  will  separate  in  the  solid  state ;  2d.  When  from  all 
or  part  of  the  elements  of  a  solid  or  liquid  mixture  a  substance 
can  be  compounded  which  is  volatile  at  the  existing,  or  induced, 
temperature,  that  substance  will  be  eliminated  in  the  gaseous 
state.  In  either  case  such  interchange  of  atoms  as  may  be  essen- 
tial to  the  formation  of  the  eliminated  substance,  takes  place,  and 
the  remaining  elements  necessarily  adjust  themselves  to  new  re- 
lations. Such  insoluble  precipitates  often  present  peculiarities  of 


416  ALKALIMETRY. 

color  or  texture  by  which  they  may  be  recognized,  and  such 
volatile  gases  may  frequently  be  identified  by  their  color,  odor, 
or  specific  gravity,  or  by  the  chemical  effects  which  they  are 
capable  of  producing.  If  every  chemical  element  was  known  to 
yield,  under  attainable  conditions,  a  characteristic  precipitate,  or 
to  evolve  a  peculiar  and  recognizable  gas,  the  analytical  chemist 
would  possess  the  means  of  detecting  every  element  with  cer- 
tainty. This  is  to  a  great  extent  the  case,  and  chemical  analysis 
is  chiefly  based  upon  a  knowledge  of  the  degrees  of  solubility 
and  volatility  which  belong  to  a  great  variety  of  chemical  sub- 
stances, with  whose  appearance  and  prominent  properties  the 
analyst  has  previously  made  himself  acquainted  ;  of  these  sub- 
stances many  are  common,  but  not  a  few  rare  and  useless,  except 
to  serve  the  purpose  of  the  analyst. 

There  exists  an  anhydrous  oxide  of  potassium,  K2O,  and  also 
a  peroxide.  The  anhydrous  oxide  K2O  is  a  gray,  inodorous,  hard, 
brittle  solid  ;  it  melts  a  little  above  a  red  heat,  but  volatilizes  only 
at  a  very  high  temperature. 

502.  Alkalimetry.  —  Since  the  value  of  the  carbonates  and 
hydrates  of  sodium  and  potassium,  as  they  are  manufactured  and 
consumed  on  the  large  scale  in  the  chemical  arts,  is  generally  de- 
pendent upon  the  amount  of  alkali  which  they  contain,  ready  to 
enter  into  chemical  combination,  it  is  important  to  have  some 
quick  and  easy  method  of  determining  how  much  available  alkali 
any  sample  of  these  substances  really  contains.  The  impurities 
which  most  frequently  contaminate  the  carbonate  of  potassium 
are  the  chlorides  and  sulphates  of  potassium  and  sodium,  silicic 
acid,  lime,  alumina,  and  the  oxides  of  iron ;  the  commonest  im- 
purities of  carbonate  of  sodium  are  the  chloride  and  sulphate  of 
sodium,  as  might  readily  be  inferred  from  consideration  of  the 
process  by  which  the  carbonate  is  manufactured.  Some  sulphite 
of  sodium  also  is  not  infrequently  present  in  commercial  soda-ash. 
Both  carbonates  are  apt  to  contain  small  proportions  of  the  hy- 
drates ;  but  as  the  hydrates  are  quite  as  valuable  for  most  uses  as 
the  carbonates,  weight  for  weight,  this  admixture  is  not  incon- 
venient. In  the  common  methods  of  testing  the  carbonates,  the 
hydrates  present  are  estimated  as  available  alkali,  but  are  made 
no  separate  account  of. 


VOLUMETRIC    ANALYSIS.  417 

It  would  be  foreign  to  our  purpose  to  enter  upon  the  details  of 
alkalimetry ;  the  process  consists  essentially  in  ascertaining  how 
much  dilute  sulphuric  acid  of  a  known  strength  is  required  to 
neutralize  exactly  a  known  weight  of  ^the  sample  examined.  The 
requisites  are  a  graduated  burette  (Appendix,  §  21),  a  standard 
acid,  pure  carbonate  of  sodium  wherewith  to  prepare  this  acid, 
and  a  colored  solution,  sensitive  to  both  acid  and  alkali,  to  indi- 
cate the  point  of  neutralization.  The  following  experiment  will 
give  some  idea  of  the  manipulation  required  in  this  sort  of  analy- 
sis, which,  on  account  of  its  rapidity,  is  of  very  general  application 
in  technical  chemistry.  The  general  method  is  called  volumetric, 
because,  when  once  the  standard  liquids  are  prepared,  quantita- 
tive results  are  obtained,  not  by  weighing,  but  by  measuring  the 
bulk  of  liquid  consumed  in  the  testing. 

Exp.  251. —  Weigh  out  accurately  5  grms.  of  pure  anhydrous  car- 
bonate of  sodium  ;  transfer  it  to  a  flask  or  beaker  having  the  capacity 
of  about  400  c.  c. ;  dissolve  it  in  about  200  c.  c.  of  water,  and  color  the 
solution  blue  with  about  2  c.  c.  of  a  violet  tincture  of  litmus.  To  pre- 
pare this  tincture,  digest  1  part  of  litmus  in  6  parts  of  water,  on  a 
water-bath,  for  several  hours ;  filter ;  divide  the  blue  liquid  into  two 
equal  portions,  and  stir  one  half  repeatedly  with  a  glass  rod  dipped  in 
very  dilute  nitric  acid,  until  the  color  just  appears  red ;  then  add  the 
other  blue  half  together  with  one  part  of  alcohol,  and  keep  the  tincture 
in  a  small  open  bottle.  In  a  stoppered  bottle  the  tincture  fades. 

Mix  about  60  grms.  of  strong  sulphuric  acid  with  500  c.  c.  of  water 
and  let  the  mixture  cool  (§  233).  Fill  a  50  c.  c.  Mohr's  burette  (Ap- 
pendix, §  21)  up  to  the  0  mark  with  the  cold  dilute  acid.  Place  the 
flask  or  beaker  containing  the  soda  solution  beneath  the  burette,  gently 
press  the  spring-clip,  and  allow  the  acid  to  flow  gradually  into  the  soda 
solution,  stirring  the  while,  until  the  color  of  the  liquid  changes  to  a 
wine-red ;  then  place  the  flask  or  beaker  over  a  lamp,  and  bring  the 
liquid  to  ebullition ;  the  dissolved  carbonic  acid  will  be  driven  out,  and 
the  liquid  will  again  become  blue  ;  more  acid  is  then  added  to  the 
nearly  boiling  fluid,  the  vessel  being  occasionally  placed  over  the  lamp,, 
until  the  color  of  the  liquid  becomes  red,  slightly  inclining  to  yellow. 
When  the  point  of  saturation  is  approaching,  add  the  acid  two  drops  at 
a  time,  and  after  each  fresh  addition  dip  a  fine  glass  rod  into  the  fluid, 
and  make  with  it  two  spots  on  a  slip  of  fine  blue  litmus-paper,  reading 
the  volume  each  time,  and  marking  the  number  of  centimetres  between 
the  two  spots.  Continue  this  operation  until  the  spots  on  the  paper 
27 


418  VALUATION    OF    POT-   AND    SODA-ASH. 

appear  distinctly  red ;  then  dry  the  paper  and  take  for  the  correct 
number  that  figure  which  stands  between  those  two  spots  which  just 
remain  red  when  dry. 

For  some  eyes,  tincture  of  cochineal  possesses  great  advantages  over 
tincture  of  litmus,  as  a  means  of  recognizing  the  point  of  neutralization. 
The  tincture  of  cochineal  is  prepared  by  digesting  3'grms.  of  powdered 
cochineal  in  a  mixture  of  50  c.  c.  of  alcohol  and  200  c.  c.  of  water,  at 
the  ordinary  temperature,  for  several  days.  The  tincture,  which  may 
be  either  decanted  or  filtered  from  the  residue,  has  a  ruby-red  color. 
The  caustic  alkalies  and  the  alkaline  carbonates  change  the  color  to  a 
violet-carmine  ;  solutions  of  strong  acids  and  acid-salts  make  it  orange  ; 
to  carbonic  acid  it  is  nearly  indifferent.  10  or  15  drops  of  the  tincture 
are  sufficient  to  color  200  c.  c.  of  liquid. 

Dilute  the  acid  which  remains  of  the  original  500  c.  c.  with  enough 
water  to  give  a  fluid,  of  which  exactly  50  c.  c.  are  required  to  saturate  • 
5  grins,  of  pure  carbonate  of  sodium.  This  dilution  is  effected  as  fol- 
lows :  —  Suppose  that  40  c.  c.  of  the  acid  have  proved  sufficient  to  neu- 
tralize 5  grms.  of  the  pure  carbonate,  then  10  c.  c.  of  water  must  be 
added  to  every  40  c.  c.  of  the  acid.  460  c.  c.  of  the  original  dilute 
acid  remain ;  now,  as  40  :  460  =  10  :  number  of  c.  c.  water  to  be  added 
=  115  c.  c.  Dilute,  therefore,  the  acid  with  115  c.  c.  of  water,  using 
some  of  this  water  to  rinse  the  burette  which  contains  the  undiluted 
acid,  the  measuring-glass  which  may  have  been  used  to  ascertain  the 
extent  of  volume  of  the  original  acid,  and  any  other  vessel  into  which 
it  may  have  been  temporarily  poured.  Of  this  diluted  acid,  50  c.  c. 
should  exactly  neutralize  5  grms.  of  pure  anhydrous  carbonate  of  so- 
dium. It  may  be  tested  by  again  weighing  out  5  grms.  of  the  pure 
carbonate,  and  repeating  the  volumetric  determination  precisely  as 
above  described.  If  the  \vork  has  been  well  done,  the  standard  acid 
will  be  found  ready  for  use. 

Weigh  out  5  grms.  of  common  soda-ash,  and  dissolve  in  about 
200  c.  c.  of  water  whatever  of  the  sample  is  soluble  ;  repeat  upon 
this  liquid,  without  filtering,  the  volumetric  operation  of  the  last  experi- 
ment. The  number  of  half  v.  c.  of  standard  acid  used  gives  directly  the 
percentage  of  pure  anhydrous  carbonate  of  sodium  which  the  sample 
contains.  If  50  c.  c.  or  100  half  c.  c.  are  used,  the  sample  is  pure  car- 
bonate of  sodium ;  if  40  c.  c.  or  80  half  c.  c.  are  used,  the  sample  is 
eighty  per  cent,  pure  carbonate,  and  twenty  per  cent,  water  and  other 
impurities. 

503. 'When  an  acid  has  been  prepared,  of  which  50  c.  c.  ex- 
actly neutralize  5  grms.  of  pure  carbonate  of  sodium,  it  is  a 
matter  of  simple  calculation  only  to  determine  how  much  hydrate 


POTASSIUM.  419 

of  sodium,  how  much  carbonate  of  potassium,  and  how  much  hy- 
drate of  potassium  of  the  same  50  c.  c.  acid  will  neutralize.  The 
following  are  the  proportions  required,  the  atomic  weight  of 
potassium  being  39.1  :  — 

106         :  80  5         :    x  =  3.773; 

Combining  weight  of  Na2C03.     Com.  wt.  of  Na2H202.     Grms.  Na2C03.      Grms.  NaJHjjOa. 

106         :  138.2     =  5         :    x  =  6.519; 

Combining  weight  of  NaaCOs-     Com.  weight  of  K2C03.     Grms.  Na^COs-      Grms.  K2C03. 

106         :  112.2     =  5        :    x  =  5.292. 

Combining  weight  of  Na2C03.    Com.  wt.  of  K2H202.      Grms.  Na^COs-      Grms.  K2H202. 

The  following  table  shows  the  quantities  of  these  four  substances 
which  are  equivalent  in  neutralizing  or  combining  power :  — 

5.000  grms.  of  carbonate  of  sodium      "i 

are  saturated  by  oO  c.  c. 
3.773       "      "  hydrate      "       "  f 

J  V    of   one    and   the  same 

b.519       "      "  carbonate  "potassium  ,     ,      ., 

5.292      "      "hydrate      «       «  j    standard  acld- 

It  is  apparent  from  this  table  that  a  given  weight  of  carbonate  of 
sodium  will  saturate  more  silicic  acid  or  more  fat,  or  will  give  off 
more  carbonic  acid  than  the  same  weight  of  carbonate  of  potas- 
sium, and  that  the  hydrate  of  sodium  is  also  more  efficient, 
gramme  for  gramme,  than  the  hydrate  of  potassium.  These 
facts  are  to  be  inferred  at  once  from  the  atomic  weights  of  so- 
dium and  potassium,  which  are  23  and  39.1  respectively  ;  they 
have  had  their  weight  in  bringing  about  the  rapid  substitution  of 
sodium-salts  for  potassium-salts  in  the  chemical  arts. 

504.  Potassium  (K).  This  element,  like  sodium,  is  made 
from  its  carbonate  by  heating  intensely  a  mixture  of  the  carbon- 
ate and  charcoal,  in  accordance  with  the  reaction, 

K2C03  +  2C  =  2K  +  3CO. 

The  apparatus  employed  is  similar  to  that  described  in  treating 
of  sodium  (§  486),  and  as  potassium  closely  resembles  sodium, 
the  same  general  method  is  followed,  and  the  same  precautions 
are  observed.  A  second  distillation  of  the  crude  potassium  is 
absolutely  essential,  because,  if  it  be  neglected,  a  black,  detonat- 
ing compound  of  uncertain  composition  is  formed,  which  explodes 
violently  upon  the  slightest  friction. 


420  POTASSIUM    DECOMPOSES    WATER. 

Potassium  is  a  silver-white  substance,  of  very  brilliant  lustre, 
which  is  brittle  at  0°,  soft  as  wax  at  ordinary  temperatures,  fuses 
at  62°. 5,  and  is  volatile  at  a  red-heat.  In  its  soft  state,  two  clean 
surfaces  can  be  welded  together  like  iron.  It  is  lighter  than 
water,  having  a  specific  gravity  of  only  0.865.  It  is  almost  in- 
stantaneously oxidized  by  air  and  water  at  the  ordinary  tem- 
perature, and,  when  heated,  by  nearly  every  gas  or  liquid  which 
contains  oxygen.  Like  sodium,  it  must,  therefore,  be  collected 
and  kept  under  naphtha,  out  of  contact  with  the  air.  At  moder- 
ate temperatures,  potassium  readily  absorbs  hydrogen,  nitrogen, 
and  carbonic  oxide,  and  enters  into  direct  combination  with 
chlorine,  bromine,  iodine,  sulphur,  selenium,  and  tellurium.  We 
have  had  occasion  to  avail  ourselves  of  its  intense  chemical 
energy  (§§  85,  97,  411). 

Exp.  252. —  Throw  a  piece  of  potassium,  as  large  as  a  small  pea,  upon 
some  cold  water  in  the  bottom  of  a  large  bottle,  and  place  a  card  or 
glass-plate  over  the  mouth  of  the  bottle.  The  potassium  decomposes 
the  water,  and  evolves  heat  enough  to  kindle  the  hydrogen  which  is 
given  off;  this  hydrogen  burns  with  a  purplish-red  color,  imparted  to 
the  flame  by  a  minute  quantity  of  vaporized  solid.  This  color  is  char- 
acteristic of  potassium  compounds,  as  a  yellow  color  is  characteristic  of 
sodium  compounds. 

505.  It  is  not  a  matter  of  indifference  from  what  kind  of  a 
mixture  of  carbonate  of  potassium  and  carbon  potassium  is  pre- 
pared. The  material  which  is  best  adapted  to  its  preparation  is 
the  potassium-salt  of  a  vegetable  acid  rich  in  carbon,  which, 
when  decomposed  by  heat  in  a  vessel  from  which  air  is  excluded, 
yields  carbonate  of  potassium  and  a  large  quantity  of  free  car- 
bon. While  grape-juice  is  being  converted  into  wine  by  fermen- 
tation, a  stony  deposit,  called  "  tartar,"  which  is  sometimes  gray 
and  sometimes  reddish,  fastens  to  the  bottom  and  sides  of  the 
casks  which  contain  the  fermented  juice.  When  freed  by  recrys- 
tallization  from  adhering  coloring  matters,  this  crystalline  and 
difficultly  soluble  substance  is  a  white  salt,  acid  and  cooling  to 
the  taste.  It  is  an  acid  tartrate  of  potassium,  and  is  commonly 
called  "  cream  of  tartar."  When  this  substance,  or  crude  tartar, 
is  heated  in  a  covered  crucible  until  it  ceases  to  emit  combustible 
vapors,  the  cooled  residue  is  found  to  consist  of  a  porous  mass  of 


CHLORIDE    OF   POTASSIUM.  421 

carbonate  of  potassium,  intimately  mixed  with  very  finely  divided 
carbon.  This  mixture  is  the  best  material  from  which  to  pre- 
pare potassium  ;  it  is  also  an  excellent  flux,  useful  in  assaying 
the  ores  of  the  common  metals.  In  wine-producing  countries 
considerable  quantities  of  excellent  carbonate  of  potassium  are 
prepared  from  the  deposits  of  the  wine-vats,  by  dissolving  the 
carbonate  out  of  the  ignited  tartar  and  purifying  the  salt,  so  ex- 
tracted, by  recrystallization.  The  carbonate  so  obtained  is  the 
purest  source  of  hydrate  of  potassium  for  laboratory  use. 

506.  Chloride  of  Potassium  (KC1).  This  salt  is  a  subordinate 
source  of  potassium  compounds.  It  is  extracted  in  considerable 
quantity  from  the  ashes  of  sea-weeds,  and  is  largely  used  in  the 
manufacture  of  common  alum,  which  is  a  sulphate  of  aluminum 
and  potassium.  It  occurs  native,  sometimes  pure,  but  more  fre- 
quently mixed  or  combined  with  other  chlorides.  The  chloride 
of  potassium  is  capable  of  uniting  with  most  other  metallic  chlo- 
rides, forming  crystallizable  double  salt?.  The  native  double 
chloride  of  potassium  and  magnesium  (KC1,  MgCl2,6H2O)  has 
become  of  late  years  a  productive  source  of  potassium-salts.  This 
mineral  is  dissolved  in  hot  water ;  from  the  cooled  solution  the 
greater  part  of  the  chloride  of  potassium  crystallizes  out,  while  the 
chloride  of  magnesium  remains  in  solution.  Chloride  of  potas- 
sium occurs  also  with  the  chlorides  of  sodium,  magnesium,  cal- 
cium, and  other  salts  in  sea-water  and  brine-springs,  and  is  ob- 
tained as  a  secondary  product  in  the  preparation  of  chlorate  of 
potassium  (§  517),  the  purification  of  nitre  (§  514),  and  in  sev- 
eral other  manufacturing  operations.  It  may  be  prepared  direct- 
ly from  potassium  and  chlorine,  or  from  the  carbonate  or  hydrate 
of  potassium  and  chlorhydric  acid. 

Chloride  of  potassium  crystallizes  in  anhydrous  cubes  ;  looks 
and  tastes  like  common  salt ;  is  not  acted  upon  by  the  air  ;  decrep- 
itates when  heated  ;  melts  at  a  low  red  heat,  and  volatilizes  un- 
changed at  a  higher  temperature.  It  is  somewhat  more  volatile 
than  common  salt,  is  more  soluble  in  water,  and  produces  a 
greater  degree  of  cold  in  dissolving.  This  chloride  enters  into 
some  highly  crystalline  compounds  of  curious  composition,  of 
which  the  product  of  the  following  experiment  may  serve  as  an 
example  :  — 


422  IODIDE    OF    POTASSIUM. 

4 

Exp.  253.  —  By  the  aid  of  a  gentle  heat,  dissolve  6  grans,  of  powdered 
red  ehromate  of  potassium  in  8  grms.  of  strong  chlorhydric  acid,  avoid- 
ing evolution  of  chlorine.  When  the  powder  is  dissolved,  allow  the 
solution  to  cool ;  flat,  red  prisms  will  crystallize  from  the  liquid.  This 
compound  answers  to  the  formula  KC1,  CrO3.  It  is  decomposed  by 
solution  in  water. 

K2O,  2CrO3          +  2HC1  =  2(KC1,  CrO3)  -f  H,0. 

Red  Chromate  of  Potassium. 

507.  Bromide  of  Potassium  (KBr)  is    a    very  soluble    salt, 
which  crystallizes  in  cubes,  and  closely  resembles  in  all  its  prop- 
erties the  chloride.     Potassium  and  bromine  unite  directly  with 
inflammation  and  violent  detonation,  the  bromide  being  the  prod- 
uct of  the  reaction.     When  bromine  is  added  to  a  solution  of 
caustic  potash,  until  the  liquid  acquires  a  permanent,  yellowish 
tinge,  a  mixture  of  bromide  and  bromate  of  potassium  is  pro- 
duced ;— 6KHO   -f-    6Br   =   oKBr   +    KBrO3  +   3H2O. 
The  mixed  salts  are  dissolved  in  water,  and  a  current  of  sulphy- 
dric  acid  is  passed  through  the  solution  to  reduce  the  bromate  ; 

KBrO3  +  3H2S  =  KBr  +  3H2O  +  3S. 
The  liquid  is  then  gently  warmed  to  expel  the  excess  of  gas, 
filtered  from  the  deposited  sulphur,  and  evaporated  until  the  bro- 
mide crystallizes.     The  salt  has  lately  come  into  use  in  medicine 
as  a  sedative. 

508.  Iodide  of  Potassium  (KI).     This  valuable  medicine  and 
photographic  material  may  be  procured  by  adding  iodine  to  a  so- 
lution of  hydrate  of  potassium,  until  the  liquid  turns  brown,  and 
gently  igniting  the  residue  obtained  by  evaporation.    The  process 
and  the  reaction  are  the  same  as  in  the  preparation  of  the  bro- 
mide, except  that  the  iodate  may  be  decomposed  by  heat  alone 
and  does  not  require  reduction  by  sulphuretted  hydrogen.     A 
better  mode  of  preparing  the  iodide  of  potassium  is  to  be  found 
in  the  decomposition  of  iodide  of  iron  by  carbonate  of  potassium. 

Digest  4  grms.  of  iodine  and  2  grms.  of  iron  filings  in  a  stoppered 
bottle  with  20  c.  c.  of  water  ;  iodide  of  iron  (FeL)  is  formed  under  these 
conditions  by  direct  union  of  the  elements.  The  liquid  is  then  trans- 
ferred to  a  flask  and  boiled,  and  a  solution  of  carbonate  of  potassium  is 
added  by  small  portions  so  long  as  a  precipitate  occurs.  The  solution, 
filtered  from  the  insoluble  carbonate  of  iron,  yields  on  evaporation 
crystals  of  iodide  of  potassium. 


CYANIDE    OF   POTASSIUM.  423 

Iodide  of  potassium  ordinarily  crystallizes  in  semi-opaque 
cubes.  It  is  permanent  in  the  air ;  has  a  sharp  taste ;  melts  be- 
low a  red  heat,  and  volatilizes  unchanged  at  a  low  red  heat.  It  is 
very  soluble  in  water,  and  in  dissolving  produces  a  considerable 
fall  of  temperature.  Alcohol  also  freely  dissolves  this  salt.  The 
facility  with  which  the  salt  is  decomposed  and  its  iodine  liberated 
by  chlorine,  ozone,  and  nitric  acid,  has  been  already  amply  illus- 
trated (Exps.  74,  76,  80). 

Iodide  of  potassium  is  much  used  in  medicine;  it  is  not  poison- 
ous, even  in  doses  of  5  to  20  grms.  Its  solution  in  water  dis- 
solves iodine  to  a  large  extent,  acquiring  thereby  a  dark-brown 
color.  This  brown  solution  is  sometimes  used  in  medicine  as  a 
vehicle  for  iodine ;  it  is  also  useful  to  the  photographer  for  re- 
moving from  the  skin  the  stains  produced  by  nitrate  of  silver. 
Iodide  of  potassium  is,  further,  employed  in  the  photographic 
process  upon  glass,  to  produce  in  the  substance  of  the  film  of  col- 
lodion a  deposit  of  iodide  of  silver  by  double  decomposition  with 
nitrate  of  silver. 

509.  Cyanide  of  Potassium  (KCN).  The  elements  which 
make  part  of  the  whitish,  soluble,  fusible,  and  deliquescent  solid 
which  bears  this  name  are  potassium,  carbon,  and  nitrogen.  At 
a  bright  red  heat  these  three  elements  will  come  together  to  form 
this  salt  out  of  quite  a  variety  of  materials,  and  under  quite  vari- 
ous circumstances.  When  nitrogen  gas  is  passed  over  a  mixture 
of  charcoal  and  hydrate  or  carbonate  of  potassium,  at  a  bright  red 
heat,  cyanide  of  potassium  is  formed.  When  nitrogenous  organic 
matters  are  fused  with  hydrate  or  carbonate  of  potassium,  the 
cyanide  is  formed.  In  presence  of  iron  scraps  or  filings,  this 
mixture  produces  a  common  cyanide,  containing  potassium,  iron, 
carbon,  and  nitrogen,  and  known  in  the  arts  as  "  yellow  prussiate 
of  potash,"  and  to  chemists  as  ferrocyanide  of  potassium.  If  this 
"  prussiate  "  is  ignited  at  a  moderate  red  heat,  it  is  decomposed 
into  cyanide  of  potassium,  nitrogen,  and  a  compound  of  iron  with 
carbon :  — 

4KCN,Fe(CN)2  =  4KCN  +  2N  +  FeC2. 
In  the  blast  furnaces  in  which  iron  ores  are  smelted  with  coal 
or  coke,  a- considerable  quantity  of  cyanide  of  potassium  is  often 
produced,  the  nitrogen  being  probably  derived  from  the  torrents 


424  SULPHIDES    OF   POTASSIUM. 

of  nitrogen  which  the  blast  of  air  carries  into  the  furnace.  In- 
stead of  igniting  the  yellow  prussiate  alone,  a  more  economical 
process  is  to  ignite  a  mixture  of  the  prussiate  with  carbonate  of 
potassium.  This  method  saves  all  the  cyanogen  in  the  prussiate, 
but  the  salt  thus  obtained  is  always  mixed  with  cyanate  and  car- 
bonate of  potassium :  — 
K4Fe(CN)6  +  K2C03  =  5K(CN)  +  K(CN)O  +  C02  +  Fe. 

The  presence  of  these  impurities  does  not  injure  the  cyanide 
for  many  of  its  uses. 

Cyanide  of  potassium  is  of  great  use  in  galvanic  gilding  and 
silvering,  since  the  cyanides  of  gold  and  silver  are  both  soluble  in 
a  solution  of  cyanide  of  potassium.  Its  solution  dissolves  the 
sulphide  of  silver,  and  has,  therefore,  been  suggested  for  house- 
hold use  in  cleaning  silver-ware  ;  photographers  sometimes  use  it 
for  removing  stains  of  nitrate  of  silver  from  the  hands ;  but  both 
these  applications  of  cyanide  of  potassium  are  dangerous  and  in- 
expedient. The  cyanide  is  intensely  poisonous,  not  only  when 
taken  internally,  but  also  when  brought  in  contact  with  an  abra- 
sion of  the  skin,  a  cut  or  scratch.  As  a  reducing  agent,  cya- 
nide of  potassium  has  great  power  ;  it  is  especially  usefuLin  blow- 
pipe reactions  (Ex.  136). 

510.  Sulphides  of  Potassium.  Potassium,  heated  in  sulphur- 
vapor,  takes  fire  readily,  and  burns  brilliantly.  There  are  sup- 
posed to  be  five  different  compounds  of  potassium  and  sulphur, 
corresponding  to  the  formulae  K2S ,  K2S2  «'K2S3 ,  K2S4 ,  and  K2S5 , 
and  there  is  a  sulphydrate  KHS,  analogous  to  the  hydrate  KHO. 

Exp.  254. —  Heat,  gently,  a  thorough  mixture  of  10  grms.  of  dry, 
powdered  carbonate  of  potassium  and  6  grms.  of  sulphur,  in  a  covered, 
earthen  or  iron  crucible,  until  effervescence  ceases  and  the  mass  flows 
quietly.  The  fused  mass  has  a  yellowish-brown  color,  and  consists  of 
tersulphide,  quinquisulphide,  and  intermediate  sulphides  of  potassium, 
mixed  with  sulphate,  and  often  with  carbonate  of  potassium  ;  it  is 
called  "  liver  of  sulphur."  This  substance,  dissolved  in  water,  gives  a 
greenish  solution,  from  which  dilute  acids  liberate  sulphuretted  hydro- 
gen, and  precipitate  milk  of  sulphur  (§  198).  The  carbonic  acid  of  the 
air  is  strong  enough  to  effect  this  decomposition  ;  hence  the  solid  sub- 
stance and  its  solution,  when  exposed  to  the  air,  smell  of  sulphuretted 
hydrogen  :  — 

K2S3  -f  H2SO4  =  K2SO4  -f  H2S  -f  2S. 


SULPHATE    OF    POTASSIUM.  425 

The  chief  use  of  liver  of  sulphur  is  in  the  medical  treatment  of  skin- 
diseases. 

oil.  When  sulphydric  acid  gas  is  passed  to  saturation  into  a 
solution  of  caustic  potash,  a  colorless  solution  is  obtained,  which 
is  supposed  to  contain  the  sulphydrate  KHS.  It  has  no  perma- 
nency, quickly  absorbing  oxygen  and  turning  yellow. 

All  the  sulphides  of  potassium  are  brown  solids,  having  an  al- 
kaline reaction  to  test-paper,  and  smelling  of  sulphydric  acid. 
Their  solutions  are  oxidized  by  exposure  to  the  air,  and  sulphur 
is  deposited  from  them. 

512.  Sulphate  of  Potassium  (K2S04).     This  anhydrous  salt 
crystallizes  in  transparent  hexagonal  prisms,  terminated  by  hex- 
agonal pyramids,  and,  consequently  bears  some  resemblance  to 
common  quartz  crystals.     The   salt    has   a   strong  tendency  to 
form  double  sulphates  ;  a  double  sulphate  of  potassium  and  mag- 
nesium is  of  importance  in  the  manufacture  of  potassium-salts 
from  sea-water.     The  salt  enters  into  the  composition  of  many 
of  the  double  sulphates  which  are  called  alums,  from  the  name 
of  the  commonest  member  of  the  class,  the  sulphate  of  aluminum 
and  potassium. 

513.  Sulphate  of  Potassium  and  Hydrogen  (KHS04).     This 
salt,  commonly  called  the  "  bisulphate,"  is  formed  on  a  large  scale 
as  a  residuary  product,  whenever  nitric  acid  is  manufactured  from 
nitrate  of  potassium.     When  ignited,  its  crystals  lose  half  their 
acid :  — 

2(KHS04)  =  K2SO4  -f  H2S04, 

and  the  salt  is  therefore  sometimes  used  as  a  flux,  in  cases  where 
the  action  of  a  strong  acid  is  required  at  a  high  temperature  upon 
salts  or  oxides  with  which  it  may  be  fused.  Platinum-tools  may 
be  cleaned  by  fusing  this  salt  in  or  upon  them. 

514.  Nitrate  of  Potassium  (KNO3).     This  valuable  salt,  com- 
monly called  saltpetre,  is  very  widely  diffused  as  a  natural  prod- 
uct, being  indeed  seldom  wholly  wanting  in  a  productive  soil,  or 
in  spring  or  river  water.      At   many  localities    it   is    found  in 
caverns  or  caves  in  calcareous  formations,  but  the  principal  com- 
mercial source  of  the  salt  is  the  soil  of  certain  tropical  regions, 
especially  of  districts  in   Arabia,  Persia,  and   India,  where  the 
nitrate  is  found  disseminated  through  the  upper  portion  of  the 


426  REFINING    OF    SALTPETRE. 

soil,  or  forming  an  efflorescence  upon  the  surface,  but  never 
extending  lower  than  the  depth  to  which  the  air  can  easily 
penetrate. 

This  natural  production  of  nitrates  appears  to  result  mainly  from  the 
putrefaction  of  vegetable  and  animal  matters,  in  presence  of  the  air 
and  of  alkaline  or  earthy  bases  capable  of  fixing  the  nitric  acid  as  soon 
as  it  is  formed.  The  well-waters  of  towns,  contaminated  by  neighbor- 
ing sewers  and  cesspools,  nearly  always  contain  nitrates.  The  decay 
of  the  luxuriant  vegetation  of  the  tropics,  promoted  by  a  hot  sun  and  a 
moist  atmosphere,  is  a  never-failing  source  of  ammonia ;  but  it  is  not 
certain  that  the  production  of  ammonia  is  a  necessary  stage  in  the  pro- 
cess of  nitrification.  In  the  artificial  production  of  nitrates  in  temper- 
ate climates,  the  supposed  natural  conditions  have  been  roughly  imi- 
tated. In  the  old  saltpetre  "  plantations "  of  European  countries, 
nitrate  of  calcium  was  produced  by  mixing  decomposing  vegetable  and 
animal  matters  with  cinders,  chalk,  marl,  and  so  forth,  moistening  the 
mass  repeatedly  with  teachings  of  manure-heaps,  exposing  it  freely  to 
the  air  for  two  or  three  years,  and  then  lixiviating.  The  nitrate  of 
calcium,  which  was  the  main  product,  was  then  converted  into  saltpetre 
by  double  decomposition  with  carbonate  of  potassium. 

The  saltpetre  is  extracted  from  the  earth  which  contains  it  by  lixiv- 
iation,  evaporation,  and  crystallization,  but  inasmuch  as  for  most  uses 
it  is  required  in  a  very  pure  state,  the  crude  salt  must  generally  be  re- 
fined. The  common  impurities  of  crude  saltpetre  are  chloride  of  so- 
dium and  chloride  of  potassium.  In  order  to  separate  these  chlorides, 
advantage  is  taken  of  the  fact  that  the  nitrate  of  potassium  is  four  times 
as  soluble  in  boiling  water  as  the  chloride  of  potassium,  and  six  times  as 
soluble  as  the  chloride  of  sodium.  The  crude  saltpetre  is  treated  with 
a  quantity  of  water,  sufficient  to  dissolve  at  boiling  all  the  nitrate  of 
potassium,  but  not  all  of  the  chloride  of  sodium  beside.  This  residual  salt 
is  scooped  out  of  the  vessel  in  which  the  solution  is  effected,  and  the  so- 
lution, after  being  somewhat  diluted,  is  boiled  with  a  little  glue,  to  co- 
agulate the  coloring  matters  and  other  soluble  dirt,  and  sweep  the  liquid 
clean  by  means  of  the  adhesive  scum  which  rises  to  the  surface.  The 
strong,  clear  solution  is  then  transferred  to  shallow  crystallizing  pans, 
and  left  at  rest  if  large  crystals  are  desired ;  if  small  crystals  are  prefer- 
able, the  liquid  is  constantly  stirred  from  the  moment  that  crystalliza- 
tion begins ;  the  saltpetre  is  then  deposited  in  a  crystalline  powder, 
called  saltpetre-flour.  The  chlorides  of  sodium  and  potassium  are 
nearly  as  soluble  in  cold  water  as  in  hot,  but  nitrate  of  potassium  is 
only  one-eighth  as  soluble  in  water  at  the  temperature  of  the  atmos- 
phere, as  in  boiling  water.  Hence  the  chlorides  remain  in  the  mother- 


PROPERTIES    OF    SALTPETRE.  427 

liquor,  while  the  nitrate  rapidly  separates  from  the  solution  as  it  cools. 
The  crystals  of  saltpetre  are  drained,  and  then  washed  with  a  solution 
of  saltpetre  saturated  in  the  cold.  This  solution  takes  up  the  adhering 
chlorides,  but  leaves  the  pure  nitrate  of  potassium  undissolved. 

Large  quantities  of  saltpetre  are  now  made  by  decomposing  nitrate 
of  sodium  with  carbonate  of  potassium.  When,  through  governmental 
interference,  the  East  Indian  supply  of  saltpetre  is  checked,  this  method 
is  resorted  to  with  advantage.  Tartar,  and  the  ashes  of  the  beet-root 
sugar  manufacture,  are  good  sources  of  potash  to  be  applied  to  this  pur- 
pose. Crude  nitrate  of  sodium  contains  so  much  chloride  of  sodium, 
that  it  is  desirable  to  purify  it  for  this  use  by  previous  recrystalliza- 
tion,  otherwise  potash  would  be  unprofitably  consumed  in  converting 
chloride  of  sodium  into  chloride  of  potassium.  One  of  the  processes  for 
converting  the  nitrate  of  sodium  into  nitrate  of  potassium  consists 
simply  in  adding  the  nitrate  of  sodium  to  a  hot,  concentrated  solution 
of  carbonate  of  potassium ;  a  precipitation  of  carbonate  of  sodium  takes 
place,  and  this  precipitate  is  removed  as  fast  as  it  forms,  until  no  more 
appears ;  from  the  cooled  liquid  saltpetre-flour  is  deposited.  The  carbon- 
ate of  potassium  may  be  replaced  in  this  process  by  chloride  of  potas- 
sium. NaNO3  -f  KC1  =  KNO,  +  NaCl. 

515.  Nitrate  of  potassium  is  white,  inodorous,  and  anhydrous, 
and  has  a  cooling,  bitter  taste.  When  pure,  it  is  permanent  in 
the  air,  —  a  fact  of  great  importance,  inasmuch  as  the  chief  use 
of  this  salt  is  in  the  manufacture  of  gunpowder.  Were  it  hygro- 
scopic, like  nitrate  of  sodium,  it  would  not  be  applicable  to  this 
use.  Saltpetre  is  one  of  those  few  potassium-salts  which  cannot 
be  wholly  replaced  in  the  arts  by  the  corresponding  sodium-salt. 
It  is  very  soluble  in  water,  especially  in  hot  water ;  it  melts  below 
a  red  heat  to  a  colorless  liquid  without  loss  of  substance,  but  at  a 
red  heat  it  gives  off  oxygen,  and  suffers  decomposition.  Its 
most  marked  chemical  characteristic  is  its  oxidizing  power.  It 
deflagrates  in  the  fire  with  charcoal,  sulphur,  phosphorus,  and 
other  combustible  bodies  ;  when  ignited  in  contact  with  copper 
or  iron  (P^xp.  49),  it  converts  these  metals  into  oxides,  and  it 
even  oxidizes  gold,  silver,  and  platinum.  It  is  on  the  oxidizing 
power  of  saltpetre  that  its  use  in  the  manufacture  of  gunpowder 
and  fire-works,  and  in  the  preparation  of  matches,  depends. 

Arrange  10  grms.  of  pure  nitrate  of  potassium  and  20  or  30  grms.  of 
thin  copper-turnings,  or  small  bits  of  sheet-copper,  in  alternate  layers, 
in  a  covered  copper  crucible,  and  expose  the  mixture  for  half  an  hour 


428  SALTPETRE    NOT    EXPLOSIVE. 

to  a  moderate  red  heat.  Dissolve  out  the  cooled  mass  with  water,  and 
let  the  liquid  stand  in  a  tall,  closed  bottle  until  the  oxide  of  copper  has 
settled  to  the  bottom.  The  supernatant  liquid  is  a  pure  solution  of 
caustic  potash ;  indeed,  this  is  an  excellent  method  of  preparing  pure 
hydrate  of  potassium  for  use  in  analysis  :  — 

2KN031  +  5Cu  +  H20  =  2KHO  +  5CuO  +  2N. 

Exp.  255. —  Heat  10  or  12  grms.  of  saltpetre,  gently,  in  a  small 
porcelain  dish,  until  it  melts ;  pour  the  melted  salt  out  on  a  cold  piece 
of  iron  or  stone ;  break  the  fused  mass  into  small  fragments,  and  fill  an 
ignition-tube,  12-15  c.  m.  long,  one-third  full  with  these  bits.  Heat  the 
tube  cautiously,  taking  pains  to  keep  all  the  salt,  when  once  melted,  in 
a  state  of  fusion.  At  a  red  heat,  oxygen,  pure  at  first,  is  slowly  evolved, 
and  may  be  collected  at  the  water-pan ;  simultaneously,  nitrite  of  po- 
tassium (KNO3)  is  formed;  at  a  second  stage,  this  nitrite  is  itself  de- 
composed, and  the  escaping  oxygen  is  then  contaminated  with  a  certain 
proportion  of  nitrogen.  A  portion  of  the  gas  collected  may  be  tested  with 
a  glowing  splinter  (Exp.  6)  ;  another  portion  may  be  mixed  with  coal- 
gas,  and,  with  the  mixture,  bubbles  may  be  blown,  as  directed  in  Exp. 
80  ;  the  mixture  will  be  found  to  be  exceedingly  explosive. 

This  experiment  proves,  in  the  first  place,  that  saltpetre  itself  is  not 
explosive  ;  and,  in  the  second  place,  affords  an  explanation  of  the  fact 
that  frightful  explosions  do  often  occur  when  storehouses,  containing 
saltpetre,  are  burned.  Carburetted  hydrogen,  such  as  wfis  obtained  in 
Exp.  156  (represented  by  the  coal-gas  in  the  last  experiment),  is 
evolved  from  the  wood- work  of  the  burning  building,  wherever  the  wood 
is  heated  out  of  contact  with  the  air ;  meanwhile,  oxygen  is  given  off 
from  the  ignited  saltpetre,  and  whenever  these  two  gases  mix  in  the 
requisite  proportions,  and  their  mixture  comes  in  contact  with  a  flame, 
a  violent  explosion  inevitably  ensues. 

Exp.  256.  —  Dissolve  5  grms.  of  saltpetre  in  20  c.  c.  of  water;  dip 
strips  of  bibulous  paper  in  the  solution,  and  dry  them;  this  paper,  once 
kindled,  will  smoulder  away  till  consumed.  It  is  used  in  connection 
with  fire-works,  and  in  the  manufacture  of  pastiles  and  aromatic  fumi- 
gating paper. 

Exp.  257. — Mix  5  grms.  of  powdered  saltpetre  with  1  grm.  of  dry, 
powdered  charcoal ;  place  the  mixture  on  a  piece  of  porcelain  and  ig- 
nite it  with  a  hot  wire.  When  the  deflagration  is  over,  a  white  solid 
will  be  found  upon  the  porcelain.  Dissolve  this  solid  in  a  few  drops  of 
water  ;  the  solution  will  be  alkaline  to  test-paper ;  add  a  few  drops  of 
a  dilute  acid  ;  a  brisk  effervescence  marks  the  escape  of  carbonic  acid. 
The  nitrate  has  oxidized  the  carbon  to  carbonic  acid,  part  of  which 


GUNPOWDER.  429 

escaped  with  the  nitrogen  during  the  deflagration,  while  part  entered 
into  combination  with  the  potassium  :  — 

4KN03  +  5C  =  2K2C03  +  SCO,  +  4N. 

Exp.  258.  — Place  30  grms.  of  saltpetre  in  a  small  beaker  with  110 
c.  c.  of  water ;  insert  a  thermometer  in  the  mixture,  and  observe  the 
very  considerable  fait  of  temperature  occasioned  by  the  solution  of  the 
salt.  In  those  countries  where  saltpetre  is  cheap  and  ice  dear,  this 
property  of  the  salt  is  availed  of  for  the  refrigeration  of  drinks. 

516.  Gunpowder  is  an  intimate  mechanical  mixture  of  soft- 
wood charcoal  (§  382),  sulphur,  and  nitrate  of  potassium,  in  the 
proportions  of  70  or  80  per  cent,  of  the  nitrate  to  10  or  12  per 
cent,  of  each  of  the  other  ingredients.  Though  it  is  in  no  sense 
a  chemical  compound,  we  may,  for  convenience'  sake,  express 
the  composition  of  gunpowder  by  the  formula  K2N206  +  S  -|-  3C, 
and  may  roughly  formulate  the  reactions  which  occur,  when  it  is 
burned,  by  the  following  equation  :  — 

KJSTA  +  S  +  3C  =  3C02  +  2N  +  K2S. 

Speaking  in  general  terms,  the  oxygen  of  the  nitrate  combines 
with  the  carbon  to  form  carbonic  acid,  or,  at  the  least,  carbonic 
oxide,  while  the  sulphur  is  retained  by  the  potassium,  and  nitro- 
gen left  free.  Gunpowder  burns  at  the  expense  of  the  oxygen 
contained  in  it :  it  has  no  need  of  air  for  its  combustion,  but  can 
be  burned  in  any  closed  space,  —  as  well,  for  example,  in  canis- 
ters under  water,  or  tightly  enclosed  in  the  chamber  of  a  gun,  as 
in  free  air. 

From  the  formula,  it  will  be  seen,  at  a  glance,  that  a  very  large 
proportion  of  gas,  as  compared  with  the  bulk  of  the  solid  powder, 
must  be  evolved  when  powder  is  burned.  But  gunpowder  burns 
rapidly  and  with  great  evolution  of  heat,  so  that  the  volume  of 
gas,  large  at  any  temperature,  is  enormously  expanded  at  the 
moment  of  its  formation  ;  hence  it  happens  that  the  gas  set  free 
in  the  barrel  of  a  gun  may  be  capable  of  occupying  a  thousand 
or  fifteen  hundred  times  as  much  space  as  the  powder  which  gen- 
erated it.  An  enormous  pressure  is  thus  engendered  at  the  spot 
where  the  powder  burns,  and  to  this  pressure  some  part  of  the 
matter,  which  confines  the  powder,  must  yield.  In  the  case  of 
the  gun-barrel,  it  is  the  bullet  which  represents  the  weakest,  or 


430  PREPARATION    OF    GUNPOWDER. 

breaking  side  of  the  chamber  in  which  the  powder  burns  ;  but 
when  rocks  are  blasted,  then  the  packing,  or  "  tamping,"  which 
represents  the  ball,  is  made  so  firm  that  it  shall  be  stronger  than 
the  rocky  sides  of  the  drill-hole,  which  is  equivalent  to  the  bar- 
rel of  the  gun.  In  case  the  walls  of  the  gun  can  be  disrupted  more 
readily  than  the  firmly  impacted  bullet  can  be  driven  out,  then, 
of  course,  the  gun  bursts  ;  and,  conversely,  the  tamping  of  a  drill- 
hole is  thrown  out  if  it  be  less  firm  than  the  rock.  In  the  case 
of  the  gun-barrel,  a  part  of  the  effect  of  the  explosion  is  felt  in 
the  kick  or  recoil  of  the  gun. 

Though  the  equation  last  given  is  useful  in  so  far  as  it  exhib- 
its the  gaseous  products  evolved  during  the  combustion  of  gun- 
powder, it  does  not  truly  express  the  solid  products  of  the  reac- 
tion. The  residue  of  the  combustion  really  contains  only  a 
comparatively  small  proportion  of  sulphide  of  potassium ;  it 
consists  mainly  of  sulphate  of  potassium  and  carbonate  of  potas- 
sium, together  with  some  hyposulphite  of  potassium,  and  a  trace 
of  unburned  carbon.  Enough  sulphide  of  potassium  is  always 
present,  however,  to  impart  the  offensive  odor  which  is  perceived 
in  washing  a  foul  gun,  and  in  powder-smoke. 

Exp.  259. — Pulverize,  separately,  23  grms.  of  nitrate  of  potassium, 
4  grms.  of  sulphur,  and  4  grms.  of  recently  ignited  charcoal.  Place  a 
drop  or  two  of  water  in  a  porcelain  mortar,  and  grind  into  it,  first,  the 
charcoal,  and  then  the  other  ingredients,  taking  care  to  add  enough 
water  to  form  a  plastic  dough.  After  the  mass  has  been  thoroughly 
kneaded,  roll  out  small  portions  of  it  between  two  pieces  of  board,  into 
long  threads,  of  the  thickness  of  a  fine  knitting-needle.  With  a  knife, 
cut  the  threads  into  small  fragments  or  granules,  and  leave  the  gran- 
ules in  a  warm  room  to  dry.  The  thoroughly-dried  product  is  gun- 
powder, and  the  manipulation  in  this  experiment  does  not  differ  essen- 
tially from  the  mode  of  manufacture  employed  in  powder-mills,  ex- 
cepting that  the  granulation  is  there  effected  by  passing  the  moist  paste 
through  cullenders. ' 

The  sulphur  in  gunpowder  acts  mainly  as  a  kindling  material  (§  200). 
In  powder  intended  for  use  in  guns,  the  proportion  of  sulphur  is  kept 
comparatively  low,  since  any  excess  of  it  would  corrode  the  metal  of  the 
gun. 

Exp.  260.  —  Knead  together,  as  in  Exp.  259,  7  grms.  of  powdered 
nitrate  of  potassium  and  1.5  grms.  of  recently-burned  and  finely-pow- 
dered charcoal.  Granulate  and  dry  the  product,  as  before,  and  'join- 


CHLORATE    OF    POTASSIUM.  431 

pare  its  inflammability  with  that  of  the  gunpowder  prepared  in  Exp. 
259,  by  touching  small  heaps  of  each  with  a  red-hot  wire.  Mixtures  of 
charcoal  and  nitrate  of  potassium,  such  as  the  foregoing,  are  much  used 
in  the  manufacture  of  fire-works. 

517.  Chlorate  of  Potassium  (KC103).  The  basis  of  the  large 
use  now  made  of  this  beautiful  salt  in  medicine,  in  calico-painting, 
in  pyrotechny,  in  the  match-manufacture,  and  in  the  chemical 
laboratory,  is  its  large  oxygen-contents.  It  is  an  oxidizing  agent 
of  the  most  vigorous  description. 

It  may  be  prepared  by  saturating  a  solution  of  1  part  of  hydrate  of 
potassium  in  3  parts  of  water,  with  chlorine,  and  heating  the  liquid 
some  time  to  the  boiling-point.  The  ultimate  result  may  be  expressed 
by  the  formula 

6KHO  +  6C1  =  KC1O3  -f  5KC1  -f  3H2O, 

but  the  process  has  two  stages,  which  are  sufficiently  described  in  §  124. 
The  hot  solution,  left  to  itself,  deposits  the  greater  part  of  the  chlorate 
in  anhydrous,  six-sided  plates  of  a  pearly  lustre  ;  the  chloride  of  potas- 
sium remains  in  the  mother-liquor.  The  chlorate  is  freed  from  adher- 
ing chloride  by  recrystallization.  The  success  of  the  process  depends 
upon  the  very  different  solubilities  of  the  chlorate  and  the  chloride  of 
potassium.  At  the  temperature  of  their  saturated  boiling  solutions  both 
salts  are  about  equally  soluble,  —  100  parts  of  water  will  dissolve  be- 
tween 60  and  67  parts  of  either  salt ;  but  at  the  ordinary  temperature  of 
the  air,  1 00  parts  of  water  will  dissolve  30-40  parts  of  chloride  of  potas- 
sium and  only  6  or  7  parts  of  chlorate  of  potassium.  We  find  here  the 
explanation  of  the  fact  that  chlorate  of  sodium  has  not  replaced  chlo- 
rate of  potassium  in  the  arts.  The  chlorate  of  sodium  is  more  soluble  in 
water  at  all  temperatures  than  the  chloride  of  sodium  is,  while  both 
are  exceedingly  soluble,  so  that  the  two  salts  cannot  be  separated  by 
crystallization.  This  process  of  crystallization  is  the  chemical  manu- 
facturer's chief  reliance  in  refining  both  his  materials  and  his  products, 
and  the  purchaser  of  chemicals  finds  his  best  guaranty  of  the  purity  of 
his  commodities  in  the  peculiar  form,  lustre,  color,  and  degree  of  trans- 
parency which  characterize  the  crystals  of  every  crystallizable  and  per- 
manent chemical  compound.  Hence,  an  easily  crystallized,  permanent 
salt,  of  characteristic  appearance,  like  chlorate  of  potassium,  will  always 
have  the  preference  over  one  which,  like  chlorate  of  sodium,  can  be 
crystallized  and  purified  only  with  difficulty,  and  is  not  permanent 
when  once  obtained.  The  chlorate  of  sodium  is  deliquescent. 

The  waste-product  in  the  making  of  chlorate  of  potassium,  by  the 
process  just  described,  is  chloride  of  potassium,  a  comparatively  dear 


432  OXIDATION    BY    CHLORATE    OF    POTASSIUM. 

salt.  An  economy  is  effected  by  substituting  hydrate  of  calcium  for 
hydrate  of  potassium,  and  thus  making  the  secondary  product  chloride 
of  calcium  instead  of  chloride  of  potassium;  one  equivalent  only  of  the 
chloride  of  potassium  is  then  required  instead  of  six  of  the  hydrate  of 
potassium.  An  excess  of  chlorine  is  passed  into  a  mixture  of  300  parts 
of  quick-lime,  154  parts  of  chloride  of  potassium,  and  100  of  water.  The 
mass  is  heated  by  steam,  stirred  with  agitators,  filtered,  and  then  evapo- 
rated nearly  to  dryness  by  steam-heat ;  the  mass  is  then  redissolved  in 
hot  water  and  set  to  crystallize  :  — 

3CaO  +  KC1  -f  6C1  =  KC1O3  +  3CaCl2. 

The  mother-liquor,  which  contains  all  the  chloride  of  calcium,  may  be 
decomposed  with  sulphate  of  potassium,  in  which  event  a  very  finely- 
divided  sulphate  of  calcium,  available  for  "  stuffing"  in  the  manufacture 
of  paper,  is  precipitated,  and  chloride  of  potassium  is  recovered,  to  be 
again  applied  to  the  production  of  the  chlorate ;  or  the  chloride  of  cal- 
cium solution  may  be  decomposed  with  carbonate  of  sodium,  in  order  to 
precipitate  a  very  finely-divided  carbonate  of  calcium,  which  is  largely 
employed  by  the  pharmaceutist  and  perfumer.  In  the  latter  case, 
chloride  of  sodium  has  to  be  thrown  away.  The  whole  manufacture  is 
a  good  example  of  a  technical  chemical  process. 

518.  Chlorate  of  potassium  is  easily  decomposed  by  heat ;  at 
a  moderate  temperature  it  yields  perchlorate  and  chloride  of  po- 
tassium (Exp.  69),  but  at  a  red  heat  it  is  resolved  into  chloride 
of  potassium  and  oxygen  (Exp.  7)  :  —  KC1O3  =  KC1  +  3O. 
Chlorate  of  potassium  is  so  prompt  an  oxidizing  agent  that  mix- 
tures of  it  with  combustible  bodies  often  detonate  violently  when 
struck  or  heated  (Exps.  117,  162).  These  combustions  are  at- 
tended with  great  danger  unless  very  small  quantities  be  used. 
Strong  acids,  like  sulphuric,  nitric,  and  cblorhydric  acids,  decom- 
pose chlorate  of  potassium  with  evolution  of  oxides  of  chlorine, 
or  of  chlorine  and  oxygen.  The  decomposition  is  often  attended 
with  decrepitation,  and  sometimes  with  a  flashing  light ;  combus- 
tibles, like  sulphur,  phosphorus,  sugar,  and  resin,  are  inflamed  by 
the  gases  evolved.  A  mixture  of  chlorate  of  potassium  and 
chlorhydric  acid  is  used  in  toxicological  investigations  as  an  oxid- 
izing agent  for  the  destruction  of  organic  matter  (§  329).  The 
following  formulae  will  elucidate  some  of  these  reactions  :  — 
3KC103  +  2H2S04r=2C102  +KC104  +2KHS04  +  H2O 
8KC1O8+  6HNO3r=:  6KNO3+  2KC104+  6C1+ 13O+  3H2O 
5KC1O3-|-12HC1  =4KC1  +3C\O2  -j-9Cl+6H2O. 


AMMONIUM-SALTS.         ,,  433 

Exp.  261. —  Pour  into  a  conical  test-glass  25-30  c.  c.  of  water,  and 
throw  into  the  water  some  scraps  of  phosphorus,  weighing  together  not 
more  than  0.3  grra.,  and  3-4  grms.  of  crystals  of  chlorate  of  potassium. 
By  means  of  a  thistle-tube  bring  5  or  6  c.  c.  of  strong  sulphuric  acid  into 
immediate  contact  with  the  chlorate  at  the  bottom  of  the  glass.  Then 
withdraw  the  thistle-tube.  In  a  moment  the  phosphorus  is  kindled,  and 
burns  with  vivid  flashes  of  light  beneath  the  water.  An  evolution  of 
chlorine  accompanies  the  reaction. 

Exp.  262.  —  Rub  4  or  5  grms.  of  clean  chlorate  of  potassium,  free 
from  dust,  to  a  fine  powder  in  a  porcelain  mortar.  In  powdering  chlo- 
rate of  potassium,  care  must  be  taken  that  the  mortar  and  pestle  are 
perfectly  clean,  and  the  salt  free  from  organic  matter,  and  that  violent 
percussion  and  heavy  pressure  upon  the  contents  of  the  mortar  be 
wholly  avoided.  Place  the  powdered  chlorate  on  a  piece  of  paper,  add 
an  equal  bulk  of  dry,  powdered  sugar  to  the  pile,  and  with  the  fingers 
and  a  piece  of  card,  mix  the  two  materials  thoroughly  together.  Mix- 
tures of  chlorate  of  potassium  and  organic  matter  are  liable  to  explode, 
if  strongly  rubbed  or  but  lightly  struck.  Wrap  the  mixture  in  a  paper 
cylinder,  and  place  the  cylinder  on  a  brick  in  a  strong  draught  of  air; 
let  fall  upon  the  mixture  a  drop  of  sulphuric  acid  from  the  end  of  a 
glass  rod  ;  a  very  vivid  combustion  will  ensue,  with  the  violet-colored 
flame  characteristic  of  potassium. 

Exp.  263.  —  Mix  together,  on  paper,  with  the  precautions  above  de- 
eribed,  1  grm.  of  black  oxide  of  copper,  1  grm.  of  sulphur,  and  2.5 
grms.  of  powdered  chlorate  of  potassium.  Place  the  mixture,  inclosed 
in  a  paper  cylinder,  on  the  top  of  a  brick,  and  touch  it  with  a  hot  wire ; 
it*  will  burn  vividly,  and  with  a  purple  color,  which  is  prized  in 
pyrotechny. 


CHAPTER    XXV. 

AMMONIUM-SALTS. 

519.  The  hypothetical  metal,  ammonium  (NH4)  is  a  device 
for  explaining  the  constitution  and  properties  of  one  well-defined 
class  out  of  the  several  classes  of  compounds  into  which  the  gas 
ammonia  enters.  This  class  of  compounds  is  that  which  results 
from  neutralizing  an  aqueous  solution  of  ammonia  with  acids,  as 
in  the  following  reactions  :  — 

28 


434  AMMONIUM-SALTS. 

NH3,  H2O  +  H2S04  =  (NH4)HS04  +  H2O. 

Sulphate  of  Ammonium  and  Hydrogen. 

NH3,  H20  -|-  HNO3  =  (NH4)NO3    +  H2O. 
Nitrate  of  Ammonium. 

According  to  this  hypothesis,  the  crystalline  salts  which  result 
from  such  neutralizations  contain  a  group  of  atoms  (NH4)  which 
is  analogous  in  its  action  to  potassium  and  sodium,  and  which 
forms  salts  analogous  in  composition  to  the  potassium-salts ;  thus, 
chloride  of  ammonium  (NH4)C1  is  analogous  to  chloride  of  potas- 
sium KC1 ;  sulphate  of  ammonium  (NH4)2iSO4  is  analogous  to 
sulphate  of  potassium  K2SO4,  and  so  forth  (§  91). 

All  the  actual  evidence  we  possess  of  the  separate  existence 
and  metallic  character  of  the  group  NH4  is  contained  in  the  fol- 
lowing curious,  but  inconclusive  experiment :  — 

Exp.  264. —  Pour  8  or  10  c.  c.  of  mercury  into  a  small  flask,  and 
warm  the  mercury  over  a  gas-lamp  ;  drop  upon  the  mercury  six  or 
eight  bits  of  metallic  sodium,  no  one  of  them  larger  than  a  hemp-seed. 
The  sodium  dissolves  with  some  spattering  in  the  warm  mercury,  and  a 
sodium  amalgam  is  thus  obtained.  Transfer  the  amalgam  to  a  tall  glass 
or  bottle  of  at  least  300  c.  c.  capacity,  and  pour  over  it  a  concentrated 
solution  of  chloride  of  ammonium.  The  amalgam  immediately  begins 
to  swell  up,  and  ultimately  increases  to  8  or  10  times  its  original  bulk, 
in  the  cold,  or  to  20  or  30  times  if  the  solution  be  hot,  assuming  a  pasty 
consistency  like  that  of  soft  butter,  but  preserving  its  metallic  lustre- 
It  begins  to  undergo  spontaneous  decomposition  as  soon  as  it  is  formed, 
and  if  it  is  placed  in  water,  this  decomposition  is  quite  rapid ;  hydrogen 
gas  is  given  off  in  minute  bubbles,  and  ammonia  is  found  in  the  solu- 
tion. This  curious  substance  has  been  supposed  to  be  a  combination 
of  ammonium  (NH4)  with  mercury ;  all  attempts,  however,  to  isolate 
the  ammonium  have  been  unsuccessful.  The  proportion  of  ammo- 
nium (?)  present  in  the  amalgam  is  extremely  minute,  notwithstanding 
the  great  change  of  bulk  and  properties  experienced  by  the  mercury. 
The  amalgam  is  said  to  contain  only  1  part  of  nitrogen  and  hydrogen  to 
1800  parts  of  mercury. 

520.  Ammonium-salts  are  generally  isomorphous  with  potas- 
sium-salts. They  have  mostly  a  pungent,  saline  taste ;  they  are 
colorless,  like  sodium  and  potassium  salts,  unless  the  acids  are 
colored  ;  the  carbonates,  and  those  salts  which,  like  the  chloride 
and  iodide,  contain  no  oxygen,  are  volatile  at  a  moderate  heat 


HYDRATE    OF   AMMONIUM.  435 

without  decomposition ;  some  salts  lose  their  ammonia  when 
heated ;  if  the  acid,  which  neutralized  this  ammonia,  is  a  non- 
volatile substance,  like  phosphoric  acid,  it  will  remain  behind  un- 
decomposed;  others,  like  the  nitrate  (Exp.  34),  yield  simpler 
gases  than  ammonia,  as,  for  example,  nitrogen  or  nitrous  oxide. 
An  aqueous  solution  of  an  ammonium-salt,  when  exposed  to  the 
air  or  evaporated,  generally  loses  ammonia  and  acquires  an  acid 
reaction  ;  hence,  in  crystallizing  an  ammonium-salt,  ammonia- 
water  must  be  occasionally  added  during  evaporation.  All  am- 
monium-salts, whether  solid  or  in  solution,  evolve  ammonia  when 
heated  with  the  hydrates  of  sodium,  potassium,  calcium,  and  a 
few  other  metals  (Exp.  51.) 

Exp.  265. — Warm  a  few  centimetres  of  a  solution  of  chloride  of 
ammonium  in  a  test-tube,  add  a  few  drops  of  a  solution  of  caustic  soda, 
and  boil  the  liquid.  The  gaseous  ammonia  can  be  detected  by  its  odor. 
If  in  any  case  the  ammonia  evolved  be  in  so  small  a  quantity  that  its 
characteristic  smell  cannot  be  detected,  it  may  be  recognized  by  its 
property  of  restoring  the  blue  color  to  reddened  litmus-paper  (§  83), 
and  of  forming  white  fumes  by  contact  with  a  rod  moistened  with 
somewhat  dilute  chlorhydric  acid  (Exp.  68).  The  reaction  may  be 
formulated  as  follows  :  —  NH4C1  -f  NallO  =  NaCl  -f  NH3  -f  H3O. 

521.  The  solution  of  ammonia  gas  in  water  (NH3,  H20)  may 
be  regarded  as  a  solution  of  hydrate  of  ammonium,  (NH4)HO, 
comparable  with  the  solution  of  caustic  soda,  NaHO,  or  caustic 
potash,  KHO.  This  solution  produces,  indeed,  many  of  the 
effects  which  the  solutions  of  the  caustic  alkalies  produce  ;  it 
neutralizes  acids,  displaces  the  oxides  of  many  metals  from  solu- 
tions of  their  salts,  and  combines  with  fats  to  form  a  soap  ;  it  is, 
in  short,  a  powerful  base. 

Exp.  266.  —  Dissolve  a  small  crystal  of  alum  in  6-8  c.  c.  of  water  in 
a  test-tube,  and  add  ammonia-water  until  the  solution,  after  being  well 
shaken,  smells  strongly  of  ammonia.  A  gelatinous  precipitate  of  the 
hydrate  of  ammonium  will  appear  in  the  liquid. 

Exp.  267.  — Dissolve  about  1  grm.  of  sulphate  of  zinc  in  6-8  c.  c. 
of  water  in  a  test-tube  ;  add  4  or  5  drops  of  ammonia-water,  and  shake 
up  the  contents  of  the  tube.  A  white,  translucent  precipitate  of  the 
hydrate  of  zinc  will  appear.  Pour  into  the  turbid  liquid  in  the  tube  3 
or  4  c.  c.  more  of  ammonia-water ;  the  precipitate  will  redissolve  and 
the  liquid  again  become  clear.  The  zinc  is  at  first  displaced  from  its 


436  CHLORIDE    OF    AMMONIUM. 

position  in  the  sulphate  by  the  group  (NH4),  but  the  hydrate  of  zinc 
thus  precipitated  is  soluble  in  an  excess  of  ammonia-water.  The  hy- 
drate of  ammonia  behaves  in  this  way  with  not  a  few  salts  of  metals. 

Ammonium-salts  are  very  numerous,  but  only  the  few  which 
are  of  present  importance  in  the  useful  arts  will  be  here  de- 
scribed. 

522.  Chloride  of  Ammonium  (NH4C1).  This  salt,  commonly 
called  sal-ammoniac,  is  found  native  in  many  volcanic  regions. 
When  nitrogenized  animal  matter  and  chloride  of  sodium  are 
distilled  together,  this  salt  sublimes  from  the  mixture  ;  the  com- 
mercial supply  of  the  salt  was  formerly  obtained  from  the  soot, 
resulting  from  the  incomplete  combustion  of  camel's  dung. 

The  raw  material,  whence  ammonium-salts  are  now  manufactured,  is 
derived  from  gas-works  and  bone-black  factories.  Coal  and  bones  con- 
tain a  portion  of  nitrogen  which,  during  the  process  of  distillation,  is 
partially  converted  into  ammonia  (§  92)  ;  this  ammonia  combines  with 
the  carbonic  acid  and  sulphydric  acid,  which  are  likewise  products  of 
the  distillation,  and  these  compounds  are  condensed  into  a  somewhat 
watery  liquor,  contaminated  with  tarry  and  oily  matters,  from  which 
the  ammonium-salts  are  subsequently  extracted.  The  impure  carbon- 
ate is  converted  into  chloride  by  the  addition  of  chlorhydric  acid,  or  of 
the  mother-liquor  from  salt-works  (a  liquor  containing  the  chlorides  of 
magnesium  and  calcium)  ;  on  evaporating  the  clarified  solution,  crystals 
of  sal-ammoniac  are  obtained,  but  they  are  generally  too  dirty  for  use. 
They  are  partly  freed  from  tarry  matters  by  heating  them  to  a  temper- 
ature a  little  below  their  subliming  point,  but  high  enough  to  drive  off 
the  tar,  and  are  then  redissolved  in  water ;  this  solution,  decolorized  by 
being  filtered  through  animal  charcoal,  is  recrystallized ;  these  crystals 
are  sometimes  further  purified  by  sublimation. 

Chloride  of  ammonium  serves  for  the  preparation  of  ammonia 
(Exp.  51),  and  of  carbonate  of  ammonium.  It  is  somewhat  em- 
ployed in  dyeing,  and  also  in  certain  processes  with  metals,  such 
as  tinning,  soldering,  and  silvering  copper  and  brass,  and  galvan- 
izing (zincing)  iron.  The  sublimed  salt  forms  semi-transparent, 
tough,  fibrous  masses ;  it  is  very  soluble  in  water,  and  a  great 
reduction  of  temperature  occurs  during  its  solution  ;  hence  it  is 
employed  as  a  refrigerant.  Its  taste  is  sharp  and  acrid.  t  When 
heated,  it  sublimes  much  below  redness,  without  undergoing 
fusion. 


NITRATE    OF    AMMONIUM.  437 

Exp.  268.  — Heat  a  bit  of  sal-ammoniac  on  a  piece  of  porcelain,  and 
observe  the  low  temperature  at  which  the  solid  is  completely  con- 
verted into  vapor. 

Exp.  269.  —  Place  a  teaspoonful  of  powdered  chloride  of  ammonium 
in  the  hollow  of  the  hand,  and  pour  upon  it  two  or  three  teaspoonfuls 
of  water.  The  cold  produced  by  the  solution  of  the  salt  will  be  yery 
perceptible. 

523.  Sulphate  of  Ammonium  (  (NH4)2SO4).   If  the  ammoniacal 
liquor  from  gas-works  or  animal  charcoal  factories  be  neutralized 
with    sulphuric   acid,  or  if  it  be  decomposed  by  gypsum  (sul- 
phate of  calcium),  the  sulphate  of  ammonium  will  be  obtained. 
In  the  latter  case,  the  impure  carbonate  of  ammonium  in  the  liquor, 
on  being  filtered  through  powdered  gypsum,  yields  carbonate  of 
calcium  and  sulphate  of  ammonium.     Another  recent  mode  of 
utilizing  the  ammoniacal  liquor  of  gas-works  yields  a  crude  sul- 
phate of  ammonium  ;  the  liquor  is  suffered  to  flow  down  the  coke- 
towers,  which  are  now  often  connected  with  sulphuric-acid  cham- 
bers (§  228),  and  there  absorbs  all  the  acid-fumes  which  escape 
from  the  chambers.    A  crude  chloride  of  ammonium  may  be  pre- 
pared in  a  similar  way  by  substituting  ammoniacal  liquor  for 
water  in  the  coke-towers  of  sulphate  of  sodium  furnaces  (§  482). 
The  absorbent  power  of  the  ammoniacal  liquor  is,  of  course,  much 
greater  than  that  of  water. 

Sulphate  of  ammonium  is  colorless,  and  has  a  very  bitter  taste  ; 
it  is  soluble  in  twice  its  weight  of  cold,  and  in  its  own  weight  of 
boiling,  water.  Its  crystalline  form  is  the  same  as  that  of  sul- 
phate of  potassium,  and  the  commercial  article  looks  very  much 
like  sand,  just  as  the  crystals  of  sulphate  of  potassium  have  a 
superficial  resemblance  to  quartz  crystals.  It  forms  a  consider-, 
able  number  of  double  salts,  which  are  isomorphous  with  the  cor- 
responding salts  of  potassium..  Sulphate  of  ammonium  is  em- 
ployed in  the  manufacture  of  ammonium-alum,  as  an  ingredient 
of  artificial  manures,  and  as  a  source  of  other  ammonium-salts. 

524.  Nitrate  of  Ammonium  (  (NH4)NO3).    The  method  of  pre- 
paring this  salt,  and  its   complete  decomposition  by  heat,  have 
been  already  described  (Exps.  33,  34,  and  §  91).     The  salt  crys- 
tallizes in  long  needles  ;  it  has  a  pungent  taste,  is  soluble  in  less 
than  half  its  weight  of  boiling  water,  and,  in  dissolving,  produces 
sharp  cold.     Between  230°  and  250°  it  is  decomposed  into  water 


438  CARBONATES    OF    AMMONIUM. 

and  nitrous  oxide ;  if  it  be  heated  hotter,  or  too  rapidly,  ammo- 
nia, nitric  oxide,  and  nitrate  of  ammonium  (NH4)N02,  are  also 
formed.  Nitrate  of  ammonium  is  formed  by  the  action  of  dilute 
nitric  acid  on  several  metals,  especially  tin. 

525.  Carbonates  of  Ammonium.  Commercial  carbonate  of 
ammonium  (sal-volatile)  .is  a  white,  semi-transparent,  fibrous 
substance,  with  a  pungent  taste  and  a  strong  ammoniacal  smell ; 
it  is  prepared,  on  a  large  scale,  by  the  dry  distillation  of  bones? 
horn,  and  other  animal  matters.  The  product  is  purified  from 
empyreumatic  substances  by  repeated  sublimation. 

Ex  p.  270. —  Mix  thoroughly  together  10  grms.  of  chloride  of  ammo- 
nium and  20  grms.  of  powdered  chalk  ;  heat  the  mixture  in  a  small 
evaporating-dish,  placed  upon  a  sand-bath.  When  white  vapors  be- 
gin to  rise  from  the  hot  mass,  place  a  wide-mouthed  bottle  over  the 
fuming  mixture.  The  white  sublimate,  which  collects  in  the  bottle,  is 
a  carbonate  of  ammonium;  chloride  of  calcium  remains  in  the  dish. 

This  experiment  illustrates  a  second,  and  very  common,  method 
of  preparing  the  commercial  carbonate,  which  simply  consists  in 
heating  to  redness  a  mixture  of  1  part  of  chloride,  or  sulphate  of 
ammonium,  and  2  parts  of  carbonate  of  calcium.  When  this 
commercial  carbonate  is  dissolved  in  strong  ammonia-water  at 
about  30°,  a  solution  is  obtained  which  yields  large,  transparent 
prismatic  crystals.  These  crystals,  however,  have  no  stability  ; 
they  are  rapidly  decomposed  in  the  air,  giving  off  water  and 
ammonia.  "  Sesquicarbonate  of  ammonia  "  is  the  name  gener- 
ally applied  to  this  substance,  —  a  name  deduced  from  the  dual- 
istic  formula  2(NH4)2O,  3CO2.  The  commercial  carbonate  ap- 
proximates to  the  composition  represented  by  this  formula ;  but  it 
is  an  impure  product,  and,  when  exposed  to  the  air,  changes 
gradually  into  a  more  stable  compound,  the  carbonate  of  ammo- 
nium and  hydrogen,  or  "bicarbonate"  (NH4)HCO3. 

This  bicarbonate  may  be  obtained  by  saturating  a  solution  of 
ammonia,  or  sesquicarbonate  of  ammonium,  with  carbonic  acid  ; 
it  forms  large,  transparent,  prismatic  crystals.  When  exposed 
to  the  air,  it  slowly  volatilizes,  giving  off  a  slight  ammoniacal 
odor.  At  the  ordinary  temperature,  it  is  soluble  in  8  parts  of 
water;  this  solution,  if  heated  above  3G°,  evolves  carbonic  acid. 
Even  at  the  ordinary  temperature,  the  solution  gradually  be- 


SULPHIDES    OF    AMMONIUM.  439 

comes  ammoniacal  on  keeping.  White,  crystalline  masses  of  this 
bicarbonate  have  hreen  found  in  guano-deposits.  It  seems  to  be 
the  most  stable  of  the  carbonates  of  ammonium,  for  the  other 
carbonates  change  into  it  if  left  to  themselves. 

526.  Sulphides  of  Ammonium.  At  a  temperature  of — 18°, 
two  volumes  of  ammonia  gas  combine  with  one  volume  of  sul- 
phydric  acid  gas  to  form  a  crystalline,  unstable  substance  of  strong 
alkaline  reaction,  which  corresponds  in  composition  with  the  sul- 
phides Na2S  and  K2S. 

2NH3  +  H2S  ==  (NH4)2S. 

Exp.  271.  —  Bass  a  slow  stream  of  washed  sulphydric  acid  through 
100  c.  c.  of  strong  ammonia-water,  until  the  solution  has  a  predomi- 
nating and  persistent  odor  of  sulphuretted  hydrogen.  This  solution  is 
at  first  comrless,  and  contains  a  sulphide  of  ammonium  and  hydrogen 
(NH4)HS;  but  when  kept  in  contact  with  air,  it  becomes  yellow, 
owing  to  the  formation  of  a  higher  sulphide  of  ammonium.  The  solu- 
tion has  the  property  of  dissolving  many  of  the  sulphides  of  the  metals, 
by  forming  with  them  double  sulphides,  and  is  a  very  useful  reagent  in 
the  analytical  laboratory. 

The  higher  sulphides  of  ammonium  are  obscure  bodies,  to 
which  the  following  formulae  baye  been  assigned,  —  (NH4)2S2, 
(NH4)2S3 ,  (NH4)2S4 ,  (NH4)2S5 ,  (NH4)2S7 .  With  the  exception 
of  the  last,  the  septisulphide,  all  these  sulphides  are  soluble  in 
water.  With  the  same  exception,  they  correspond  with  the  sul- 
phides of  sodium  and  potassium.  They  are,  in  general,  unstable  ; 
the  most  permanent  is  the  septisulphide,  which  forms  ruby-red 
crystals,  capable  of  resisting  temperatures  below  300°,  and  only 
slowly  decomposable  by  water  and  chlorhydric  acid. 


CHAPTER    XXVI. 

LITHIUM RUBIDIUM CAESIUM THALLIUM. 

SPECTRUM     ANALYSIS. 

527.  Lithium  (Li).  This  rare  metal  occurs  as  a  constituent 
of  not  a  few  minerals,  especially  micas  and  feldspars,  but  does 
not  form  a  large  percentage  of  any  of  them.  The  minerals 


440  LITHIUM. 

lepidolite,  triphylline,  and  petalite  usually  contain  from  3.5  to  5 
per  cent,  of  lithium,  and  are  the  chief  sources  of  the  element.  In 
much  smaller  proportion,  it  has  been  recognized  in  sea-water, 
mineral-waters,  and  almost  all  spring-waters,  in  milk,  tobacco, 
and  human  blood.  It  is,  therefore,  a  widely-diffused,  but  not 
abundant,  substance.  The  metal,  which  is  obtained  by  decom- 
posing the  fused  chloride  by  the  galvanic  current,  has  the  color 
and  lustre  of  silver  on  a  freshly-cut  surface,  but  quickly  tarnishes 
on  exposure  to  the  air.  It  is  harder  and  less  fusible  than  sodium 
and  potassium,  but  softer  than  lead ;  it  may  be  welded,  by  pres- 
sure, at  ordinary  temperatures.  It  floats  on  naphtha,  and  is  the 
lightest  of  all  known  solids  which  include  no  air,  its  specific 
gravity  being  only  0.59.  The  atomic  weight  of  the  element  is 
also  low,  namely,  7. 

In  its  chemical  relations,  lithium  closely  resembles  sodium  and 
potassium,  but  is  somewhat  less  energetic;  it  combines  with  the 
same  elements  to  form  analogous  compounds  to  those  of  sodium 
and  potassium  ;  but  the  properties  of  these  compounds,  while  pre- 
senting a  striking  general  resemblance  to  those  of  the  sodium  and 
potassium  compounds,  nevertheless  offer  some  special  points  of 
divergence  from  them.  Thus,  the  hydrate  of  lithium  (LiHO) 
has  the  same  taste,  causticity,  and  alkalinity  as  the  hydrates  of 
sodium  and  potassium,  but  is  much  less  soluble  in  water.  The 
fused  hydrate  attacks  platinum  more  energetically  than  caustic 
soda  and  potash  do.  The  carbonate  and  phosphate  of  lithium 'are 
rather  sparingly  soluble  in  water,  while  the  corresponding  sodium 
and  potassium-salts  are  exceedingly  soluble.  The  chloride  of 
lithium  (LiCl),  produced  when  lithium  burns  in  chlorine,  or 
when  the  hydrate  or  carbonate  of  lithium  is  dissolved  in  chlor- 
hydric  acid,  crystallizes  in  cubes,  and  has  the  taste  of  common 
salt,  but  it  is  more  volatile  than  the  chloride  of  potassium,  and  it 
deliquesces  faster  than  any  other  known  salt;  whereas  the  chlo- 
rides of  sodium  and  potassium  are  almost  permanent  when  pure. 
All  the  volatile  lithium-compounds  color  a  gas,  alcohol,  or  blow- 
pipe flame  carmine-red.  The  most  delicate  reaction,  for  the  de- 
tection of  lithium,  the  test  which  has  revealed  its  existence  in  a 
great  variety  of  substances  which  were  never  imagined  to  con- 
tain it,  is  the  presence  of  one  bright  line,  of  a  peculiar  red,  in  the 


SPECTRUM    ANALYSIS.  441 

spectrum,  seen  on  looking  through  a  glass  prism,  at  a  flame 
colored  with  a  lithium-compound. 

528.  Spectrum  Analysis.  We  have  had  occasion  to  observe 
that  certain  chemical  substances,  like  boracic  acid  and  salts  of 
sodium,  potassium  and  lithium,  impart  peculiar  colors  to  the 
blow-pipe  flame,  or  to  any  other  hot  and  colorless  flame.  If 
these  colored  flames  are  looked  at  through  a  prism,  a  narrow 
pencil  of  the  colored  light  being  directed  through  a  slit  upon 
the  prism,  it  will  be  seen  that  each  different  flame  produces  a 
peculiar  spectrum,  consisting  of  one  or  more  distinct  bright  lines 
of  colored  light,  and  bearing  no  resemblance  to  the  continuous 
band  of  rainbow-colors  which  constitutes  the  common  spectrum 
produced  by  a  pencil  from  any  source  of  white  light.  Thus,  the 
spectrum  of  the  yellow  sodium  flame  consists  of  a  single,  bright, 
yellow  line ;  the  purple  potassium  flame  gives  a  spectrum  con- 
taining two  bright  lines,  one  lying  at  the  extreme  red  and  the 
other  at  the  extreme  violet  end,  and  a  second,  fainter  red  line ; 
while  the  lithium  spectrum  consists  of  a  very  characteristic  red 
line  and  a  fainter  orange  line. 

These  peculiar  lines  which  characterize  the  spectrum  of  any 
element  are  invariably  produced  by  that  element,  and  never  by 
any  other  substance,  and  not  only  the  color  and  number  of  lines, 
but  their  position  in  the  normal  spectrum,  always  remain  unaltered. 
When  the  spectrum  of  a  flame,  colored  with  a  mixture  of  sodium 
and  potassium  salts  is  examined,  the  yellow  line  of  sodium  is 
seen  in  its  place,  and  the  red  and  purple  lines  of  potassium  are 
as  visible  in  their  respective  positions  as  if  no  sodium  had  been 
present.  This  example  illustrates  one  great  advantage  which 
the  use  of  the  prism  gives,  —  the  unaided  eye  cannot  distinguish 
the  potassium  color  in  the  presence  of  the  intense  sodium- 
yellow,  the  brighter  color  hiding  the  paler ;  but  with  the  prism 
it  is  easy  to  detect  each  of  several  ingredients  of  a  mixture  by 
the  appearance  of  its  characteristic  lines.  Now,  every  element- 
ary substance,  whether  metallic  or  non-metallic,  solid,  liquid,  or 
gaseous,  when  heated  to  the  point  at  which  its  vapor  becomes 
luminous,  emits  a  peculiar  light  produced  by  it  alone,  and  the 
bright  lines  of  the  spectrum  of  this  light  are  characteristic  of  this 
element  in  number,  color,  and  position.  Many  metals  require 


442  SPECTRUM   ANALYSIS. 

a  much  higher  temperature  than  that  of  the  common  gas-flame 
to  convert  them  into  luminous  vapors;  but  by  the  use  of  the 
electric  lamp  all  the  metals,  even  gold,  silver,  and  platinum,  may 
be  made  to  yield  peculiar  spectra.  The  permanent  gases  also 
give  characteristic  spectra  when  they  are  heated  by  the  passage 
of  the  electric  spark  ;  the  spectrum  of  hydrogen,  for  example, 
consists  of  one  red,  one  green,  and  one  blue  line. 

A  new  method  of  analysis,  of  extreme  delicacy,  is  based 
upon  these  facts.  Spectrum  analysis  is  competent  to  detect  the 
?.Tsi.oo<y.o<j(yth  °f  a  gramme  of  sodium,  or  the  ^o.o^o.uooth  of 
a  gramme  of  lithium,  and  many  other  elements  in  incredibly 
small  proportions.  So  extreme  is  the  delicacy  of  the  method 
that  it  brings  into  plain  sight  minute  quantities  which  altogether 
escape  the  coarser  process  of  analysis,  and  reveals,  as  substances 
common  in  familiar  things,  elements  which  were  long  supposed 
to  be  of  extreme  rarity.  Thus,  the  presence  of  lithium,  formerly 
considered  a  rare  element  peculiar  to  a  few  obscure  minerals, 
has  been  demonstrated  by  spectrum  analysis  in  many  drink- 
ing-waters, in  tea,  tobacco,  milk,  and  blood.  A  still  more 
striking  illustration  of  the  value  of  spectrum  analysis  is  to  be 
found  in  the,  discovery  of  four  new  elementary  bodies  by  its 
means. 

529.  Two  new  elements  which  closely  resemble  sodium  and 
potassium,  arid  are  in  nature  associated  with  these  alkali  metals, 
have  been  found  in  certain  mineral-waters,  and  in  the  mineral 
lepidolite.  One  of  these  elements  gives  a  spectrum  containing, 
among  others  of  less  mark,  a  superb,  double,  red  line,  and  has 
thence  been  called  Rubidium ;  the  other  produces  a  spectrum 
characterized  by  two  beautiful  blue  lines,  and  has  thence  been 
called  Ccesium.  These  two  new  metals  resemble  potassium  so 
closely  in  all  their  chemical  properties,  that  it  would  have  been 
nearly  impossible  to  detect  them  by  the  common  analytical  pro- 
cesses, yet  their  spectra  are  in  the  highest  degree  characteristic, 
exhibiting  bright  bands  which  exist  neither  in  the  potassium  spec- 
trum, nor  in  any  other  known  spectrum.  The  recently  discovered 
metal,  Thallium,  was  discovered  and  traced  to  its  source  in  certain 
kinds  of  pyrites  by  observing  a  splendid  green  line  which  did 
not  belong  to  any  known  substance.  The  new  metal,  Indium, 


RUBIDIUM    AND    CESIUM.  443 

was  also  detected,  traced  to  its  source  in  certain  zinc  ores,  and 
successfully  isolated  by  the  help  of  a  dark-blue  line  which  had 
not  been  previously  observed. 

The  methods  and  processes  of  spectrum  analysis  are  not 
applicable  to  colored  artificial  lights  alone ;  they  have  been 
applied  with  encouraging  success  to  the  lights  of  various  quality 
which  emanate  from  the  sun,  the  stars,  and  the  nebuke ;  but 
the  details  of  these  observations  belong  rather  to  physics  than 
to  chemistry. 

530.  Rubidium  and  Ccesium  (Rb  and  Cs).  These  two 
elements  are  always  found  together,  and  in  association  with 
potassium.  Though  extensively  diffused,  they  generally  occur 
in  very  minute  quantities.  Rubidium  seems  to  be  rather  the 
most  abundant.  Ten  kilogrammes  of  the  mineral  water  in  which 
these  metals  were  first  discovered  yield  not  quite  two  milli- 
grammes of  chloride  of  cresium,  and  about  two  and  a  half 
milligrammes  of  chloride  of  rubidium.  Since  v  the  original  dis- 
covery of  the  elements,  they  have  been  found  in  many  other 
springs,  in  several  kinds  of  mica,  and  in  other  silicates,  and 
in  the  ashes  of  beet-root,  tobacco,  coffee,  and  grapes.  To 
separate  the  metals  from  potassium  the  analyst  relies  on  the 
greater  insolubility  of  the  double  chlorides  which  they  form  with 
platinum  ;  if  a  mixture  of  the  chlorides  of  potassium,  rubidium, 
and  ccesium  be  completely  precipitated  by  chloride  of  platinum, 
arid  the  yellow  precipitate  be  repeatedly  treated  with  boiling 
water,  the  insoluble  residue  will  contain  the  new  metals.  The 
caesium  is  separated  from  the  rubidium  by  converting  a  mixture 
of  their  carbonates  into  their  tartrates,  the  rubidium  into  the 
acid,  or  bitartrate,  the  caesium  into  the  neutral  tartrate,  and 
then  exposing  this  mixture  to  very  moist  air.  The  neutral  tar- 
trate of  cesium  deliquesces,  the  acid  tartrate  of  rubidium  remains 
solid,  and  the  two  salts  are  separated  by  filtration.  Most  of  the 
salts  of  rubidium  and  caesium  are  isomorphous  with  the  corre- 
sponding potassium-salts.  Their  hydrates,  RbHO  and  CsHO, 
are  caustic  alkalies,  soluble  in  water  and  alcohol.  Their  carbon- 
ates are  fusible,  deliquescent,  and  strongly  alkaline ;  the  nitrates 
(RbNO3  and  CsNO3)  and  sulphates  (Rb2S04  and  Cs2SO4)  are 
anhydrous,  crystalline  salts,  soluble  in  water;  the  sulphates  form 


444  THALLIUM. 

alums  with  sulphate  of  aluminum.  The  chloride  of  caesium, 
CsCl,  is  a  deliquescent  salt,  like  chloride  of  lithium ;  the 
chloride  of  rubidium,  RbCl,  is  permanent,  like  the  chlorides 
of  sodium  and  potassium.  The  fused  chlorides  are  easily  decom- 
posed by  the  galvanic  current. 

The  metal  rubidium  is  white,  and  has  the  brilliant  lustre  of 
silver,  but  it  rapidly  oxidizes  in  the  air;  its  specific  gravity  is 
1.52  and  its  atomic  weight  85.7.  It  may  be  prepared  either  by 
the  electrolysis  of  its  chloride,  or,  like  potassium,  by  the  reduc- 
tion of  its  carbonate  by  ignition  with  carbon  and  chalk. 

The  properties  of  caesium  have  only  been  studied  in  the 
amalgam  with  mercury  resulting  from  the  electrolysis  of  its 
chloride ;  the  metal  itself  has  not  been  isolated.  Its  atomic 
weight,  deduced  from  the  analysis  of  its  chloride,  is  133.  There 
can  be  no  question  that  the  properties  of  both  rubidium  and 
caesium  differ  from  those  of  sodium  and  potassium,  not  in  kind 
but  only  in  degree.  They  are  therefore  classed  with  sodium  and 
potassium  as  alkali-metals. 

531.  Thallium  (Tl).  This  metal  was  discovered,  by  means 
of  spectrum  analysis,  in  Lipari  sulphur  and  in  the  deposit  in  the 
flue  of  a  pyrites-burner,  —  a  furnace  in  which  iron  pyrites  are 
roasted  for  the  sake  of  the  sulphurous  acid  they  yield.  The 
element  is  found  to  occur  in  not  inconsiderable  quantities  in 
many  specimens  of  iron  pyrites,  and  appears  to  take  the  place  of 
arsenic,  which  is  a  common  impurity  of  this  mineral.  Thallium 
presents  the  external  characters  of  lead ;  it  is  heavier  than  lead, 
having  a  specific  gravity  of  11.85,  and  it  is  so  soft  that  the 
thumb-nail  can  indent  it ;  it  is  very  malleable,  and  ductile 
enough  to  be  drawn  into  wire ;  it  melts  at  200°  and  volatilizes 
at  redness ;  its  freshly-cut  surface  has  a  bluish-white  lustre, 
but  it  quickly  tarnishes,  and  is  gradually  oxidized  in  the  air,  so 
that  it  is  best  preserved  under  water.  Water  is  not  decomposed 
by  it  even  at  boiling.  When  strongly  heated  in  oxygen,  it  takes 
fire  and  burns  with  a  bright  green  flame. 

Thallium  dissolves  in  dilute  acids,  with  evolution  of  hydro- 
gen. There  are  several  oxides  of  this  metal,  of  which  the 
most  important  is  the  oxide  T12O,  corresponding  in  composition, 
and,  to  some  extent,  in  properties,  with  the  oxide  of  sodium 


SILVER.  445 

Na20.  This  oxide  is  somewhat  soluble  in  water,  and  yields  a 
caustic  alkaline  solution,  which  absorbs  carbonic  acid  from  the 
air,  and  forms  a  well-defined  series  of  salts.  The  sulphate, 
T12S04,  is  a  soluble  salt,  which  forms  an  alum  with  sulphate  of 
aluminum  ;  the  chloride,  T1C1 ,  is  only  slightly  soluble  in  water, 
resembling,  in  this  respect,  the  chloride  of  lead,  and  being  quite 
unlike  ,the  soluble  chlorides  of  sodium,  potassium,  rubidium,  and 
cassium.  The  carbonate  of  thallium  is  a  soluble  salt,  but  the 
sulphide  of  thallium,  T12S ,  is  an  insoluble  black  powder,  which 
resembles  the  sulphide  of  lead,  but  is  entirely  unlike  the  sulphides 
of  the  alkali-metals.  The  soluble  salts  of  thallium  are  very  poi- 
sonous. In  general,  the  properties  of  thallium  are  intermediate 
between  those  of  lead  and  those  of  sodium  and  potassium.  Like 
the  alkali-metals,  it  replaces  hydrogen  atom  for  atom ;  its  atomic 
weight  is  204. 


CHAPTER    XXVII. 

SILVER  THE    ALKALI-METALS  —  QUANT1VALENCE. 

532.  Silver  is  a  widely-diffused  and  quite  abundant  element, 
but  in  its  mode  of  occurrence  it  differs  widely  from  the  alkali- 
metals  which  we  have  just  been  studying.  In  the  first  place,  it 
frequently  occurs  native,  both  pure,  and  alloyed  with  mercury, 
copper,  and  gold,  —  a  mode  of  occurrence  quite  impossible  for 
the  alkali-metals,  because  of  their  readiness  to  combine  with  the 
elements  of  air  and  water.  Native  silver  is  found  in  various 
forms,  sometimes  crystallized  in  cubes,  orvoctohedrons,  sometimes 
in  filaments,  both  coarse  and  fine,  and  sometimes  in  shapeless 
masses.  The  metal  more  commonly  occurs  in  combination  with 
sulphur,  mixed  with  sulphides  of  lead,  antimony,  copper,  and  iron. 
It  is  from  argentiferous  sulphides  that  the  larger  part  of  the  sil- 
ver of  commerce  is  extracted,  and,  among  ores  of  this  kind,  the 
argentiferous  sulphide  of  lead  (galena)  is  the  most  abundant. 
Combinations  of  silver  with  selenium,  tellurium,  chlorine,  bro- 
mine, and  iodine  are  also  to  be  enumerated  among  silver-con- 


446  EXTRACTION    OF    SILVER. 

taining  minerals  ;  of  these  the  chloride  (horn-silver)  occurs  in 
quantities  large  enough  to  make  it  valuable  as  an  ore  of  the 
metal.  It  is  noticeable,  that  the  only  elements  which  are  ex- 
tracted in  any  quantity  from  their  chlorides  as  ores,  are  sodium, 
potassium,  and  silver.  The  chlorides  of  copper,  mercury,  and 
lead  do,  indeed,  occur  as  natural  minerals,  but  as  sources  of  those 
metals,  they  have  no  significance.  A  small  proportion  of  silver 
exists  in  sea-water  (about  1  milligramme  in  100  litres),  and  its 
presence  has  been  recognized  in  common  salt,  in  chemical  prod- 
ucts, in  the  making  of  which  salt  is  used,  in  various  sea-weeds, 
in  the  ashes  of  land-plants,  in  the  ash  of  ox-blood,  and  probably 
also  in  coal.  In  sea-water  it  exists,  as  sodium  and  potassium  do, 
in  the  form  of  ctrloride. 

When  silver  is  extracted  from  argentiferous  sulphide  of  lead,  the  ore 
is  first  treated  for  lead,  precisely  as  it  would  be  if  it  contained  no  silver. 
The  lead,  so  reduced,  contains  all  the  silver  originally  present  in  the 
quantity  of  ore  treated.  The  subsequent  separation  of  the  metallic 
silver  from  the  metallic  lead  depends  upon  the  chemical  properties  of 
lead  rather  than  of  silver,  for  the  silver  remains  unaltered  during  the 
whole  process ;  this  separation  will,  therefore,  be  described  in  the  next 
chapter. 

The  mixed  sulphides  which  contain  silver  have  been  heretofore  gen- 
erally reduced  by  a  complicated  process  which  depends  ultimately  on 
an  amalgamation  of  the  silver  with  mercury.  The  ore,  after  thorough 
washing  and  grinding,  is  mixed  with  a  portion  of  common  salt,  and 
roasted  for  several  hours  ;  during  this  roasting,  white  fumes  of  arsenious 
and  antimonious  acids  are  expelled,  the  sulphides  of  copper  and  iron 
are  partially  converted  into  oxides,  chlorides,  and  sulphates,  and  chlo- 
ride of  silver  and  sulphate  of  sodium  are  formed.  The  roasted  product 
is  then  reduced  to  a  very  fine  powder,  and  agitated  in  revolving  casks 
with  water  and  iron  filings,  or  scraps,  to  which  mercury  is  soon  added. 
This  operation  lasts  about  20  hours ;  during  it,  the  iron  decomposes  the 
chloride  of  siver,  and  the  mercury  dissolves  the  silver  to  an  amalgam ; 
from  this  amalgam  the  excess  of  mercury  is  first  squeezed  out  through 
leather  or  cloth-filters,  and  the  remainder  is  driven  off  by  distillation. 
The  residual  spongy  mass  is  silver,  alloyed  with  a  variable  proportion 
of  copper,  derived  from  the  ore  and  reduced  to  the  metallic  state  by 
the  same  steps  which  have  reduced  the  silver. 

This  process  is  European ;  the  process  of  amalgamation,  as  prac- 
tised in  Mexico  and  South  America,  is  quite  different,  and  the  reactions 
which  it  depends  upon  are  somewhat  obscure.  The  ore  is  not  roasted 


PROPERTIES    OF    SILVER.  447 

but  after  being  ground  to  powder,  moistened  with  water,  and  mixed 
with  from  1  to  5  per  eent.  of  common  salt,  it  is  suffered  to  lie  undis- 
turbed for  some  days.  From  ^  to  1  per  cent,  of  roasted  copper  pyrites 
is  then  added,  together  with  a  considerable  proportion  of  metallic  mer- 
cury, and  the  mass  is  worked  together  and  commingled  by  the  tram- 
pling of  mules  or  horses.  After  an  interval  of  two  or  three  weeks, 
a  second  dose  of  mercury  is  given,  and,  after  a  still  longer  interval,  a 
third.  By  this  last  addition,  a  fluid  amalgam  is  obtained,  which  is  sep- 
arated by  washing  and  filtering,  and  distilled  for  the  recovery  of  a 
portion  of  the  mercury  employed,  and  the  isolation  of  the  silver.  In 
this  process  there  is  a  great  waste  of  mercury,  because  much  of  it  is 
converted  into  a  chloride  of  mercury  (calomel),  and  lost.  The  recom- 
mendations of  the  process  are  mainly  these,  —  that  it  requires  no  fuel, 
except  for  the  distillation  of  the  amalgam,  and  that  it  leaves  the  silver 
in  a  condition  of  great  purity.  The  whole  process,  though  far  from  eco- 
nomical from  the  point  of  view  of  the  theoretical  chemist,  was  doubtless 
a  legitimate  outgrowth  of  the  conditions  under  which  it  took  birth. 

Various  processes  have  been  patented  for  the  extraction  of  silver 
without  the  use  of  the  costly  mercury,  some  of  which  have  been  suc- 
cessfully practised  on  a  large  scale.  They  depend,  for  the  most  part, 
either  on  the  comparative  stability,  in  the  fire,  of  sulphate  of  silver 
when  once  formed,  as  compared  with  the  sulphates  of  iron  and  copper, 
and  the  consequent  possibility  of  dissolving  sulphate  of  silver  out  of  the 
roasted  ore,  or  upon  the  fact  that  the  chloride  of  silver,  which  results 
from  the  roasting  of  the  ore  with  chloride  of  sodium,  may  be  dissolved 
in  solutions  of  the  alkaline  chlorides,  and,  indeed,  in  aqueous  solutions 
of  a  great  many  other  soluble  salts,  though  it  is  by  itself  insoluble  in 
water.  Any  aqueous  solution,  containing,  among  other  things,  a  silver- 
salt,  whether  in  the  condition  of  chloride,  sulphate,  or  nitrate,  is  indif- 
ferent, may  be  decomposed  by  digestion  with  metallic  copper;  the 
silver-salt  will  be  decomposed,  the  corresponding  copper-salt  formed 
and  dissolved,  and  the  metallic  silver  will  be  precipitated. 

533.  Silver  (Ag).  The  element,  silver,  is  much  more  famil- 
iarly known  than  any  of  its  compounds ;  known  from  the  earliest 
ages,  this  metal  has  always  been  prized  as  much  for  its  beauty 
as  for  its  rarity.  White,  brilliantly  lustrous,  susceptible  of  an- 
admirable  polish,  wonderfully  malleable  and  ductile,  the  best 
known  conductor  of  heat  and  electricity,  fusible  only  at  a  very 
elevated  temperature  and  permanent  in  the  air,  whether  hot  or 
cold,  wet  or  dry,  it  represents  and  embodies  in  the  completes! 
sense  all  that  is  commonly  understood  by  the  term  metal. 


448  THE    TERM    METAL. 

This  word  metal  cannot  be  strictly  defined;  it  is  a  conventional 
term,  vaguely  used  because  expressing  a  vague  idea.  Thus 
metals  would  all  be  solid  were  not  mercury,  and  perhaps  caesium, 
fluid ;  they  are  generally  heavy,  but  lithium,  sodium,  and  potas- 
sium float  upon  water;  they  have  all  a  peculiar  lustre,  called 
metallic;  but  this  lustre  does  not  characterize  metals  alone,  for 
coke  and  graphite,  galena,  molybdenite,  and  many  other  minerals 
often  exhibit  a  similar  lustre ;  they  may  all  be  said  to  be  opaque, 
but  gold  may  be  beaten  out  so  thin  as  to  transmit  a  greenish 
light.  While  it  is  not  possible  to  define  the  term  metal  with 
precision  from  chemical,  any  more  than  from  physical  properties, 
one  general  chemical  fact  deserves  attention  in  this  connection. 
We  have  seen  that  bodies  which  contain  a  large  proportion  of 
oxygen,  such  as  SO3 ,  P2O5 ,  N2O5 ,  and  CO  ,  have  a  common 
tendency  to  unite  with  other  bodies  which  are  alike  in  that  they 
contain  a  much  smaller  proportion  of  oxygen,  such  as  K2O, 
Na2O,  PbO,  and  CaO,  to  form  more  or  less  stable  saline  sub- 
stances. The  first  class  of  bodies,  which  are  usually  rich  in 
oxygen,  have  been  called-  acids ;  the  second  class,  which  are 
usually  poor  in  oxygen,  have  been  designated  collectively  as 
bases.  Now,  those  elements  which  unite  with  oxygen  to  form 
acids  alone  are,  as  a  rule,  non-metallic,  and  those  elements  which 
unite  with  oxygen  to  form  bases  are,  in  the  chemical  sense  of  the. 
term,  the  metals  ;  but  no  sharp  line  of  division  between  metallic 
and  non-metallic  elements  can  be  established  on  this  principle, 
inasmuch  as  some  elements,  which  possess  the  other  character- 
istics of  a  metal,  form  no  basic  oxide,  while  some  metals,  like  an- 
timony, form  oxides  which  are  at  one  time  bases  and  at  another 
time  acids.  The  metal  arsenic,  for  example,  forms  no  basic  oxide, 
and  we  shall  hereafter  meet  with  another  illustration  of  the  same 
difficulty  of  classification  in  the  element  tungsten. 

Melted  silver  possesses  the  curious  property  of  absorbing  a 
large  volume  of  oxygen  (twenty -two  times  its  bulk),  from  the 
air,  while  it  is  liquid.  This  gas  it  gives  out  again  on  solidifying. 
When  a  globule  of  molten  silver  is  cooled  suddenly,  the  film  of 
solid  metal  which  forms  upon  its  surface  is  burst  open  by  the 
escaping  gas,  and  the  liquid  silver  within  is  apt  to  be  projected 
outwards  with  the  gas  ;  this  phenomenon  is  called  spitting  ;  it 


SILVER    COIN.  449 

often  occasions  a  loss  in  silver  assays.  When  heated  on  lime 
before  the  oxyhydrogen  blow-pipe,  silver  gives  off  vapors  which 
become  oxidized  if  the  blast  of  gas  contain  an  excess  of  oxygen ; 
a  fine  silver  wire  is  dispersed  in  greenish  vapors  when  a  very 
powerful  electric  discharge  is  sent  through  it.  Silver  combines 
slowly  with  chlorine,  bromine,  and  iodine,  and  promptly  with 
sulphur.  The  tarnishing  of  silver  is  due  to  the  formation  of  a 
thin  film  of  the  black  sulphide  over  the  metallic  surface,  by  com- 
bination between  the  silver  and  the  sulphur  of  the  sulphuretted 
hydrogen  which  is  often  present  in  the  air  of  towns  and  houses. 

The  best  solvent  for  silver  is  nitric  acid  diluted  with  two  or 
three  parts  of  water ;  nitric  oxide  is  evolved,  and  nitrate  of  silver 
remains  in  solution  :  — 

3Ag  +  4HN03  =  3AgN08  +  NO  +  2H20. 
Chlorhydric   acid  acts   upon  it   but  slowly,  for  the  chloride  of 
silver  is  but  slightly  soluble  in  chlorhydric  acid,  whether  strong 
or  dilute.     Boiling  sulphuric  acid  dissolves  silver,  and  forms  the 
sulphate,  sulphurous  acid  being  evolved  during  the  reaction :  —  * 

2  Ag  +  2  H,S04  =  Ag,S04  +  2  H2O  +  S02. 
Neither  the  alkalies  nor  their  nitrates  have  much  effect  on 
silver,  whether  they  are  in  solution  or  are  fused  by  heat;  hence 
a  silver  dish  is  used  in  concentrating  the  caustic  alkalies,  and 
'a  silver  crucible  for  fusing  refractory  minerals  with  the  hydrate 
of  sodium  or  potassium.  The  specific  gravity  of  silver  is  10.5 
and  its  atomic  weight  108. 

534.  The  physical  and  chemical  qualities  of  silver  fit  it  to 
serve  as  a  medium  of  exchange,  and  as  the  material  of  jewellery 
and  plate.  But  as  the  pure  metal  would  be  rather  too  soft  for  " 
ordinary  use,  it  is  hardened  by  combining  with  it  a  small  propor- 
tion of  copper.  The  proportion  of  copper  in  the  "  standard " 
silver  employed  for  coinage  varies  in  different  countries;  — 
in  the  United  States  and  in  France  it  is  10  per  cent. ;  in  Great 
Britain  it  is  7.5  per  cent. ;  in  Germany  it  is  25  per  cent. 

Exp.  272.  —  Place  one  or  two  dimes  in  a  small  flask,  and  cover  them 
with  nitric  acid  diluted  with  two  parts  of  water.    Warm  the  flask  gently 
in  a  place  where  there  is  a  good  draught  of  air ;  the  coins  will  gradu- 
ally dissolve,  with  evolution  of  nitric  oxide,  which,  on  contact  with  air, 
29 


450  NITRATE    OF    SILVER. 

produces  the  abundant  red  fumes  which  escape  from  the  flask  ;  add 
more  nitric  acid,  from  time  to  time,  if  necessary  to  complete  the  solu- 
tion. The  blue  solution  contains  both  the  silver  and  the  copper  dis- 
solved in  nitric  acid. 

Place  in  the  blue  solution  two  or  three  copper  cents,  and  leave  the 
flask  at  rest  for  some  days  in  a  warm  place.  Then  collect  the  little 
plates  of  pure  silver,  which  have  separated  from  the  solution,  upon  a 
filter,  and  wash  them,  first  with  water,  and  then  with  ammonia-welter, 
until  the  ammonia-water  no  longer  shows  any  tinge  of  blue.  This  sil- 
ver, washed  finally  with  water  and  dried,  is  well-nigh  pure  ;  two-thirds 
of  it  may  be  again  dissolved  in  nitric  acid  ;  the  solution  will  contain 
pure  nitrate  of  silver. 

535.  Nitrate  of  Silver  (AgN03).  This  salt,  as  we  have 
already  seen,  is  obtained  in  solution ^by  dissolving  silver  in  nitric 
acid.  When  such  a  solution  is  evaporated  to  the  point  of  crys- 
tallization, the  nitrate  is  obtained  in  transparent,  anhydrous, 
tabular  crystals,  which  are  soluble  in  their  own  weight  of  cold 
water,  and  in  half  their  weight  of  hot  water.  The  salt  fuses 
easily,  and  when  cast  into  cylindrical  sticks  is  used  in  surgery  as 
a  caustic,  under  the  name  of  lunar  caustic. 

Nitrate  of  silver,  when  pure,  is  not  altered  by  exposure  to 
sunlight ;  but  if  it  be  in  contact  with  organic  matter,  light  readily 
decomposes  it,  and  a  black,  insoluble  product  is  formed  of  no 
ordinary  stability.  Hence  the  solution  of  the  nitrate  stains  the 
skin  black,  and  the  salt  forms  the  basis  of  an  indelible  ink  used 
for  marking  linen  and  other  fabrics. 

Exp.  273.  — Dissolve  8  grms.  of  crystallized  carbonate  of  sodium  and 
1  grm.  of  gum  arabic  in  1 6  c.  c.  of  hot  water.  Moisten  a  bit  of  linen 
or  cotton-cloth  with  this  preparatory  solution,  dry  it,  and  press  it  smooth 
with  a  hot  iron. 

Dissolve  1  grm.  of  nitrate  of  silver  and  0.5  grm.  of  gum  arabic  in 
1.75  c.  c.  of  water,  previously  colored  with  India-ink. 

Write  with  this  silver  solution  upon  the  prepared  surface  of  cloth, 
and  expose  the  writing  to  the  direct  rays  of  the  sun  for  a"  few  hours. 
Then  wash  out  the  gum  and  carbonate  of  sodium  with  water ;  a  very 
durable  mark,  which  neither  soap  nor  "  soda  "  will  obliterate,  will  re- 
main on  the  cloth. 

Nitrate  of  silver  is  even  more  completely  decomposed  by  a  red 
heat  than  nitrate  of  potassium,  for  nothing  but  metallic  silver 
remains  behind  ;  in  decomposing,  it  gives  up  a  large  quantity  of 


OXIDES    OF    SILVER.  451 

oxygen ;  hence  mixtures  of  combustibles,  like  sulphur,  phospho- 
rus, and  charcoal,  with  nitrate  of  silver,  detonate,  explode,  or 
deflagrate,  when  struck  sharply  with  a  hammer,  or  touched  with 
a  hot  wire.  (Compare  §§  515,  516.)  Phosphorus,  mercury, 
charcoal,  grape  sugar,  certain  essential  oils,  and  many  other 
organic  substances  reduce  metallic  silver  from  solutions  of 
nitrate  of  silver.  Nitrate  of  silver  is  the  material  from  which 
most  other  silver  compounds  are  artificially  prepared.  It  is 
largely  consumed  in  photography. 

The  precipitation  of  metallic  silver  in  a  beautiful  arborescent  form 
is  accomplished  as  follows  :  —  Dissolve  2  grms.  of  nitrate  of  silver  in  60 
c.  c.  of  water,  and  place  the  solution  in  a  test-glass ;  pour  2  grms.  of 
mercury  into  the  liquid,  and  let  the  glass  stand  at  rest  for  several  hours. 
The  nitrate  of  silver  may  be  recovered  by  dissolving  the  precipitated 
silver  in  nitric  acid,  and  evaporating  the  solution. 

To  illustrate  the  decomposition  of  a  silver  solution  by  an  organic  sub- 
stance, dissolve  2  grms.  of  nitrate  of  silver  in  60  c.  c.  of  water,  and  im- 
merse in  the  solution  a  horn  or  ivory  paper-knife,  which  has  been 
cleansed  from  grease  with  ammonia-water  and  rinsed  in  fresh  water. 
Let  the  knife  remain  in  the  solution  about  an  hour  ;  it  will  turn  yellow ; 
take  it  out,  rinse  it  in  water,  and  expose  it  to  the  direct  rays  of  the  sun 
until  it  turns  jet  black  ;  then  burnish  it  with  a  piece  of  leather,  and  the 
silver  will  appear  in  the  metallic  state. 

Exp.  274. —  Wrap  a  piece  of  phosphorus,  no  bigger  than  a  pin's 
head,  with  a  small  crystal  of  nitrate  of  silver,  in  a  bit  of  paper ;  place 
the  packet  on  an  anvil  and  strike  it  with  a  hammer.  The  explosion 
will  be  sharp.  The  student  will  remember  that  nitrate  of  silver  stains 
the  fingers. 

Exp.  275.  —  Mix  1  grm.  of  powdered  nitrate  of  silver  with  0.2  grm. 
of  dry,  powdered  charcoal ;  place  the  mixture  on  a  piece  of  porcelain,  . 
and  touch  it  with  a  red-hot  wire.     The  mixture  deflagrates,  and  there 
remains  behind  metallic  silver. 

Exp.  276.  —  Add  to  a  solution  of  nitrate  of  silver  a  solution  of  caus- 
tic soda,  until  no  further  precipitate  is  produced.  The  brownish  pre- 
cipitate is  a  hydrated  oxide  of  silver. 

536.  Oxides  of  Silver.  Silver  probably  forms  three  oxides, 
Ag4O,  Ag2O  and  Ag2O2 .  The  first  is  a  very  unstable  black 
powder ;  the  second  forms  with  acids  the  ordinary  silver  salts ; 
the  third  is  a  crystalline  body  obtained  by  electrolysis  of  nitrate 
of  silver.  The  precipitate  obtained  in  the  last  experiment  is 


452  FULMINATING    SILVER. 

the  hydrate  of  the  oxide  Ag2O ;  this  precipitate  readily  parts 
with  its  water,  and  at  a  temperature  much  below  100°  becomes 
anhydrous.  Unlike  the  oxides  of  sodium  and  potassium,  this 
oxide  of  silver  yields  up  its  oxygen  below  a  red  heat,  and 
metallic  silver  remains,  as  may  be  demonstrated  by  heating  the 
product  of  the  last  experiment;  light  also  reduces  it,  and  hydro- 
gen even  at  100°  has  the  same  effect.  Oxide  of  silver  bears, 
however,  certain  striking  resemblances  to  the  oxides  of  the 
alkali-metals ;  thus,  it  is  a  strong  base,  uniting  with  strong  acids 
to  form  salts  which  are  neutral  to  test-paper,  and  which  are  in 
some  cases  isomorphous  with  the  corresponding  salts  of  sodium. 
The  oxide  is  slightly  soluble  in  water,  and  the  solution  has 
a  feeble  alkaline  reaction. 

The  oxide  is  freely  soluble  in  ammonia-water,  and  the  solution 
deposits,  on  exposure  to  the  air,  a  black,  micaceous  powder 
which  has  received  the  name  of  fulminating  silver,  because 
of  its  explosive  character.  The  same  dangerous  compound  is 
formed  when  freshly  precipitated  oxide  of  silver  is  digested  for 
some  hours  in  ammonia-water,  and  it  is  also  produced  when  an 
ammoniacal  solution  of  chloride  or  nitrate  of  silver  is  precipitated 
with  a  solution  of  hydrate  of  sodium  or  potassium.  It  is  neces- 
sary to  be  aware  of  these  facts  in  order  to  avoid  the  risk  of 
producing  by  accident  this  exceedingly  dangerous  substance. 
Its  composition  is  not  accurately  known  ;  friction  or  slight  pres- 
sure, even  under  water,  may  cause  it  to  explode.  The  student 
should  never  venture  to  prepare  this  substance. 

Exp.  277. — 'Fill  three  test-tubes  one-third  full  of  water,  and  pour 
into  each  a  few  drops  of  a  moderately  strong  solution  of  nitrate  of  sil- 
ver. Add  to  the  first  test-tube  2  or  3  c.  c.  of  a  solution  of  chloride  of 
sodium,  and  shake  the  tube  violently ;  a  dense,  white,  curdy  precipitate 
of  the  chloride  of  silver  will  be  produced.  Add  to  the  second  test-tube 
2  or  3  c.  c.  of  a  solution  of  bromide  of  potassium,  and  shake  the  tube ; 
a  yellowish  precipitate  of  bromide  of  silver  will  be  thrown  down.  Add 
to  the  third  test-tube  1  or  2  c.  c.  of  a  solution  of  iodide  of  potassium, 
and  shake  up  the  liquid  ;  a  pale-yellow  flocculent  deposit  of  iodide  of 
silver  will  be  formed. 

Withdraw  from  each  test-tube  a  portion  of  the  precipitate  it  contains, 
and  try  to  dissolve  each  precipitate  in  strong  nitric  and  chlorhydric 
acids  ^  the  attempt  will  fail,  for  these  silver  salts  are  insoluble  in  acids. 


PHOTOGRAPHY DAGUERREOTYPE.  453 

Withdraw  from  each  test-tube  another  portion  of  the  precipitate  it 
contains,  and  treat  each  precipitate  with  ammonia-water ;  the  chloride 
of  silver  will  dissolve  easily,  the  bromide  less  easily,  the  iodide  with 
difficulty.  Lastly,  pour  upon  the  remnants  of  the  original  precipitates 
in  the  three  test-tubes  a  moderately  strong  solution  of  hyposulphite  of 
sodium  (§  495) ;  all  three  precipitates  will  immediately  dissolve. 

Exp.  278. —  Precipitate  some  curdy  chloride  of  silver  by  adding 
chloride  of  sodium  solution,  or  chlorhydric  acid,  to  a  solution  of  nitrate 
of  silver,  so  long  as  any  precipitate  is  produced.  Throw  the  precipitate 
upon  a  filter,  and  wash  it  with  water ;  then  open  the  filter,  spread  the 
chloride  evenly  over  it,  and  place  it  in  direct  sunlight.  The  white  pre- 
cipitate rapidly  changes  to  violet  on  exposure  to  the  sun's  rays,  the 
depth  of  shade  increasing  as  the  action  of  the  light  continues.  This 
coloration  arises  from  a  partial  decomposition  of  the  chloride  of  silver, 
the  change  of  color  being  accompanied  by  a  loss  of  chlorine.  Upon  the 
facts  illustrated  in  this  and  the  preceding  experiments  the  main  pro- 
cesses of  photography  depend. 

537.  Photography.  The  chemical  changes  which  the  salts  of 
silver  undergo,  when  exposed  to  light,  are  the  basis  of  the  art  of 
photography,  not  because  these  are  the  only  salts  which  are 
affected  by  light,  but  because  none  are  so  advantageous,  on  the 
whole.  There  are  three  different  kinds  of  photographic  process, 
—  that  on  silver,  that  on  glass,  and  that  on  paper. 

To  produce  a  photograph  on  silver,  —  a  daguerreotype,  —  a 
highly  polished  silver  plate  is  exposed  in  a  dark  box  to  the  diluted 
vapor  of  a  mixture  of  bromine  and  iodine.  A  bronze-yellow 
film  of  brom-iodide  of  silver  is  thus  produced  upon  the  plate, 
which,  at  a  certain  stage,  possesses  a  high  degree  of  sensitive- 
ness to  light.  The  plate  is  then  transferred  to.  a  camera,  and 
exposed  at  the  fpcus  of  the  lens  to  the  light  radiated  from  the ' 
object  to  be  copied.  After  remaining  a  few  seconds  in  the 
camera,  it  is  withdrawn,  and  immediately  exposed  in  a  warm 
box  to  the  vapor  of  metallic  mercury.  When  the  plate  is  taken 
from  the  camera,  the  film  looks  as  uniform  as  ever,  and  no 
image  is  visible  upon  it;  but  the  exposure  to  mercury  vapor 
immediately  brings  out  an  image.  The  mercury  fixes  itself 
strongly  upon  those^  parts  which  have  received  the  light,  while 
it  takes  no  hold  upon  those  parts  of  the  film  which  the  light  has 
not  decomposed.  A  strong  solution  of  hyposulphite  of  sodium  is 
then  poured  over  the  plate,  in  order  to  dissolve  off  the  undecom- 


454  PHOTOGRAPHY    ON    GLASS. 

posed  brom-iodide.  The  highly  polished  silver,  beneath,  forms 
the  shades,  and  the  amalgam  of  mercury  with  silver  forms  the 
lights.  The  plate  is  washed,  and  a  very  dilute  solution  of 
chloride  of  gold  in  hyposulphite  of  sodium  is  poured  over  its 
surface  and  gently  warmed.  A  thin  film  of  gold,  which,  as  it 
were,  varnishes  the  picture,  is  thus  deposited  upon  the  plate ; 
another  washing  completes  the  operation.  The  daguerreotype 
is  the  most  perfect  of  photographs,  but  the  polish  of  the  surface 
prevents  the  image  from  being  seen  in  all  lights,  and  the  plate  is 
liable  to  be  tarnished  and  ruined  by  sulphuretted  gases. 

In  order  to  get  a  photograph  upon  glass,  a  transparent  film 
capable  of  holding  the  necessary  silver-salt   must  first  be  at- 
tached to  the  glass  plate.     Collodion,  a  solution  of  a  variety  of 
gun-cotton  in  a  mixture  of  alcohol   and   ether,  is  the  material 
of  this  film.     To  the  collodion  is  added  a  solution  of  an  iodide, 
either  of  potassium,  cadmium,  or  ammonium,  or  a  mixture  of 
these ;  the  bromides  of  ammonium  and  cadmium,  or  one  of  them, 
added  in  the  proportion  of  one  part  of  the  bromides  to  three  or 
four  of  the  iodides,  render  the  film  more  sensitive  to  yellow  and 
red  rays,  —  a  point  of  importance  in  cloudy  weather  or  smoky 
towns.     The    collodion    thus    prepared   is  poured   rapidly  over 
a  clean    and  dry  surface  of  plate-glass ;   the   volatile    solvents 
evaporate  rapidly,  and  as  soon  as  the  film  is  coherent,  the  glass 
is  plunged  into  a  bath  of  nitrate  of  silver  very  slightly  acidified 
with  acetic  or  dilute  nitric  acid.     This  bath  must  be  in  a  dark 
place  ;  the  plate  remains  in  it  for  several  minutes.     A  yellow 
layer  of  iodide  or  brom-iodide  of  silver  is  produced  in  the  film, 
and  nitrate  of  potassium,  cadmium,  or  ammonium  dissolves  in  the 
bath.     The   plate   is    then    exposed   in    the    camera   for  'a  few 
seconds.     When  removed  no  image  is  perceptible,  but  on  pour- 
ing  over  the   film   a   solution  of  gallic    or   pyrogallic   acid   in 
alcohol  and  acetic  acid,  or  a  solution  of  the   green  sulphate  of 
iron,  mixed  with  a  few  drops  of  a  weak  solution  of  nitrate  of 
silver,  the  image  will  be  developed,  slowly  or  rapidly,  according 
to  the  nature  and  strength  of  the  developing  liquid,  the  degree 
of  exposure,  and  the  intensity  of  the  light.    The  illuminated  por- 
tions of  the  picture  will  appear,  under  the  action  of  the  devel- 
oper, more  or  less  black,  while  the   shaded  portions  will  retain 


PHOTOGRAPHY.  455 

the  yellow  color  of  the  iodide.  As  soon  as  the  details  of  the 
shaded  portions  appear,  the  liquid  is  washed  off  and  the  develop- 
ment arrested.  A  saturated  solution  of  hyposulphite  of  sodium 
is  then  poured  over  the  film  to  dissolve  off  the  yellow  iodide  of 
silver  where  it  has  not  been  affected  by  the  light;  only  the 
reduced  portions  of  silver  remain,  and  they  appear  more  or  less 
opaque.  The  plate  must  finally  be  very  thoroughly  washed  to 
remove  all  traces  of.  the  hyposulphite,  and  then  dried  and 
varnished  on  the  collodion  side  to  protect  the  film  from  injury. 

Concerning  the  nature  of  the  change  which  a  film  of  iodide  of 
silver  undergoes  when  exposed  to  light,  we  cannot  be  said  to 
have  any  exact  knowledge.  There  is  no  perceptible  alteration 
in  the  film ;  there  is  no  loss  of  iodine ;  the  iodide  retains  its 
solubility  in  hyposulphite  of  sodium;  yet  the  impression  is  not 
of  a  temporary  kind,  for  the  invisible  image  produced  on  a  plate 
may  be  developed  many  hours  afterwards  if  the  plate  is  kept  in 
the  dark  during  the  interval. 

The  photograph  on  collodion  may  be  employed  directly  as  a 
positive  picture,  if  not  too  strongly  developed,  by  placing  it  on 
a  black  background.  Those  portions  which  are  opaque  to  light, 
or  in  other  words  those  in  which  silver  is  deposited,  will  reflect 
light,  and  furnish  the  lights  of  the  picture ;  while  those  on  which 
the  light  did  not  act,  and  which  are  therefore  transparent,  will 
appear  black  from  the  nature  of  the  background,  and  these  will 
form  the  shades  of  the  picture.  In  the  daguerreotype  the 
finished  picture  is  inverted,  —  in  the  collodion  positive  it  is  not 
inverted.  If  the  development  be  pushed  further,  the  image 
becomes  so  strongly  defined  that  the  deposited  silver  will  more 
or  less  completely  intercept  the  light.  The  collodion  side  of  the 
plate  is  then  placed  in  contact  with  the  sensitive  side  of  paper 
impregnated  with  chloride  of  silver,  by  a  process  immediately  to 
be  described,  and  exposed  to  light  in  a  pressure-frame.  The 
light  is  arrested  by  the  altered  parts  of  the  collodion,  but  is 
freely  transmitted  by  the  other  portions  ;  upon  the  paper,  there- 
fore, the  lights  of  the  real  object  are  light  and  the  shades  are 
dark.  Such  a  negative  collodion  picture  may  of  course  be 
copied  on  a  second  sensitive  collodion  film. 

Two  developing  solutions,  used  one  after  the  other,  produce 


456  PHOTOGRAPHY    ON    PAPER. 

a  better  effect  than  one.  The  green  sulphate  of  iron  may  be 
used  first,  and  pyrogallic  acid  with  nitrate  of  silver  subse- 
quently; the  iron  solution  must  be  completely  washed  off  before 
the  other  is  added.  The  picture  may  even  be  intensified  by  pyro- 
gallic acid  after  the  plate  has  been  washed  in  hyposulphite  of 
sodium. 

Photographs  were  made  on  paper  long  before  the  film  on  glass  came 
into  use,  but  the  paper  process  is  now  chiefly  confined  to  the  printing 
of  positive  impressions  from  collodion  negatives  on  glass.  The  silver- 
salt,  which  is  preferred  for  photographic  paper,  is  the  chloride,  with  or 
without  albumen,  but  always  accompanied  with  free  nitrate  of  silver. 
The  paper  is  floated  for  five  minutes  on  a  solution  of  chloride  of  sodium 
or  ammonium ;  when  dried,  it  is  floated  in  a  dark  room,  for  five  min- 
.utes,  on  its  salted  surface,  in  a  solution  of  nitrate  of  silver;  again  dried, 
it  is  fit  for  use.  When  such  paper  is  used  to  obtain  a  positive  impres- 
sion from  a  collodion  negative,  or  from  a  paper  negative  made  trans- 
parent with  wax,  or  a  mixture  of  wax  and  paraffine,  it  is  exposed  to 
light  under  the  negative  to  be  copied,  until  the  lights  of  the  picture  are 
of  a  pale  lilac  hue,  and  the  shades  of  a  deep  bronze  color.  After  being 
thoroughly  washed,  the  paper  is  transferred  to  a  "  toning  "-bath,  which 
consists  of  a  very  dilute  solution  of  bicarbonate  of  sodium,  with  a  minute 
proportion  of  chloride  of  gold.  The  picture  is  kept  in  motion  while  in 
this  bath ;  it  remains  there  until  its  shades  have  acquired  a  deep  purple- 
black  color.  It  is  only  in  those  parts  of  the  picture  in  which  the  silver 
has  been  well  reduced  that  this  toning  effect  is  produced.  The  picture 
is  again  washed  in  water,  and  soaked  for  fifteen  minutes  in  a  solution 
of  hyposulphite  of  sodium,  in  order  to  remove  all  the  chloride  of  silver 
which  is  contained  in  the  substance  of  the  paper.  Finally,  the  picture 
must  be  soaked  for  twenty-four  hours  in  water  wliich  is  constantly  re- 
newed, in  order  to  wash  away  every  trace  of  the  compound  hyposul- 
phite of  sodium  and  silver.  No  photograph  will  keep  long,  unless  the 
chloride  of  silver  has  been  completely  dissolved  by  the  hyposulphite, 
and  the  compound  hyposulphite  washed  away  with  a  thoroughness  that 
leaves  no  trace  behind.  If  the  first  condition  is  not  fulfilled,  diffused 
daylight  will  alter  the  picture ;  if  the  second  condition  is  not  complied 
with,  yellow  or  brown  stains  will  ultimately  destroy  the  picture. 

As  in  every  other  art  which  embraces  many  details,  and  demands  a 
trained  eye  and  hand,  eminent  skill  in  photography  can,  as  a  rule,  be 
acquired  only  by  long  practice. 

538.  Chloride  of  Silver  (AgCl).  Native  chloride  of  silver 
occurs,  sometimes  in  cubical  crystals,  sometimes  in  compact, 


CHLORIDE    OF    SILVER.  457 

semi-transparent  masses,  which  are  sectile,  and,  from  their  gen- 
eral appearance,  have  given  the  mineral  the  name  of  horn-silver. 
Chloride  of  silver  may  be  precipitated  from  any  soluble  silver- 
salt  by  adding  to  the  silver  solution  chlorhydric  acid,  or  the  solu- 
tion of  any  soluble  chloride;  or  it, may  be  obtained  by  passing 
over  a  dry  silver-salt  a  stream  of  dry  chlorine  gas.  This  last  re- 
action is  the  basis  of  a  method  of  preparing  anhydrous  nitric  acid. 
When  a  stream  of  dry  chlorine  is  made  to  pass  over  perfectly 
dry  nitrate  of  silver,  heated  to  50°  or  60°,  the  following  reaction 
takes  place :  — 

Ag2N806  +  2C1  =  2AgCl  +  N203  +  0. 

The  characteristics  of  precipitated  chloride  of  silver  have  been 
already  described  (Exp.  277).  The  presence  of  an  extraordi- 
narily minute  proportion  of  chloride  of  silver  renders  a  clear 
liquid  opalescent.  It  is  easy  to  detect  silver  in  a  solution,  of 
which  it  forms  only  the  ^oo^oo^1  Part>  by  adding  to  the  solution 
a  drop  of  chlorhydric  acid  or  of  a  soluble  chloride.  An  admira- 
ble method  of  determining  the  amount  of  silver  present  in  any 
solution  depends  upon  the  insolubility  of  chloride  of  silver,  the 
density  and  peculiar  curdy  quality  of  the  precipitate,  and  the 
visibility  of  the  smallest  trace  of  it  in  a  clear  fluid.  This  method, 
now  generally  employed  in  mints  and  assay-offices,  is  applicable 
to  the  quantitative  analysis  of  silver  alloys ;  it  is  volumetric,  and 
depends  upon  the  measurement  of  the  amount  of  a  standard  so- 
lution of  chloride  of  sodium  which  is  required  to  effect  the  com- 
plete precipitation,  as  chloride,  of  the  silver  contained  in  a  given 
weight  of  the  alloy.  In  a  solution  which  is  acidulated  with  nitric 
acid,  and  which  contains  no  excess  of  the  soluble  chlorides,  the 
chloride  of  silver  is  easily  coagulated  into  dense  flocks  by  agita- 
tion, so  that  the  exact  point  at  which  the  precipitate  ceases  to  be 
formed  is  readily  perceived. 

Chloride  of  silver  melts  at  about  2GO°.  It  is  not  decomposed 
when  heated  with  carbon,  but  is  easily  reduced  by  hydrogen 
when  heated  in  a  current  of  the  gas ;  zinc  and  iron  reduce  moist 
chloride  of  silver  to  metallic  silver ;  when  heated  with  carbonates 
or  hydrates  of  sodium,  potassium,  or  calcium,  chloride  of  silver 
gives  its  chlorine  to  the  other  metal,  and  pure  silver  is  set 
free. 


458  ATOMIC    WEIGHT   OF    SILVER. 

These  methods  of  reducing  chloride  of  silver,  except  that  by  hydrogen, 
are  turned  to  account  in  the  refining  of  silver  on  a  large  scale.  The 
coin,  or  bullion,  to  be  refined,  is  dissolved  in  nitric  acid,  and  to  the  so- 
lution chloride  of  sodium  is  added  ;  the  precipitated  chloride  of  silver 
is  washed  until  the  washings  are  tasteless,  and  is  then  slightly  acidu- 
lated with  sulphuric  acid  ;  bars  of  zinc  are  placed  in  the  moist  mass, 
and  the  whole  left  at  rest  for  two  or  three  days.  Chloride  of  zinc  and 
metallic  silver  are  the  products.  As  soon  as  the  reduced  silver  is  en- 
tirely soluble  in  nitric  acid,  the  reduction  is  complete.  The  reduced 
metal  is  digested  for  two  or  three  days  in  dilute  sulphuric  acid,  to  re- 
move adhering  zinc-salts,  and  is  then  thoroughly  washed,  dried,  and 
finally  melted  and  cast  into  ingots.  If  an  absolutely  pure  metal  is  de- 
sired, the  first  reduction  should  be  made  with  pure  zinc,  and  this  re- 
fined silver  may  be  again  dissolved  in  nitric  acid,  thrown  down  as  chlo- 
ride, and  reduced  again  from  the  washed  chloride  by  fusion  with  car- 
bonate of  calcium. 

539.  The  reduction  of  chloride  of  silver  by  hydrogen  is  the 
basis  of  one  of  the  several  determinations  of  the  atomic  weight  of 
silver,  and  since  silver  forms  a  large  number  of  anhydrous  salts 
with  acids,  and  has  little  or  no  tendency  to  form  more  than  one 
salt  with  each  acid,  the  silver-salt  is  often  the  best  one  to  prepare 
and  analyze,  whenever  the  combining  weight  of  an  acid  is  to  be 
determined.  But  it  is  clear  that  the  accuracy  of  these  determi- 
nations depends  upon  the  accuracy  with  which  the  atomic  weight 
of  silver  is  known,  —  hence  extraordinary  pains  have  been  taken 
to  arrive  at  the  true  atomic  weight  of  silver.  It  has  been  found, 
by  the  most  careful  experiment,  by  heating  chloride  of  silver  in 
a  current  of  hydrogen,  that  in  132.856  parts  of  that  compound, 
100' parts  of  silver  are  united  with  32.856  of  chlorine.  If  the 
atomic  weight  of  chlorine  be  accepted  at  35.5,  a  simple  propor- 
tion leads  to  the  atomic  weight  of  silver. 

32.856         :         35.5     ==  100         :     x    =     108.07. 

Amount  of  Cl.     At.  Weight  of  Cl.     Ami.  o/Ag.     At.  Weight  of  Silver. 

An  entirely  different  experiment  verifies  this  result ;  — by  burn- 
ing finely  divided  silver  in  a  current  of  perfectly  dry  chlorine,  it 
is  proved  that  108  parts  of  silver  combine  with  35.505  of  chlo- 
rine. The  following  round  of  experiment  and  simple  calculation 
will  illustrate  the  manner  in  which  one  atomic  weight  is  derived 
from  another,  and  all  are  verified  by  mutual  comparison.  Chlo- 


CYANIDE    OF    SILVER.  459 

rate  of  potassium,  when  heated,  gives  off  all  its  oxygen  and 
chloride  of  potassium  remains.  Assuming  that  the  formula  of 
chlorate  of  potassium  is  KC1O3  and  that  the  atomic  weight 
of  oxygen  is  1 6,  we  derive  the  following  proportion  from  the 
fact  of  experiment  that  100  parts  of  KC103  yield  39.209  parts 
of  oxygen. 

39.209       :       48     =       60.791         :       x    =     74.4208. 
Amount  of  O.         3O.          Amt.ofKCl         Molecular  Weight  of  KCL 
It   is  another  experimental  fact  that  100   parts  of  chloride  of 
potassium    produce,    when    precipitated    with   nitrate   of  silver, 
192.75  of  chloride  of  silver. 

100       :         192.75       =       74.4208     :     x     =     143.446. 
Amt.of.KC\.    Ami.  of  AgCl.    M.WeightofKCl.    Molecular  Wt.  A«Cl. 
But  it  has  been  determined,  as  above  stated,  that  132.856  parts 
of  chloride  of  silver  contained  32.856  parts  of  chlorine,  and 
accordingly 

132.856     :         32.856         =     143.446     :     x    =     35.476. 
Amt.  of  AgCl.      A  mt.  of  Cl.     M.  Weight  of  AgCl.        At.  Weight  of  Cl. 

But  if  the  molecular  weight  of  ^chloride  of  silver  is  .  143.446 
we  may  deduct  the  atomic  weight  of  chlorine,  .  .  35.476 
and  so  obtain  the  atomic  weight  of  silver;  .  .  .  107.970 

and  if  the  molecular  weight  of  chloride  of  potassium  is     74.4208 
we  may  deduct  the  atomic  weight  of  chlorine,     .         .     35.476 
and  so  obtain  the  atomic  weight  of  potassium.     .         .     38.9448 

These  numbers  will  be  found  to  be  very  nearly  coincident  with 
those  previously  given  as  the  accepted  atomic  weights  of  these 
three  very  important  elements. 

540.  Bromide    and   Iodide  of  Silver  (AgBr   and  Agl)    are 
two   rather   rare    minerals,  usually  associated  with  chloride    of 
silver  or  with  native   silver.     Their  artificial  preparation  and 
such  of  their  properties  as  have  present  importance  have  been 
already  alluded  to   (Exp.  277).     They  are  both  easily  fusible 
and  insoluble  in  water,  but  soluble  in  concentrated  solutions  of 
the  bromide  and  iodide  of  potassium. 

541.  Cyanide  of  Silver  (AgCN)  is  a  white  powder,  insoluble 
in  water   but    soluble    in    ammonia-water,  obtained   by  precip- 


460  SULPHIDE    OF    SILVER. 

itating  nitrate  of  silver  with  a  soluble  cyanide  like  the  cyanide 
of  potassium.  Cyanide  of  silver  is  soluble  in  solutions  of  the 
cyanides  of  sodium,  potassium,  calcium,  and  other  metals,  form- 
ing double  cyanides  of  the  formula  MAgG,N2.  When  such 
a  solution  is  subjected  to  the  action  of  a  galvanic  battery,  metallic 
silver  is  deposited  at  the  negative  pole  in  a  compact,  adherent, 
layer,  while  at  the  positive  pole,  where  a  strip  or  plate  of  metallic 
silver  is  placed,  a  quantity  of  the  metal  equal  to  that  which  is 
deposited  at  the  negative  pole  continually  dissolves.  A  solution 
which  contains  ^  of  its  weight  of  silver  is  found  to  be  of  con- 
venient strength  for  the  ordinary  operations  of  electro-plating. 

542.  Sulphide  of  Silver  (Ag2S).     This  compound  is  a  princi- 
pal ore  of  silver.     The  native  mineral  is  sometimes  crystallized, 
in  cubes  or  octohedrons,  and  sometimes  massive  ;  it  has  a  leaden 
lustre  and  color,  and  it  is  so  soft  that  a  knife  will  cut  or  a 
die  impress  it ;  it  is  fusible,  and  wlien  roasted  in  the  air,  yields 
silver,  which  remains  in  the  metallic  state,  and  sulphurous  acid, 
the  product  of  the  combination  of  its  sulphur  with  the  oxygen  of 
the  air.     The  pure  mineral  is  very  easily  recognized  by  these 
marked  characteristics.      Silver1  is  readily  tarnished  by  contact 
with  moist   gaseous  sulphydric  acid,  or   with  a   solution   of  a 
soluble  sulphide  ;  this  tarnish  is  the  sulphide  of  silver  (§  533). 
The    sulphide   may  be   artificially  prepared    by  transmitting   a 
stream  of  sulphuretted  hydrogen  through  a  solution  of  a  salt  of 
silver. 

Exp.  279.  —  Place,  in  a  test-glass,  8  or  10  c.  c.  of  water,  to  which  20 
or  30  drops  of  a  solution  of  nitrate  of  silver  has  been  added,  and  pass 
through  the  dilute  solution  a  slow  stream  of  sulphuretted  hydrogen. 
The  black  precipitate  is  the  sulphide  of  silver. 

Strong  acids,  especially  when  hot,  dissolve  or  decompose  this 
sulphide.  It  is  not  soluble  in  solutions  of  the  sulphides  of  the 
alkali  metals,  but  by  fusion  it  may  be  made  to  unite  with  many 
other  sulphides  of  metals. 

543.  Sulphate  of  Silver  (Ag.2S04).    When  silver  is  boiled  with 
strong  sulphuric  acid,  the  silver  gradually  dissolves,  and  there  are 
formed  the  sulphate  of  silver,  water  and  sulphurous  acid  :  — 

2  Ag  +  2  H2S04  =  Ag2S04  +  2  H2O  +  S(V. 
The  sulphate  is  dissolved  by  the  excess  of  acid,  but  it  is  deposited 


THE    ALKALI    GROUP.  461 

in  great  part  on  the  addition  of  water,  for  it  is  but  slightly 
soluble  in  water.  As  gold  is  not  soluble  in  sulphuric  acid,  small 
quantities  of  gold  may  be  separated  from  large  quantities  of 
silver  or  silver  alloys,  by  boiling  the  metal,  finely  granulated,  in 
cast-iron  vessels  with  oil  of  vitriol ;  silver  and  copper  dissolve, 
and  the  gold  is  left  behind  in  a  fine  powder.  The  solution  of 
silver  is  subsequently  diluted,  and  the  silver  precipitated  from 
the  solution  in  the  metallic  state  by  means  of  metallic  copper. 
(Exp.  27?.)  Old  silver  coin,  containing  not  more  than  ^oWh 
of  gold,  has  .been  profitably  worked  over  by  this  process. 

544.  The  Alkali  group.  The  metals  which  must  plainly  be 
classed  together  under  this  head  are  sodium,  potassium  (ammo- 
nium), lithium,  rubidium,  and  caesium.  Two  other  metals  are 
better  classed  with  this  group  than  elsewhere,  but  their  likeness 
to  the  alkali  metals  is  but  partial,  and  in  many  respects  their 
properties  are  quite  unlike  those  of  the  six  metals  just  enumer- 
ated; these  two  metals  are  silver  and  thallium.  The  common 
properties  of  the  alkali  metals  are  mainly  these  ;  —  they  have  the 
lustre  of  silver,  are  soft,  easily  fusible,  and  volatile  at  high  tem- 
peratures ;  they  unite  greedily  with  oxygen,  and  decompose 
water  with  facility,  forming  basic  hydrates  which  are  very  caustic 
and  intensely  alkaline  bodies,  not  to  be  decomposed  by  heat ; 
their  carbonates,  sulphates,  sulphides,  and  chlorides,  and  indeed 
the  vast  majority  of  their  salts,  are  soluble  in  water,  and  each 
metal  forms  but  one  chloride,  one  bromide,  and  one  iodide ; 
they  all  form  basic  oxides,  and  never  an  acid  oxide;  they  occur 
in  nature  in  modes  analogous  though  not  the  same  ;•  their  corre- 
sponding salts  are  often,  though  not  always,  isomorphous ;  lastly, 
there  ?s  a  general,  though  not  absolute,  uniformity  among  the 
formula?  of  the  compounds  into  which  these  elements  enter,  so 
that  if  a  compound  of  a  given  composition  is  proved  to  exist  for 
one  of  these  elements,  the  strong  presumption  is  that  analogous 
compounds  with  all  the  other  elements  of  the  group  exist  like- 
wise with  properties  similar,  though  not  identical. 

Silver  and  thallium  present,  on  the  whole,  so  few  points  of 
resemblance  to  the  alkali  metals  that  they  would  not  be  compre- 
hended in  the  same  group  with  them  were  it  not  for  one  consid- 
eration weighty  enough  to  turn  the  balance  when  the  discussion 


462  QUANTIVALENCE. 

of  other  properties  leaves  the  matter  in  doubt.  Sodium,  potas- 
sium (ammonium),  lithium,  caesium,  rubidium,  silver,  and  thal- 
lium all  replace  hydrogen,  atom  for  atom.  All  these  elements 
are  exchangeable  for  hydrogen  and  with  each  other,  atom  for 
atom,  and  in  the  present  state  of  the  science  they  must  be 
regarded  as  the  only  metals  thus  equivalent  to  hydrogen.  The 
atom  of  the  elements  of  the  chlorine  group,  including  fluorine  in 
that  designation,  and  of  the  seven  elements  above  enumerated,  is 
exchangeable  for  one  atom  of  hydrogen ;  it  is  worth  one  in 
exchange,  and  these  elements  are  therefore  said  to  be  univalent, 
or  with  less  verbal  precision,  monatomic. 

545.  Quantivalence.  The  chemical  elements  have  not  all  the 
same  atom-fixing  power;  thus,  while  an  atom  of  chlorine  combines 
with  only  one  atom  of  hydrogen,  an  atom  of  oxygen  has  the 
power  to  drag  two  atoms  of  hydrogen  into  a  molecule  ;  an  atom 
of  nitrogen  holds  three  atoms  of  hydrogen  in  firm  chemical  com- 
bination, and  an  atom  of  carbon  four  hydrogen-atoms.  In  all 
double  decompositions  an  atom  of  sodium,  potassium,  or  silver 
replaces  one  atom  of  hydrogen,  but  an  atom  of  calcium  or  lead 
two  atoms  of  hydrogen  (§  82).  To  indicate  conveniently  the 
atom-fixing  power  of  each  element  a  sign  is  needed  and  a  name. 
The  conventional  sign  is  a  Roman  numeral,  or  an  equivalent  num- 
ber of  accents,  placed  above  and  at  the  right  of  the  symbol  of  the 
element,  in  case  its  atom  be  worth  more  than  one  of  hydrogen  ; 
and  for  the  name  to  d  -note  this  atom-fixing  power  of  the  ele- 
ments the  word  "  quantivalence "  may  be  used,  or  the  less 
descriptive  word  "  atomicity."  The  elements  are  called  univa- 
lent,  bivalent,  trivahnt  and  quadrivalent,  or  monatomic,  diatomic, 
triatomic  and  tetratomic,  according  as  their  respective  atoms 
are  capable  of  saturating,  or  holding  in  firm  chemical  combina- 
tion, 1,  2,  3,  or  4  atoms  of  hydrogen.  Thus,  while  the  simple 
symbols  Cl,  Br,  K,  Ag  indicate  that  chlorine,  bromine,  potas- 
sium and  silver  are  univalent,  the  symbols  of  nitrogen,  antimony, 
/and  other  trivalent  elements  may  be  written  N'",  Sb'",  etc.  In 
the  same  way  the  symbols  O"  and  Ca"  indicate  that  oxygen  and 
calcium  are  bivalent,  and  the  symbol  C""  shows  that  carbon  is 
quadrivalent. 

The  quantivalence  of  many  of  the  elements  is  not  yet  deter- 


QUANT1VALENCE.  463 

mined  with  certainty,  but  the  classification  into  groups  of  the  ele- 
ments we  have  thus  for  studied,  rests  upon  the  quanti valence  of 
the  elements,  as  well  as  upon  the  other  chemical  resemblances, 
which  have  been  dwelt  upon  in  connection  with  each  group. 
The  elements  of  the  chlorine-group  and  the  alkali-group  are  uni- 
valent ;  the  elements  of  the  sulphur-group  and  the  majority  of 
the  metals,  hereafter  to  be  studied,  are  bivalent ;  the  elements  of 
the  nitrogen-group  are  trivalent,  and  of  the  carbon  group  quadri- 
valent. 

It  must  not  be  supposed  that  the  atom-fixing  power  of  the  ele- 
mentary bodies  is,  under  all  circumstances,  and  in  all  compounds, 
invariably  exerted  to  the  fullest  extent.     Were  the  combination 
of  the  elements  governed  by  any  such  law  as  this,  it  would  evi- 
dently be  impossible  for  any  two  elements  to  unite  in  more  than 
one  proportion.     Thus    trivalent  nitrogen  and  bivalent  oxygen 
could  only  combine  in  the  proportions  represented  by  the  formula 
N2///08",  proportions   which  completely  satisfy  the   atom-fixing 
power  of  both  elements.     But  we  know  that  these  two  elements 
actually  form  no  less  than  five  different  compounds  (§§  75,  76), 
of  which  only  one  is  marked  by  the  complete  equilibrium  of  the 
two  elements,  and  this  one  is  by  no  means  the  most  stable  mem- 
ber of  the  series;  on  the  contrary,  it  is  about  the  most  unstable. 
The  student  must  not  imagine  that  a  bivalent  element  has  twice 
as  strong  affinities  as  a  univalent  element ;  the  atom-fixing  power 
of  an  element  is  no  test  or  index  of  the  avidity  with  which  it 
seeks   combination.      Chlorine,   which   holds   but   one  atom   of 
hydrogen,  is  competent   to  decompose   sulphuretted   hydrogen, 
ammonia,  and  marsh-gas,  although  sulphur  unites  by  preference 
with  two,  nitrogen  with  three,  and  carbon  with  four  atoms  of 
hydrogen. 


464  CALCIUM. 

CHAPTER    XXVIII. 

CALCIUM STRONTIUM BARIUM LEAD. 

CALCIUM. 

546.  This  metal  is  a  constituent  of  several  of  the  commonest 
and  most  important  minerals  ;  it  forms  a  very  considerable  portion 
—  perhaps  as  much  as  one-sixteenth  —  of  the  solid  crust  of  the 
earth.     Before  considering  the  properties  of  the  metal  itself,  let 
us  examine  some  of  its  familiar  compounds. 

547.  Carbonate  of  Calcium  (CaC03)  occurs  in  nature  in  many 
different  forms,  called  by  a  great  variety  of  names,  among  which 
may  be  mentioned  limestone,  chalk,  marble,  calc-spar,  and  coral. 
There  are  whole  ranges  of  mountains  composed  almost  entirely 
of  limestone,  while  hi  many  extensive  tracts  of  country  the  soil  is 
calcareous  and  reposes  upon  limestone  rocks.    The  shells  of  shell- 
fish are  almost  entirely  composed  of  it,  and  it  is  an  important 
constituent  of  dolomite,  marl,   and  many  other  rocks  and  min- 
erals.    It  is  formed  artificially,  as  has  been  seen  (Exp.  172), 
when  carbonic  acid  is  brought  in  contact  with  lime-water ;  but  it 
is  noteworthy  that  carbonic  acid  will  not  unite  with  the  anhydrous 
oxide  of  calcium  (quick-lime). 

Carbonate  of  calcium,  though  tasteless,  is  slightly  soluble  in 
water,  and  the  solution  exhibits  a  faint  alkaline  reaction  ;  it  is, 
however,  rather  freely  soluble  in  water  charged  with  carbonic 
acid •(§  403). 

Exp.  280.  —  Place  in  a  test-tube  20  or  30  drops  of  lime-water,  and 
as  much  pure  water ;  immerse,  in  the  mixture,  the  delivery-tube  of  a 
bottle  from  which  carbonic  acid  gas  is  being  evolved  (Exp.  1 75).  Car- 
bonate of  calcium  will  be  thrown  down  at  first,  but  after  a  while,  as  the 
water  in  the  test-tube  becomes  saturated  with  carbonic  acid,  the  pre- 
cipitated carbonate  will  redissolve,  and  there  will  be  obtained  a  per- 
fectly clear  solution,  which,  in  spite  of  the  large  proportion  of  carbonic 
acid  contained  in  it,  has  a  decided  alkaline  reaction.  By  boiling  the 
solution,  so  that  a  portion  of  its  carbonic  acid  may  be  expelled,  the  car- 


CALCAREOUS    PETRIFACTIONS.  465 

bonate  of  calcium  can  be  again  precipitated.  So,  too,  if  the  liquid  be 
left  exposed  to  the  air,  it  will  gradually  give  off  carbonic  acid,  and  be- 
come turbid  from  deposition  of  carbonate  of  calcium. 

The  phenomena  illustrated  in  this  experiment  often  occur  in  nature. 
In  many  districts  where  limestone  is  abundant,  the  well  and  river- 
waters  are  highly  charged  with  carbonate  of  calcium  held  dissolved  by 
carbonic  acid ;  the  water  is  thus  made  "  hard  "  (see  §  560),  and  is,  com- 
paratively speaking,  unfit  for  washing  and  for  many  other  purposes. 
When  employed  as  a  source  of  steam-power,  such  waters  deposit  car- 
bonate of  calcium  as  an  incrustation  upon  the  sides  of  the  boilers  as 
fast  as  the  excess  of  carbonic  acid  is  expelled  by  boiling.  This  scale, 
or  incrustation,  forms  a  more  or  less  coherent  coating  upon  the  inner 
surface  of  the  boiler,  and,  being  a  very  poor  conductor  of  heat,  it  greatly 
interferes  with  the  heating  of  the  water ;  the  scale  keeps  the  water 
away  from  the  iron  sides  of  the  boiler,  and  the  metal,  being  thus  un- 
duly heated,  is  rapidly  oxidized,  or  "  burnt  out,"  as  the  fireman  cor- 
rectly states  it. 

The  formation  of  calcareous  petrifactions,  of  stalactites  and  stalag- 
mites, of  the  stones  called  tufa  and  travertine,  and  of  many  deposits  of 
crystallized  carbonate  of  calcium,  is  directly  referable  to  the  escape  of 
carbonic  acid  from  calcareous  waters.  Whenever  water,  charged  with 
carbonate  of  calcium,  flows  out  from  the  earth  into  the  open  air,  or 
trickles  into  hollows  or  caverns  within  the  earth,  carbonic  acid  is  given 
off  in  the  gaseous  state,  and  carbonate  of  calcium  is  deposited.  Stalac- 
tites are  the  pendent  masses,  like  icicles,  which  hang  from  the  roofs  of 
caverns,  and  the  walls  of  cellars,  bridges,  and  like  covered  ways; 
stalagmites  are  the  opposite  masses  which  grow  up  out  of  the  drops  of 
water  which  fall  from  the  stalactites  above  them,  before  all  the  carbon- 
ate has  been  deposited  from  them.  The  waters  of  some  mineral  springs 
are  so  highly  charged  with  carbonate  of  calcium  that,  on  being  exposed 
to  the  air,  they  quickly  deposit  a  considerable  quantity  of  it  upon  any 
solid  substance  with  which  they  come  in  contact.  Jn  case  such  waters 
flow  over  pieces  of  wood  or  other  organic  matter,  the  form  of  the  wood 
will  be  preserved  in  the  cast  or  "  petrifaction,"  long  after  the  wood  it- 
self has  decayed  and  disappeared.  Wliere  such  deposits  are  formed 
upon  a  scale  so  large  as  to  be  of  geological  importance,  as  is  the  case 
in  some  of  the  volcanic  districts  of  Italy,  the  rock  formed  is  called  tufa 
when  porous,  and  travertine  if  compact. 

548.  Carbonate  of  calcium  dissolves  also  in  aqueous  solutions 
of  several  of  the  salts  of  ammonium,  such  as  the  chloride, 
nitrate,  and  sulphate,  especially  if  it  has  only  recently  been 
precipitated  and  is  still  moist  and  incoherent. 

30 


466  CARBONATE    OF    CALCIUM   NOT    INSOLUBLE. 

Exp.  281. —  Through  2  or  3  c.  c.  of  lime-water,  contained  in  a  test- 
tube,  blow,  by  means  of  a  glass  tube,  a  quantity  of  air  from  the 
lungs ;  to  the  milky  liquid  obtained,  add,  drop  by  drop,  a  cold,  satu- 
rated aqueous  solution  of  chloride  of  ammonium,  until  the  cloudiness  in 
the  lime-water  has  disappeared,  —  that  is,  until  the  carbonate  of  calcium 
has  all  been  dissolved. 

Ex/).  282.  —  Place  a  drop  or  two  of  a  solution  of  chloride  of  calcium 
in  a  test-tube,  pour  upon  it  several  drops  of  a  strong  solution  of  chlo- 
ride of  ammonium ;  shake  the  mixture,  and  then  add  to  it  a  few  drops 
of  a  solution  of  carbonate  of  ammonium,  and  also  a  few  drops  of  ammo- 
nia-water. If  enough  chloride  of  ammonium  has  been  added  to  the 
liquid,  no  precipitate  will  be  formed  in  it,  though,  in  the  absence  of 
chloride  of  ammonium,  a  precipitate  will  at  once  be  produced  on  mix- 
ing the  other  ingredients.  A  precipitate  may,  however,  always  be  ob- 
tained by  boiling  the  mixed  solutions,  unless  a  large  excess  of  chloride 
of  ammonium  be  present,  or  unless  the  chloride  of  calcium  solution  be 
very  dilute. 

By  repeating  this  experiment  under  varied  conditions,  taking  note, 
in  each  case,  of  the  number  of  drops  of  the  solutions  of  chloride  of  am- 
monium, chloride  of  calcium,  and  of  water  employed,  and  methodically 
increasing  or  diminishing  each  of  these,  the  student  will  quickly  per- 
ceive the  real  significance  of  the  solvent  power  of  the  ammoniacal-salt, 
and  will  appreciate  the  fact,  that,  in  testing  for  small  quantities  of 
either  lime  or  carbonic  acid,  it  is  necessary  for  the  analyst  to  exclude 
ammonium-salts  from  his  solutions  as  far  as  may  be  practicable. 

When  boiled  with  solutions  of  the  salts  of  ammonium, — with  chloride 
of  ammonium,  for  example,  —  carbonate  of  calcium  is  rapidly  decom- 
posed and  dissolved  ;  carbonate  of  ammonium  being  given  off  while  the 
chloride,  or  some  other  salt,  of  calcium  remains  in  solution. 

549.  Carbonate  of  calcium  is  remarkable  not  only  for  the  very 
great  diversity  of  external  appearance  which  is  presented  by  its 
several  massive  and  amorphous  varieties,  but  it  is  likewise  found 
in  a  greater  variety  of  regular  crystalline  forms  than  any  other 
substance;  more  than  150  native  varieties  of  it  have  been 
observed  by  mineralogists.  As  calc-spar,  it  occurs  in  rhombo- 
hedrons  and  other  derivative  forms  of  the  sixth  or  hexagonal 
system  (§  191),  but  it  is  found  also  as  the  mineral  arragonite,  in 
forms  of  the  trimetric  system,  and  is  consequently  dimorphous. 

The  two  forms  of  carbonate  of  calcium,  calc-spar  and  arra- 
go.iite,  present  many  differences  in  their  physical  properties. 
Some  specimens  of  calc-spar,  called  Iceland  spar,  are  perfectly 


CALC-SPAR    AND    ARRAGONITE.  467 

transparent  and  colorless,  and  exhibit  to  a  remarkable  degree 
the  phenomena  of  double  refraction.  Transparent  crystals  of 
arragonite  exhibit  also  the  phenomena  of  double  refraction,  but 
arragonite  has  two  axes  of  double  refraction ;  calc-spar  only  one. 
Crystals  of  calc-spar  are  cleavable  parallel  to  the  faces  of  the 
rhombohedron  which  is  the  primary  form  of  the  mineral,  and 
masses  of  it  may  often  be  broken  up  into  more  or  less  perfect 
rhombohedrons.  Arragonite,  on  the  contrary,  presents  two  direc- 
tions of  distinct  cleavage  parallel  to  the  faces  of  a  right  rhombic 
prism.  The  fractures  of  the  two  minerals  are  therefore  quite 
unlike.  The  specific  gravity  of  calc-spar  ranges  from  2.7  to  2.75, 
while  the  specific  gravity  of  arragonite  is  generally  between  2.9 
and  3.3.  Arragonite  is  considerably  harder  than  calc-spar,  but 
its  specific  heat  (0.1966)  is  less.  When  carbonate  of  calcium 
crystallizes  from  hot  solutions  it  takes  the  form  of  arragonite,  but 
from  cold  solutions  it  crystallizes  as  calc-spar.  In  like  manner 
the  precipitate  formed  by  mixing  boiling  solutions  of  chloride  of 
calcium  and  carbonate  of  ammonium  is  seen  under  the  micro- 
scope to  consist  of  acicular  crystals  of  arragonite,  while  the  pre- 
cipitate obtained  from  cold  solutions  of  the  same  salts  is  amor- 
phous. In  either  case,  however,  if  the  moist  precipitate  be  left 
to  itself  for  some  time  in  the  cold,  it  will  gradually  assume  the 
rhombohedral  form  of  calc-spar,  no  matter  whether  it  was  at  first 
acicular  or  amorphous. 

In  all  its  varieties  carbonate  of  calcium  is  readily  attacked  by 
acids,  even  if  these  be  dilute;  the  action  is  attended  with 
effervescence,  owing  to  the  expulsion  of  carbonic  acid  and  the 
escape  of  this  gas  through  the  liquid  :  — 

CaO,  C02  +  2HC1  =  CaCl2  +  C02  +  H2O. 

Limestone  is  readily  distinguished  by  this  reaction  from  other 
rocks. 

550.  Oxide  of  Calcium  (CaO).  On  being  heated,  carbonate  of 
calcium  begins  to  give  off  carbonic  acid  at  a  low  red  heat,  as  has 
been  seen  in  Exp.  174,  and  at  full  redness  is  completely  resolved 
into  oxide  of  calcium,  commonly  called  quick-lime,  and  carbonic 
acid. 

Exp.  283.  —  Place  a  small  fragment  of  marble  upon  a  piece  of  char- 


468  OXIDE    OF    CALCIUM. 

coal  and  heat  it  strongly  in  the  blow-pipe  flame  during  several  minutes. 
Or  throw  a  lump  of  limestone  upon  an  anthracite  fire,  and  leave  it  there 
for  half  an  hour  or  more.  In  either  case,  it  will  be  found,  upon  exam- 
ination, that  the  calcined  product  has  lost  the  property  of  effervescing 
with  acids ;  that  it  weighs  less  than  the  original  limestone,  and  that  it 
exhibits  a  distinct  alkaline  reaction  when  placed  on  wet  test-paper. 

For  use  in  the  arts,  limestone  is  burned  in  special  furnaces,  of 
peculiar  construction,  called  lime-kilns,  some  of  which  are  so 
arranged  that  they  may  be  kept  in  operation  for  years  without 
intermission.  When  carbonate  of  calcium,  instead  of  being 
heated  merely  in  quiescent  air,  is  heated  in  a  current  of  air,  or 
of  any  other  gas,  such  as  steam  for  example,  it  will  give  off  all 
its  carbonic  acid  very  easily.  It  has  been  found  in  practice  that 
limestone  fresh  from  the  quarry  can  be  more  readily  burned 
than  that  which  has  been  long  dug  out  of  the  ground  and  has  so 
lost  its  natural  moisture;  in  damp  weather, moreover, the  burning 
is  said  to  go  on  more  satisfactorily  than  when  the  atmosphere  is 
dry.  But,  on  the  other  hand,  if  carbonate  of  calcium  be  ignited 
in  a  tube  of  iron,  or  other  metal  closed  hermetically,  so  that  no 
carbonic  acid  can  escape  from  the  tube,  the  carbonate  will  fuse 
without  undergoing  decomposition,  and  on  cooling  it  will  often 
solidify  to  a  fine-grained  crystalline  mass,  like  marble.  Under 
these  conditions  the  carbonate  of  course  disengages  some  car- 
bonic acid  at  first,  but  the  gas  being  unable  to  escape  from  the 
tube  soon  exerts  so  great  a  pressure  upon  that  portion  of  the 
carbonate  which  is  left  that  all  further  decomposition  is  arrested, 
and  the  carbonate  remains  as  such  even  at  temperatures  high 
enough  to  melt  it. 

Of  the  anhydrous  oxide  of  calcium  little  need  here  be  said ; 
it  is  infusible  at  the  most  intense  heat  at  our  present  command, 
and  is,  therefore,  used  for  making  crucibles  in  which  the  most 
refractory  metals  are  melted  by  the  aid  of  the  compound  blow- 
pipe. It  has  no  power  to  unite  with  dry  carbonic  acid ;  but  it 
unites  with  water  very  energetically,  and  the  product  of  this 
union  combines  readily  with  carbonic  acid.  When  lumps  of 
quick-lime  are  exposed  to  the  air  they  slowly  absorb  both  water 
and  carbonic  acid,  and  after  a  while  fall  to  powder.  This 
powder  is  known  as  air-slaked  lime;  its  composition  may  be 


HYDRATE    OP    CALCIUM.  469 

represented  by  the  formula  Ca,H,CO5,  or,  dualistic,  CaO,  C02; 
CaO,H2O. 

551.  Hydrate  of  Calcium  (CaH2O2).    When  water  is  brought 
in  contact  with  oxide  of  calcium,  the  latter  swells  up  and  falls  to 
powder ;  a  large  amount  of  heat  is  evolved,  and  there  is  obtained 
a  compound  of  calcium,  hydrogen,  and  oxygen,  commonly  called 
slaked  lime,  or  in  chemical  language  hydrate  of  calcium :  — 

CaO  -f  H2O  =  CaH2O2- 

Exp.  284.  —  Place  a  lump  of  recently-burned  quick-lime,  weighing 
about  30  grms.,  upon  a  large  earthen  plate ;  pour  upon  the  lime  some 
15  or  20  c.  c.  of  water,  and  observe  how  much  the  lime  increases  in 
bulk  as  it  is  converted  into  hydrate  of  calcium.  The  heat  of  the  mass 
may  be  shown  by  thrusting  an  ordinary  friction -match  into  the  middle 
of  it ;  or,  in  case  a  considerable  quantity  of  quick-lime  has  been  em- 
ployed,^ by  excavating  a  small  hole  in  the  dry  powder  and  throwing  in 
a  few  grains  of  gunpowder,  inflammation  will  ensue  in  both  cases. 
That  much  heat  is  evolved,  may  be  shown  also  by  covering  the  moist- 
ened quick-lime  with  a  not  too  tall  inverted  beaker-glass  or  bottle,  and 
observing  that  after  a  considerable  amount  of  aqueous  vapor  has  been 
condensed  upon  the  walls  of  the  glass,  the  space  within  the  latter  will 
at  last  become  filled  with  a  hot,  invisible  atmosphere  of  steam ;  when 
the  bottle  is  lifted,  and  the  steam  thus  brought  in  contact  with  the  cold 
external  air,  a  dense  cloud  or  fog  is  immediately  formed. 

So  much  heat  is  developed  during  the  union  of  water  with  lime,  that 
wood  will  quickly  be  brought  to  the  kindling  temperature  and  inflamed, 
if  it  happen  to  be  in  contact  with  large  masses  of  these  substances  re- 
acting upon  one  another.  Fires  are  very  frequently  occasioned  by  the 
access  of  water  to  ships  or  warehouses  in  which  quick-lime  is  stored. 
It  has  been  noticed,  when  large  quantities  of  quick-lime  are  slaked  in 
a  dark  place,  that  light  as  well  as  heat  is  evolved  from  the  lime.  Even 
when  quick-lime  is  brought  in  contact  with  ice,  so  much  heat  is  evolved 
that  the  mixture  sometimes  becomes  hot  enough  to  boil  water. 

552.  When   hydrate  of  calcium   is  stirred  into  water,  there 
is  formed  not  only  a  true  solution,  lime-water,  which  may  be 
obtained  clear  and  colorless  by  filtration  (See  Exp.  172),  but  also 
a  turbid  liquor  consisting  of  particles  of  solid  hydrate  of  calcium 
diffused  through  the  lime-water  ;  this  liquor  is  known  as  milk  or 
cream  of  lime,  according  to  its  consistency.    In  slaking  lime,  only 
about  half  a  part  of  water  is  really  needed  to  convert  one  part 
of  quick-lime  into  hydrate  of  calcium ;  but  in  all  cases  where  a  fine, 


470  SLAKED-LIME. 

smooth  paste  is  desired,  as  in  the  preparation  of  milk  of  lime, 
or  of  mortar,  and  in  general  whenever  hydrate  of  calcium  is 
required  in  a  very  finely  divided  condition,  it  is  best  to  pour  two 
or  three  parts  by  weight  of  water  upon  one  part  of  quick-lime, 
so  that  the  slaking  may  be  quickly  effected.  By  using  hot  water 
the  process  may  be  still  further  accelerated.  The  proportions 
of  material  given  at  the  beginning  of  the  experiment  are  better 
adapted  than  these  last  for  illustrating  the  evolution  of  heat ;  but 
if  too  little  water  be  employed,  the  hydrate  of  calcium  formed  is 
liable  to  be  granular,  and  crystalline  rather  than  powdery.  Both 
milk  of  lime  and  dry  powdery  hydrate  of  calcium  are  largely 
employed  for  purifying  the  illuminating  gas  made  from  coal. 
They  remove  from  the  gas  sulphydric  and  carbonic  acids. 

Exp.  285.  —  Provide  two  gas-bottles,  one  arranged  for  generating 
sulphydric  acid  (Exp.  90),  the  other  for  generating  carbonic  acid 
(Exp.  1 75).  Connect,  with  one  of  the  gas-bottles,  a  tube  filled  loosely 
with  dry  hydrate  of  calcium  (Appendix,  fig.  15),  and,  with  the  other, 
a  small  bottle  containing  milk  of  lime.  Pour  chlorhydric  acid  into  the 
gas-bottles,  so  that  sulphydric  and  carbonic  acids  shall  be  freely  evolved, 
and  test,  from  time  to  time,  with  lead-paper  (Exp.  94),  and  with  lime- 
water  (Exp.  174),  as  to  whether  these  acids  are  completely  absorbed 
by  the  dry  hydrate  of  calcium  and  the  milk  of  lime.  After  a  while, 
change  the  places  of  the  absorbing  tube  and  bottle,  so  that  the  milk  of 
lime  shall  now  be  where  the  dry  hydrate  was  before,  and  again  test 
the  efficiency  of  the  absorption,  with  lead-paper  and  lime-water.  In 
actual  practice  it  is  found  that,  while  the  dry  hydrate  is  a  more  efficient 
absorbent  of  carbonic  acid  than  milk  of  lime,  the  latter  is  capable  of 
taking  up  far  more  sulphydric  acid  than  the  former. 

553.  Hydrate  of  calcium  may  be  obtained  crystallized,  in 
hexagonal  prisms,  by  evaporating  lime-water  in  the  dry  ex- 
hausted receiver  of  an  air-pump.  At  a  red  heat  it  gives  off  its 
water,  and  is  reconverted  into  quick-lime.  The  residue  in  this 
case  is  left  in  an  open,  porous  condition  which  well  fits  it  for 
many  chemical  purposes  (§  120). 

It  is  noteworthy  that  hydrate  of  calcium  is  somewhat  less 
soluble  in  hot  than  in  cold  water.  If  a  cold,  saturated  solution 
of  lime-water  be  boiled,  nearly  half  of  its  solid  contents  will  be 
deposited,  and  in  case  none  of  the  water  has  been  driven 
off,  the  matter  thus  precipitated  will  slowly  dissolve  again  after 


MORTAR.  471 

the  liquid  above  it  has  become  cold.  In  studying  this  point,  the 
experimenter  must  take  care  that  the  solution  is  kept  out  of  con- 
tact with  the  air,  lest  it  absorb  some  of  the  carbonic  acid  which 
is  always  present  in  the  atmosphere,  and  become  turbid  from 
deposition  of  carbonate  of  calcium.  A  familiar  instance  of  this 
absorption  is  seen  in  cases  where  milk  of  lime  is  employed  for 
whitewashing:  the  loosely  adherent  white  coating,  left  after  the 
liquid  has  become  thoroughly  dry,  is  no  longer  hydrate  of  cal- 
cium, but  carbonate  of  calcium  in  a  more  or  less  pure  condition. 

554.  Slaked  lime  is  very  largely  employed  for  making  mor- 
tar, as  an  ingredient  of  various  cements,  and  for  plastering. 
When  mixed  with  enough  water  to  form  a  thick  paste,  it  is  de- 
cidedly plastic,  and  admits  of  being  spread  and  moulded  like  wax 
or  clay.  This  paste  sets,  as  it  dries,  to  a  firm,  solid  mass,  which, 
when  in  thin  layers,  adheres  firmly  to  any  rough  surfaces  upon 
which  it  may  have  hardened.  When,  however,  any  considerable 
mass  of  the  moist  paste  is  allowed  to  solidify  by  itself,  the  dry 
product  will  gradually  crack  and  fall  to  pieces.  Lime-paste  can- 
not, therefore,  be  employed  as  a  mortar  unless  it  be  mixed  with 
some  substance  like  sand,  which  shall  present  numerous  surfaces 
upon  which  the  hardened  product  may  adhere ;  by  the  addition 
of  sand,  moreover,  the  moist  lime  is  prevented  from  shrinking  too 
much  as  it  becomes  dry. 

Mortar  is  commonly  prepared  by  mixing  1  part  of  quick-lime 
with  water  enough  to  form  a  thin  paste,  then  adding  3  or  4  parts 
of  coarse,  sharp  sand,  and  thoroughly  incorporating  these  ingre- 
dients. The  paste  thus  obtained  is  applied  as  a  thin  layer  to  the 
moistened  surfaces  of  the  bricks  or  stones  to  be  united.  The 
pasty  mortar  soon  sets  to  the  hard  mass  above  described,  and,  on 
continued  exposure  to  the  air,  it  slowly  absorbs  carbonic  acid  at 
its  surface,  and  is  there  converted  into  a  compact  compound  of 
hydrate  and  carbonate  of  calcium.  The  stone-like  mass,  thus  ob- 
tained, binds  firmly  together  the  bricks  or  stones  between  which 
it  has  been  interposed.  It  has  been  asserted  that  the  original 
mortar-paste  sets  more  firmly  if  it  contain  a  certain  admixture  of 
carbonate  of  calcium,  than  if  it  contain  only  the  pure  hydrate  ; 
this  admixture  is,  of  course,  produced  when  mortar  is  left  for 
some  time  in  contact  with  the  air  before  being  used.  In  the 


472  CAUSTIC    LIME. 

course  of  time  chemical  combination  occurs,  to  a  limited  extent, 
between  the  silicic  acid  of  the  sand  and  the  oxide  of  calcium  in 
the  hardened  mortar,  though  the  process  goes  on  but  slowly ; 
each  grain  of*  sand  finally  becomes  covered  with  a  thin  layer  of 
hydrated  silicate  of  calcium,  which  contributes  materially  to  the 
solidity  of  the  mortar.  The  mortar  taken  from  old  buildings  yields 
a  certain  proportion  of  gelatinous  silica,  on  being  treated  with 
chlorhydric  acid  (§  466). 

The  conversion  of  the  original  mortar  into  hydro-carbonate  and 
silicate  of  calcium  is  never  completely  accomplished  ;  in  the  cen- 
tral portions  of  the  mass,  free  hydrate  of  calcium  will  still  be 
found  after  the  lapse  of  many  centuries.  Samples  of  mortar,  re- 
cently taken  from  the  Great  Pyramid,  were  found  on  analysis 
to  contain  a  large  proportion  of  the  free  hydrate. 

555.  The  plastering  used  for  finishing  the  walls  and  ceilings  of 
rooms  is  mortar,  to  which  a  quantity  of  hair  has  been  added  to 
increase  its  tenacity ;  in  drying,  it  is,  of  course,  subject  to  the 
same  chemical  changes  as  ordinary  mortar.     By  absorbing  car- 
bonic acid  from  the  air,  it  is  gradually  converted,  in  part,  into 
carbonate  of  calcium,  while  water  is  set  free  :  — 

CaO,  H20  +  C02  =  CaO,  CO2  +  H2O. 

Consequently,  the  walls  of  recently  plastered  rooms  cannot  be- 
come permanently  dry,  until  enough  carbonic  acid  has  been  ab- 
sorbed to  expel  the  chemically  combined  water  from  their  outside 
surfaces  ;  hence,  the  dampness  so  often  perceived  in  new  houses, 
when  carbonic  acid  first  comes  to  be  freely  generated  in  them  by 
respiration  and  by  burning  lamps.  In  order  to  dry  plastering,  it 
would,  doubtless,  be  better  to  employ  open  fires  of  charcoal,  or  of 
coke,  and  to  deliver  the  products  of  the  combustion  directly  into 
the  room  which  is  to  be  dried,  instead  of  relying  solely  upon  hot 
air,  as  is  now  usual. 

556.  Hydrate  of  calcium,  like  the  hydrates  of  sodium  and  of 
potassium,  exhibits  a  strong  alkaline  reaction  when  tested  with 
moistened  litmus-paper,  and  exerts  a  corrosive  action  upon  most 
organic  substances  ;  hence,  it  is  often  called  caustic  lime. 

Exp.  286.  —  Add  a  few  drops  of  water  to  a  small  quantity  of  dry 
hydrate  of  calcium,  and  rub  it  to  a  paste  between  the  fingers.  It  will 


SULPHATE    OF    CALCIUM.  473 

be  felt  that  the  alkali  acts  upon  the  skin ;  a  little  of  the  cuticle  is  really 
dissolved. 

Exp.  28  7.  —  Wrap  a  handful  of  dry  hydrate  of  calcium  in  a  paper, 
— or,  better,  in  a  piece  of  linen  or  cotton  cloth,  —  and  set  the  packet 
aside  for  a  week  or  two.  After  a  while,  the  cloth  or  paper  will  become 
rotten  and  friable  :  the  caustic;  lime,  as  the  common  phrase  is,  has  eaten 
away  their  more  corruptible  portions,  and  has  so  destroyed  the  integ- 
rity of  the  whole.  As  a  preliminary  operation  in  tanning  leather,  hides 
are  soaked  in  milk  of  lime  to  loosen  the  hair,  so  that  it  may  be  readily 
scraped  off.  The  value  of  lime,  as  an  ingredient  of  composts  to  be 
used  as  manure,  appears  to  depend,  in  great  measure,  upon  its  power 
of  hastening  the  decay  and  disintegration  of  organic  matter. 

Lime  has  been  found  to  be  specially  valuable  as  manure  when  ap- 
plied to  soils  rich  in  vegetable  matter.  The  organic  matters  are  decom- 
posed or  oxidized  into  carbonic  and  various  other  organic  acids,  which 
unite  with  the  lime ;  sometimes,  under  special  conditions,  more  or  less 
nitrate  of  calcium  is  found  among  the  products. 

Lime  is  important,  also,  from  being  not  only  the  cheapest  al- 
kali, but  the  cheapest  of  all  the  bases.  Since  its  compounds  with 
carbonic  and  sulphuric  acids  are  nearly  insoluble  in  water,  it  is 
largely  employed  for  removing  these  acids  from  solutions  in  which 
their  presence  is  not  desired ;  it  may  itself  be  removed  from  any 
solution  by  means  of  the  acids  in  question.  It  is  used  in  the 
manufacture  of  the  caustic  alkalies,  soda  and  potash  ;  of  am- 
monia-water, and  of  bleaching-powders  ;  as  a  flux  in  many  metal- 
lurgical operations:  in  the  refining  of  sugar;  for  preparing  a 
lime-soap  in  the  manufacture  of  stearine-candles,  and  for  number- 
less other  purposes.  A  noteworthy  property  of  slaked-lime  is  its 
power  of  dissolving  freely  in  solutions  of  common  sugar. 

557.  Sulphate  of  Calcium  (CaS04)  is  found  native  in  large 
quantities,  as  the  minerals  gypsum  and  alabaster.  These  miner- 
als contain  one-fifth  their  weight  of  water  ;  their  composition  may 
be  represented  by  the  formula  CaSO4  -f-  2H20.  The  same  1  y- 
drated  salt  may  be  obtained  by  adding  sulphuric  acid,  or  the 
solution  of  some  sulphate,  to  a  strong  aqueous  solution  of  almost 
any  of  the  salts  of  calcium.  This  hydrated  compound  is  the  sub- 
stance commonly  meant  when  sulphate  of  calcium  is  spoken  of. 
The  anhydrous  compound  is  also  important :  it  is  sometimes 
found  in  nature  as  the  mineral  anhydrite,  and  may  be  readily  pre- 


474  PLASTER    OF    PARIS. 

pared  bj  heating  the  hydrated  salt.  There  is  still  a  third  com- 
pound, the  composition  of  which  may  be  represented  by  the 
formula  2CaSO4  -f-  H2O,  of  which,  however,  but  little  is  known. 

Exp.  288. —  Place  in  a  porcelain  evaporating-dish,  —  or,  better,  in  an 
iron  pan,  —  two  table-spoonfuls  of  powdered  gypsum ;  heat  the  gyp- 
sum moderately  over  the  flame  of  the  gas-lamp,  and  observe  the  move- 
ment of  ebullition  occasioned  by  the  escaping  water ;  stir  the  mixture 
as  long  as  the  vapor  of  water  is  seen  to  escape,  and  then  set  the  residue 
aside  to  cool.  The  dry  product  is  known  as  calcined  gypsum,  or  plaster 
of  Paris. 

As  much  as  nine-tenths  of  the  water  which  the  gypsum  contains  may 
be  readily  expelled  at  temperatures  between  100°  and  120°;  but,  in 
order  to  drive  off  the  last  portions  of  the  water,  a  temperature  of 
nearly  300°  is  required.  If  the  dry  compound  be  heated  to  tempera- 
tures much  higher  than  300°,  its  particles  appear  to  become  aggluti- 
nated, and  the  chemical  properties  of  the  substance  are  somewhat 
changed  ;  the  gypsum  is  then  said  to  be  over-burned.  At  the  tempera- 
ture of  redness,  sulphate  of  calcium  melts  without  decomposition,  and, 
on  cooling,  assumes  a  crystalline  structure  similar  to  that  of  native  an- 
hydrite. 

558.  When  powdered  sulphate  of  calcium,  which  has  been 
made  anhydrous  at  a  comparatively  low  temperature,  is  made 
into  a  paste  with  water,  and  then  left  to  itself,  it  soon  sets  or 
hardens  into  a  compact,  coherent  mass.  This  solidification  is  a 
consequence  of  the  reassumption  by  the  sulphate  of  calcium  of 
the  two  molecules  of  water  of  crystallization  which  were  driven 
off  by  heat  when  the  substance  was  made  anhydrous. 

Exp.  289.  —  Place  a  small  coin  at  the  bottom  of  a  cylindrical  paste- 
board pill-box,  a  little  wider  than  the  coin  ;  smear  the  coin  and  the  in- 
terior of  the  box  with  a  thin  film  of  oil.  Mix  intimately  two  or  three 
teaspoonfuls  of  the  calcined  gypsum,  of  Exp.  288,  with  about  half 
their  volume  of  water,  in  a  small  porcelain  dish,  and  quickly  pour  the 
mixture  into  the  box,  so  that  the  coin  shall  be  completely  covered  by 
it.  The  mixture,  Avhich  is  of  the  consistence  of  cream,  should  then  be 
immediately  stirred  or  puddled  with  a  hair-pencil,  or  with  a  tuft  of  cot- 
ton tied  upon  a  stick,  or  with  the  end  of  the  finger,  so  that  the  bubbles 
of  air  which  remain  adhering  to  the  surface  of  the  coin  may  be  pressed 
out,  and  the  moist  paste  made  to  come  everywhere  into  contact  with 
the  metal.  In  the  course  of  a  few  minutes  the  paste  will  solidify  and 
become -so  hard  that  the  pasteboard  envelope  may  be  torn  away  from 


PLASTER-CASTS.  475 

it,  and  the  coin  removed.  A  perfect  cast  or  copy  of  the  stamp  upon 
the  coin  will  be  found  impressed  upon  the  hardened  gypsum.  The 
impression  in  this  first  cast  is,  of  course,  reversed,  but  by  smearing  it 
with  oil  and  then  pouring  over  it  a  new  portion  of  the  gypsum-paste, 
precisely  as  was  done  with  the  coin,  a  fac-simile  of  the  original  coin 
may  be  obtained. 

Plaster  of  Paris  is  largely  used  in  this  way  for  taking  accurate  copies 
of  a  great  variety  of  objects.  Thus,  in  the  process  known  as  stereotyp- 
ing, a  thin  paste  of  plaster  is  poured  upon  the  surface  of  the  printer's 
types,  after  they  have  been  set  up  and  made  ready  for  printing ;  the 
mould  thus  formed  is  dried  and  baked  to  expel  the  water  from  the 
gypsum,  and  is  then  plunged  into  a  bath  of  a  melted  alloy  of  lead,  anti- 
mony, and  tin,  known  as  stereotype  metal,  in  such  manner  that,  on 
withdrawing  the  mould  and  allowing  the  metal  within  it  to  cool,  there 
is  obtained  a  fac-simile  of  the  original  types.  From  this  durable  metal- 
lic casting  the  page  is  finally  printed. 

As  has  been  said  above,  the  moist  paste  sets  as  soon  as  the  water, 
which  has  been  mechanically  mixed  with  the  anhydrous  sulphide  of 
calcium,  enters  into  chemical  combination  with  it.  As  in  all  other  in- 
stances of  chemical  action,  so  here,  heat  is  evolved  as  the  water  and 
plaster  combine,  as  may  readily  be  appreciated  by  operating  upon  con- 
siderable quantities  of  the  materials.  Since  the  plaster  assumes  crys- 
talline form  as  it  becomes  hydrated,  the  paste  increases  in  bulk  as  it 
hardens,  and  is  thus  pressed  into  the  finest  interstices  of  the  moulds. 

Gypsum  sets  the  more  quickly  in  proportion  as  the  temperature  at 
which  it  has  been  dehydrated  is  low.  After  it  has  been  heated  above 
300°,  it  will  no  longer  set  on  being  mixed  with  water.  Besides  its  use 
in  taking  casts,  plaster  of  Paris,  on  account  of  this  power  of  combining 
with  water,  is  largely  employed  in  the  preparation  of  stucco  and  of 
various  imitations  of  marble.  The  hydrated  compound  finds  applica- 
tion also  as  a  manure,  in  the  manufacture  of  ammoniacal-salts,  and  for 
various  other  purposes. 

Exp.  290.  —  That  the  plaster-paste  expands  considerably  at  the  mo- 
ment of  solidification  may  be  shown  as  follows :  —  Procure  a  cracked 
test-tube,  or  small  flask,  and  fill  it  completely  with  a  paste  made  of  cal- 
cined gypsum  and  water,  in  the  proportions  of  12  pts.,  by  weight,  of  the 
former,  to  5  of  the  latter.  In  the  course  of  15  or  20  minutes  it  will  be 
seen  that  the  original  crack  in  the  glass-vessel  has  extended  in  various 
directions,  in  consequence  of  the  expansion  of  the  mass  within  it.  It 
will  be  noticed,  also,  that  the  vessel  feels  warm  to  the  hand  (compare 
Exp.  289).  Finally,  by  breaking  away  the  glass-envelope,  there  may 
be  obtained  a  cast  of  the  glass  vessel. 


476  BOILER-SCALE. 

i 

Exp.  291.  — The  power  of  sulphate  of  calcium  to  take  up  water,  — 
to  solidify  water  as  it  unites  with  it  to  form  the  crystalline  compound 
CaH4SO6,  —  can  be  made  manifest  as  follows: — Prepare  two  table- 
spoonfuls  of  a  saturated  aqueous  solution  of  chloride  of  calcium,  by  dis- 
solving 1  pt,  by  weight,  of  the  dry  chloride  in  1.5  parts  of  water ;  also, 
prepare  the  same  quantity  of  a  saturated  solution  of  sulphate  of  sodium 
(Exp.  233),  and,  finally,  mix  the  two  solutions.  Sulphate  of  calcium 
will  be  formed,  in  accordance  with  the  reaction,  — 

CaCl2  -f  Na.2SO4  =  CaSO4  -f  2NaCl, 

and  will  unite  with  the  water  in  which  the  ingredients  from  which  it 
has  been  formed  were  previously  held  in  solution,  so  that  an  almost 
solid  mass  of  CaSO4,  2H2O  will  take  the  place  of  the  two  liquids. 

Ordinary  hydrated  sulphate  of  calcium  is  soluble  in  about  400 
parts  of  water,  at  the  ordinary  temperature  of  the  air  ;  but,  like 
hydrate  of  calcium,  sulphate  of  sodium,  and  a  few  other  salts,  it 
is  less  soluble  in  hot  water  than  in  cold.  When  an  aqueous  so- 
lution of  sulphate  of  calcium  is  heated  to  100°  or  more,  a  precip- 
itate will  soon  be  formed  in  it,  even  if  the  solution  be  very  dilute ; 
and  at  temperatures  as  high  as  140°  or  150°  the  anhydrous  com- 
pound is  completely  insoluble  in  water.  In  the  same  way  as  with 
sulphate  of  sodium  (Exp.  233),  it  appears  that  the  bihydrated 
sulphate  of  calcium  cannot  exist  at  temperatures  much  superior  to 
100°,  and  that  above  that  temperature  we  have  to  deal  with  other 
compounds  of  different  solubility.  In  other  words,  the  water 
which  is  held  in  chemical  combination  in  ordinary,  unburned 
gypsum  may  be  expelled  by  heat  even  when  the  gypsum  is 
dissolved  in  water.  Whenever  water,  containing  sulphate  of 
calcium  in  solution,  is  strongly  heated,  as  in  steam-boilers, 
there  is  precipitated  the  half-hydrated  compound,  of  composition 
2CaSO4  -|-  H2O,  which  has  been  mentioned  above.  Hence  the 
formation  of  incrustations,  or  scale,  of  sulphate  of  calcium  upon 
the  walls  of  boilers  fed  with  sea-water,  or  with  other  water  con- 
taining the  sulphate.  It  should  be  remarked  that  the  incrustation 
in  this  case  does  not  depend  upon  evaporation ;  the  sulphate  of 
calcium  will  be  deposited  the  more  rapidly  in  proportion  as  the 
water  of  the  boiler  is  hot,  and  as  more  of  the  impure  feed-water 
is  pumped  into  the  boiler. 

559.  Besides  occurring  in  sea-water,  sulphate  of  calcium  is  a 


TESTING    WATER.  477 

very  common  impurity  in  spring-water.  Water  which  contains 
much  of  it  is  "  hard,"  and  is  not  well  adapted  either  for  washing 
or  for  cooking. 

Exp.  292.  —  Dissolve  a  small  bit  of  soap  in  hot  water,  and  add  to  the 
solution  an  equal  bulk  of  a  solution  of  sulphate  of  calcium.  The  mix- 
ture immediately  becomes  turbid,  and  after  a  few  moments  there  will 
be  formed  a  greasy,  flocculent,  adhesive  scum  upon  the  surface  of  the 
liquor.  This  precipitate  is  a  lime-soap  formed  by  the  union  of  the  fatty 
ingredients  of  the  soap  and  the  base  of  the  sulphate  of  calcium.  Com- 
mon soap  is  a  compound  of  one  or  more  organic  acids,  known  as  fatty 
acids,  with  caustic  soda.  This  soda-soap  is  soluble  in  water,  but  lime- 
soap  is  insoluble ;  hence,  when  a  soluble  salt  of  calcium  is  added  to  a 
solution  of  soap,  precipitation  occurs.  When  soap  is  added  to  hard 
water,  it  will  neither  produce  any  permanent  froth  nor  cleansing  effect, 
until  the  sulphate,  or  other  lime-salt  present,  has  all  been  decomposed  ; 
with  such  waters,  much  soap  is  consumed  in  removing  the  calcium  com- 
pound, before  the  proper  detergent  action  of  the  soap  can  be  brought 
into  play. 

560.  An  excellent  process  for  determining  the  relative  hard- 
ness of  several  samples  of  water  has  been  founded  upon  the 
behavior  of  water  towards  soap,  as  set  forth  in  the  foregoing 
experiment:  — 

Exp.  293.  —  Prepare  a  sample  of  water,  of  standard  hardness,  as  fol- 
lows :  —  Dissolve  0.5  grm.  of  white  marble,  or  other  pure  carbonate  of 
calcium,  in  dilute  chlorhydric  acid,  evaporate  to  dryness,  in  order  to 
expel  the  excess  of  acid,  and  dissolve  the  pure  chloride  of  calcium  ob- 
tained in  2  litres  of  water.  Next  prepare  a  solution  of  soap  by  digest- 
ing 7  grms.  of  Castile  soap,  —  or,  better,  white  curd  soap, —  in  1120 
grms.  of  a  mixture  of  3  parts  of  alcohol,  of  0.83  specific  gravity,  and  1 
of  pure  water,  until  no  more  soap  dissolves ;  filter  the  solution,  and 
preserve  it  in  a  tight  bottle.  Measure  off'  100  c.  c.  of  the  water,  of 
standard  hardness,  place  it  in  a  bottle  of  200  or  250  c.  c.  capacity,  and 
by  means  of  a  graduated  burette  (Appendix,  §  21),  or  pipette,  add  to 
it  the  solution  of  soap  by  portions  of  1  c.  c.  each.  After  the  addition 
of  each  c.  c.  of  the  soap  solution,  replace  the  stopper  in  the  bottle,  and 
shake  the  latter  violently,  then  place  the  bottle  upon  its  side,  and 
observe  whether  the  bubbles,  which  form  upon  the  surface  of  the 
liquid,  quickly  disappear.  So  long  as  the  bubbles  disappear  immedi- 
ately, new  portions  of  the  soap-liquor  must  be  added  ;  but  as  soon  as 
a  permanent  froth  is  formed,  the  operation  is  finished.  It  is  customary 
to  consider  the  operation  completed  when  the  bubbles  persist  during 


478  PHOSPHATES    OF    CALCIUM. 

three  minutes.  The  number  of  c.  c.  of  soap-liquor,  which  has  been  em- 
ployed in  producing  this  result,  is  then  carefully  recorded. 

Samples  of  well  and  river  water  may  readily  be  compared  with  the 
water  of  known,  standard  hardness.  We  have  only  to  measure  off  100 
c.  c.  of  the  well-water,  place  it  in  the  small  bottle,  as  above,  and  add  to 
it  the  soap-liquor,  whose  value  has  been  determined,  until  a  persistent 
froth  is  produced.  If  it  be  assumed  that  the  standard  chloride  of  cal- 
cium water  represent  100°  of  hardness,  the  comparative  hardness  of 
any  other  sample  of  water  will  follow  from  the  proportion :  —  As  the 
quantity  of  soap-liquor  required  to  produce  persistent  bubbles  in  the 
standard  water  is  to  100,  so  is  the  quantity  of  soap-liquor  which  produces 
bubbles  in  any  given  sample  of  water  to  the  relative  hardness  of  the 
sample. 

When  the  water  under  examination  has  a  much  higher  degree  of 
hardness  than  100°,  it  is  necessary  to  dilute  it  with  from  1  to  5  times  its 
volume  of  distilled  water  before  adding  the  soap-liquor ;  for  the  curdy 
precipitate,  which  would  form,  if  soap  were  added  to  the  undiluted 
liquid,  would  interfere  with  the  formation  of  froth,  and  so  make  it  diffi- 
cult to  determine  when  a  sufficient  quantity  of  the  soap-liquor  had  been 
used. 

On  being  ignited  in  an  atmosphere  of  hydrogen,  or  in  contact 
with  substances  containing  carbon,  gypsum  may  readily  be  deox- 
idized and  converted  into  sulphide  of  calcium  :  — 

CaSO4  +  4C  =  CaS  +  4CO. 

This  reduction  is  readily  effected,  also,  when  aqueous  solutions  of 
gypsum  are  left  in  contact  with  decaying  vegetable  matter  ;  since, 
in  this  case,  carbonic  acid  will  necessarily  come  in  contact  with 
the  sulphide  of  calcium  as  soon  as  it  is  formed,  sulphuretted 
hydrogen  gas  will  be  set  free,  as  may  be  perceived  wherever  the 
mud  of  docks  and  marshes  is  wet  with  sea-water :  — 

CaS  -f  H2O  +  CO2  =  CaCO3  +  H2S. 

561.  Phosphates  of  Calcium.  There  are  several  of  these  phos- 
phates, comparable  respectively  with  the  various  phosphates  of 
sodium  (§  489)  ;  the  most  remarkable  among  them  is  the  triphos- 
phate  (3CaO,  P2O5),  commonly  called  bone-phosphate,  from  being 
found  in  bones.  It  is  the  chief  of  the  inorganic  constituents,  of 
which  the  skeletons  of  animals  are  composed.  Small  portions  of 
it  are  found  in  most  rocks  and  soils  (§  262),  it  being  a  very  widely 


CHLORIDE    OF    CALCIUM.  479 

diffused,  though  nowhere  a  very  abundant,  substance.  Consider- 
able masses  of  it  have  been  found,  however,  in  Spain,  New  Jer- 
sey, and  Canada,  and  it  is  the  principal  ingredient  of  some  kinds 
of  guano.  No  matter  whence  obtained,  it  is  a  valuable  manure 
when  reduced  to  a  fine  powder.  Though  as  good  as  insoluble  in 
water,  it  dissolves  readily  in  acids  and  in  solutions  of  various  or- 
ganic substances. 

562.  Chloride  of  Calcium  (CaCl2)  may  be  prepared  by  dis- 
solving chalk  or  marble  in  chlorhydric  acid  (as  in  Exp.  175), 
and  evaporating  the  solution  to  dryness.     It  is  produced  in  large 
quantities  in   the  arts  by  heating  chloride  of  ammonium  with 
slaked  lime  in  the  preparation  of  ammonia-water  (Exp.  51)  :  — 

2NH4C1  +  CaH2O2  =  CaCl2  +  2NH3  +  2H20. 

When  dried  at  about  200°,  chloride  of  calcium  is  left  as  a  porous 
mass,  which  is  largely  employed  in  chemical  laboratories  for  dry- 
ing gases  (Appendix,  §  15).  It  absorbs  water  with  great  avidity, 
and  is  one  of  the  most  deliquescent  substances  known.  When 
exposed  to  air  at  the  ordinary  temperature,  it  soon  absorbs  so 
much  water  that  it  dissolves  completely.  At  a  low  red-heat  the 
anhydrous  chloride  melts  to  a  clear  liquid ;  if  ignited  for  any 
length  of  time  in  contact  with  the  air,  it  suffers  decomposition  to 
a  slight  extent,  a  little  oxide  and  carbonate -of  calcium  being 
formed.  From  highly-concentrated  aqueous  solutions  there  may 
be  obtained  crystals  of  the  hydrated  compound  CaCl2  +  6H20. 

Slaked  lime  may  be  dissolved  in  considerable  quantity  in  a 
boiling,  aqueous  solution  of  chloride  of  calcium,  and  the  filtered 
solution  deposits,  on  cooling,  long,  thin  crystals  of  a  compound 
known  as  oxychloride  of  calcium  (CaCl2,  3CaO-f-  16H2O),  which 
is  immediately  decomposed  when  treated  with  pure  water. 

563.  Hypochlorite  of  Calcium  (CaCl20.,),  as  has  been  shown 
in  §  120,  is   a   component  of  the    substance   commonly  called 
**  chloride  of  lime."     This  important  bleaching  agent  is  prepared 
by  passing   chlorine   gas   into   chambers   filled   with   layers  of 
finely-powdered  slacked  lime,  in  accordance  with  the  reaction  al- 
ready set  forth.     Chloride  of  lime,  or  bleaching-powder,  is  a  dry, 
white  powder,  smelling  feebly  of  hypochlorous  acid  ;  it  always 
contains  a  certain  excess  of  hydrate  of  calcium  which  has  been 


480  HYPOCHLORITE    OF    CALCIUM. 

unacted  upon  by  chlorine  ;  it  is,  therefore,  only  partially  soluble 
in  water.  When  exposed  to  the  air,  it  slowly  absorbs  carbonic 
acid,  and,  at  the  same  time,  evolves  chlorine  ;  hence  its  employ- 
ment as  a  disinfecting  agent.  If,  instead  of  being  left  to  be 
slowly  acted  upon  by  the  carbonic  acid  of  the  air,  it  be  treated 
with  a  dilute  acid,  —  such  as  vinegar,  —  a  copious  evolution  of 
chlorine  will  immediately  occur. 

Exp.  294. —  Place  half  a  teaspoonful  of  bleach! ng-powder  in  a  test- 
glass,  cover  the  powder  with  water,  and  stir  in  enough  of  a  solution  of 
blue  litmus  to  distinctly  color  the  mixture.  By  means  of  a  glass  tube, 
blow  into  the  mixture  air  expired  from  the  lungs,  and  observe  that  the 
blue  color  of  the  litmus  will  soon  be  destroyed.  The  carbonic  acid  from 
the  lungs  decomposes  the  hypochlorite  of  calcium,  and  the  chlorine  set 
free  destroys  the  color. 

Exp.  295. —  At  the  bottom  of  a  large,  tall  beaker,  or  other  wide- 
mouthed  glass-vessel,  of  the  capacity  of  two  or  three  litres,  place  a 
small  bottle  containing  15  or  20  grms.  of  bleachi ng-powder.  Cover  the 
beaker  with  a  glass-plate,  or  sheet  of  pasteboard,  provided  with  a  small 
hole  at  the  centre ;  through  this  hole  in  the  cover  pass  a  thistle-tube 
down  into  the  bottle  of  bleachi  ng-powder,  and  pour  upon  it  several 
small  successive  portions  of  sulphuric  acid  diluted  with  an  equal  volume 
of  water.  Chlorine  gas  will  immediately  be  set  free  from  the  bleach- 
ing-powder,  in  accordance  with  the  reaction, 

CaCl2,  CaCl2O2  +  2H2SO4  —  2CaSO4  +  2H2O  +  4C1, 
and,  falling  over  into  the  bottom  of  the  large  beaker,  will  gradually 
press  out  and  displace  the  arir  therein  contained,  so  that  after  a  short 
time  the  beaker  will  be  seen  to  be  completely  filled  with  the  green  gas. 
This  is  by  far  the  easiest  and  most  expeditious  method  of  preparing 
chlorine.  If  desirable,  the  bleaching-powder  may,  of  course,  be  placed 
in  a  flask,  together  with  the  acid  and  the  evolved  gas  collected  at  will, 
by  means  of  suitable  delivery-tubes ;  but  many  of  the  experiments  of 
Chapter  VIII.  may  be  performed  perfectly  well  in  the  jar  of  chlorine 
obtained  as  above.  The  heavy  gas  may  be  ladled  out  of  the  jar  with 
a  dipper  made  of  any  small  bottle,  and  poured  upon  a  solution  of  indigo 
to  show  its  bleaching  power. 

It  will  be  noticed,  in  the  above  reaction,  that  by  the  addition  of  an 
acid  all  the  chlorine  of  the  bleaching-powder  is  expelled.  The  point 
is  important  as  bearing  upon  the  practical  use  of  this  agent. 

Exp.  296. —  Soak  a  bit  of  printed  calico  in  a  half-litre  of  water,  into 
which  10  or  15  grms.  of  bleaching-powder  have  been  stirred.  Observe 
that  the  color  of  the  calico  slowly  undergoes  change  ;  then  transfer  the 


OXYGEN    FROM    BLEACHING    POWDER.  481 

cloth  to  another  bottle  filled  with  very  dilute  chlorhyclric  or  sul- 
phuric acid,  and  take  note  of  the  rapidity  with  which  the  color  is 
discharged.  If  need  be,  again  immerse  the  calico  in  the  bleaching 
bath,  and  afterwards  in  the  dilute  acid.  Finally,  wash  the  whitened 
cloth  thoroughly  in  water.. 

564.  When  heated,  bleaching  powder  gives  off  oxygen, 
while  chloride  of  calcium  is  left  as  a  residue.  The  reaction 
furnishes  a  cheap  and  convenient  method  of  obtaining  oxy- 
gen. Another  method  of  procuring  oxygen  from  the  hypo- 
chlorite  is  to  mix  a  solution  of  the  latter  with  black  oxide  of 
manganese,  red  oxide  of  mercury,  oxide  of  iron,  or  oxide  of 
copper,  or,  better,  with  hydrated  sesquioxide  of  iron,  hydrate 
of  copper,  of  nickel,  or  of  cobalt,  and  to  gently  warm  the  mix- 
ture. 

Exp.  297. —  Fill  an  ignition  tube  one-third  full  of  bleaching  pow- 
der, and  arrange  the  apparatus  so  that  the  gas  may  be  collected 
over  water.  Heat  the  tube,  and  observe  that  the  gas  is  expelled  at 
a  comparatively  low  temperature.  1  grin,  of  bleaching  powder 
yields  40  or  50  c.  c.  of  oxygen  gas. 

Exp.  298.  —  Take  as  much  bleaching  powder  as  was  employed  in 
Exp.  297,  dissolve  it  in  a  small  quantity  of  water,  filter  the  solution, 
and  place  it  in  a  small  flask  provided  with  a  delivery  tube.  Add  to 
the  contents  of  the  flask  two  or  three  drops  of  the  solution  of  a  co- 
balt salt,  connect  the  flask  with  an  inverted  bottle  of  water  upon 
the  water  pan,  by  means  of  the  delivery  tube,  then  heat  the  flask 
to  70°  or  80°,  and  observe  that  oxygen  is  freely  evolved. 

The  cobalt  solution  employed  amounts  to  the  same  thing  as  hy- 
drated oxide  of  cobalt,  since  the  latter  is  immediately  precipitated 
from  the  cobalt  salt  by  the  caustic  lime  in  the  bleaching  powder. 
The  action  of  the  oxide  of  cobalt,  or  other  metallic  oxide  in  this  ex- 
periment, appears  to  be  somewhat  analogous  to  that  of  the  higher 
o*xides  of  nitrogen  in  the  manufacture  of  sulphuric  acid  (§228). 
The  oxide  of  cobalt  probably  takes  oxygen  from  the  solution  of 
bleaching  powder,  and  combines  with  it  to  form  a  high,  unstable 
oxide  which  immediately  decomposes  again  with  evolution  of  oxy- 
gen. The  oxidation  and  deoxidation  of  the  cobalt  compound  thus 
goes  on  incessantly,  and  a  very  small  quantity  of  the  latter  is  suffi- 
cient to  decompose  any  desired  amount  of  bleaching  powder.  It 
is  important  that  the  solution  of  the  hypochlorite  should  be  filtered 
as  above  directed,  lest  a  quantity  of  it  be  lost  by  foaming  over  out 
of  the  flask.  31 


484  PEROXIDE    OF   BARIUM. 

Most  of  the  compounds  of  strontium  and  barium  are  closely 
analogous  to  the  corresponding  compounds  of  calcium.  The 
oxides,  peroxides,  hydrates,  carbonates,  sulphates,  nitrates, 
phosphates,  chlorides,  sulphides,  etc.,  resemble  in  the  main  the 
corresponding  calcium  salts.  The  hydrates  of  strontium  and 
barium  are,  however,  more  readily  soluble  in  water  than 
the  hydrate  of  calcium,  while  their  sulphates,  nitrates,  and 
chlorides  are  less  soluble  than  those  of  calcium.  Sulphate 
of  barium  is  almost  absolutely  insoluble  in  water,  and  sul- 
phate of  strontium  is  only  very  slightly  soluble.  Sulphate 
of  barium  is  found  native,  sometimes  in  considerable  masses, 
as  a  very  heavy  white  mineral  called  barytes,  which,  when 
powdered,  is  largely  employed  for  adulterating  white  lead. 
The  name  barium  comes  from  a  Greek  word  meaning  heavy. 

From  the  carbonates  of  strontium  and  barium  the  carbonic 
acid  cannot  readily  be  driven  off  by  heat  alone,  though  when 
mixed  with  charcoal,  and  then  ignited,  these  carbonates  may 
be  reduced  to  oxides.  A  better  way  of  preparing  the  oxides  is 
to  heat  the  nitrates  strongly  in  a  porcelain  crucible  or  retort. 
Unlike  hydrate  of  calcium,  hydrate  of  barium  does  not  give 
off  its  water  at  the  temperature  of  redness,  but  melts  without 
undergoing  decomposition.  From  hydrate  of  strontium  the 
water  may  be  expelled  by  heat,  though  with  difficulty.  Perox- 
ide of  barium  (BaO2)  is  of  interest,  since  by  means  of  it 
peroxide  of  hydrogen  (§  61)  and  antozone  (§  177)  may  be 
prepared. 

568.  In  order  to  obtain  peroxide  of  barium,  a  mixture  of  1 
part  of  oxide  of  barium,  and  4  parts  of  chlorate  of  potassium 
may  be  thrown,  little  by  little,  into  a  crucible  heated  to  low 
redness,  and  the  fused  mass  subsequently  washed  with  water  to 
remove  chloride  of  potassium  ;  or,  a  current  of  oxygen  gas,  or 
of  air,  may  be  made  to  flow  over  oxide  of  barium  heated  to  low 
redness  in  a  porcelain  tube.  As  thus  prepared  the  peroxide 
is  never  pure,  being  mixed  with  more  or  less  protoxide.  Perox- 
ide of  barium  decomposes,  with  evolution  of  oxygen,  at  the 
temperature  of  bright  redness,  and  in'  view  of  this  fact  it  was 
at  one  time  proposed  to  employ  the  substance  as  a  means  of 


STRONTIUM.  SALTS.  485 

obtaining  pure  oxygen  from  the  air  upon  the  large  scale.  A 
considerable  number  of  tubes  charged  with  protoxide  of 
barium,  having  been  suitably  arranged  in  furnaces,  half  of  the 
tubes  were  heated  to  dull  redness,  and  a  current  of  air  was 
made  to  flow  through  them,  until  the  protoxide  had  been  con- 
verted into  peroxide  ;  the  current  of  air  was  then  transferred 
to  the  other  tubes,  while  the  first  series  was  .put  in  connection 
with  a  gas-holder  and  heated  to  bright  redness,  until  the 
second  atom  of  oxygen  had  been  driven  out.  The  second 
series  of  tubes  were  next  deprived  of  oxygen,  while  the  tubes 
of  the  first  series  were  put  to  their  old  work  of  absorbing 
oxygen  from  the  air.  The  process  thus  became  a  continuous 
one,  and  was  really  capable  of  furnishing  large  quantities  of 
oxygen  ;  it  has,  however,  been  superseded  by  cheaper  methods 
(§'242). 

Strontium  salts  are  commonly  prepared  from  the  native 
carbonate,  a  mineral  called  strontianite,  while  the  various  salts 
of  barium  are  obtained  either  from  the  native  carbonate 
(witlierite) ,  or  more  commonly  from  the  sulphate ;  the  finely 
powdered  sulphate,  after  having  been  mixed  with  powdered 
charcoal  and  oil,  is  strongly  heated  in  a  covered  crucible,  and 
so  reduced  to  the  condition  of  sulphide  of  barium  :  — 

BaSO4  +  4C  =  BaS  +  4CO. 

Sulphide  of  barium  is  readily  soluble  in  water,  and  on 
being  treated  with  chlorhydric,  nitric,  or  ariy  other  acid,  it  de- 
composes, sulphuretted  hydrogen  is  given  off,  and  there  is 
formed  chloride  of  barium,  or  some  other  salt,  according  to 
the  acid  employed  :  — 

BaS  +  2HC1  =  BaCl2  +  H2S. 

Several  of  the  compounds  of  barium  are  useful  reagents  in 
the  chemical  laboratory.  Sulphate  of  barium  is  employed  as  a 
pigment  by  artists  in  water  colors,  under  the  name  permanent 
white;  also,  in  the  finishing  of  paper,  pasteboard,  etc.,  and  for 
adulterating  white  lead.  As  a  water  color  it  is  valuable,  since 
it  is  scarcely  at  all  acted  upon  by  any  chemical  agent,  but 
when  ground  with  oil  it  becomes  translucent,  and  seriously 


486  THE    CALCIUM   GROUP. 

impairs  the  opacity  or  covering  power  of  the  better  pigments 
with  which  it  is  mixed. 

Compounds  of  barium  and  of  strontium  are  employed  in 
the  preparation  of  fire-works,  for  obtaining  green  and  crim- 
son flames  respectively  :  — 

The  green  barium  flame  may  be  well  shown  by  mixing  with  the 
fingers  a  gramme  of  powdered  chlorate  of  barium  with  half  a  gramme 
of  flowers  of  sulphur,  and  strewing  the  mixture  upon  a  glowing 
coal.  The  green  fire  of  the  pyrotechnists  may  be  prepared  by  mix- 
ing together  58  parts  of  nitrate  of  barium,  13  parts  of  sulphur,  6 
parts  of  chlorate  of  potassium,  and  2  parts  of  charcoal. 

To  exhibit  the  red  strontium  flame,  a  mixture  may  be  prepared 
by  rubbing  together  in  a  mortar  30  parts  of  anhydrous  nitrate  of 
of  strontium,  10  parts  of  powdered  sulphur,  and  3  parts  of  sulphide 
of  antimony,  and  to  this  mixture  may  be  added,  with  the  hand, 
taking  care  to  avoid  all  violent  friction,  7  parts  of  powdered,  fused 
chlorate  of  potassium.  The  mixture  may  then  be  shaken  loosely 
upon  a  piece  of  sheet  iron  and  touched  with  a  lighted  stick  or  glow- 
ing coal. 

Or  the  color  may  be  shown  upon  a  smaller  scale  by  operating  as 
follows :  — 

Exp.  299. —  By  means  of  iron  wire,  suspend  three  small  bullets 
of  well  burned  coke  from  a  ring  of  the  iron  stand.  Heat  the 
fragments  in  turn  with  the  flame  of  the  gas  lamp,  and  observe  the 
slightly  yellowish  flame  which  will  be  produced  in  each  case ;  then 
moisten  one  of  the  pieces  of  coke  with  a  solution  of  chloride  of 
calcium,  the  second  with  a  solution  of  chloride  of  barium,  and  the 
third  with  a  solution  of  nitrate  of  strontium,  and  again  heat  them 
in  turn  with  the  gas  flame.  The  calcium  salt  will  impart  a  reddish 
yellow  color  to  the  flame,  the  barium  salt  a  green  color,  and  the 
strontium  salt,  a  beautiful  crimson.  Instead  of  the  bits  of  coke, 
platinum  wire  might  of  course  be  employed,  as  in  Exp.  206. 

569.  As  appears  abundantly  from  the  foregoing,  the  three 
elements,  calcium,  strontium,  and  barium,  are  intimately  re- 
lated one  with  the  other,  and  are,  as  a  family,  clearly  distin- 
guished in  several  important  particulars  from  the  metals  of 
the  preceding  group.  Even  before  the  metals  of  this  family 
were  discovered  and  isolated,  it  had  long  been  customary 
among  chemists  to  speak  of  the  oxides  of  calcium,  strontium, 


THE    ALKALINE    EARTHS.  487 

and  barium,  as  the  alkaline  earths,  in  contradistinction  from 
the  "  alkalies,"  potash,  and  soda,  upon  the  one  hand,  and  the 
"  earths,"  such  as  the  oxides  of  magnesium  and  aluminum, 
upon  the  other. 

Each  of  the  members  of  the  alkaline-earthy  group  now  in 
question  decomposes  water,  even  at  the  ordinary  temperature, 
taking  away  its  oxygen  ;  and  each  of  them  forms  two  oxides, 
a  neutral,  insoluble  binoxide,  belonging  to  the  class  of  ant- 
ozonides  (§  182),  and  a  more  or  less  soluble  protoxide,  acting 
as  a  powerful  base  ;  their  carbonates  and  sulphates  are  all 
difficultly  soluble,  and,  like  the  other  compounds  of  the  three 
metals,  are  isomorphous  with  one  another.  In  all  their  com- 
pounds, there  may  be  seen  the  same  progression  of  properties 
which  has  been  met  with  in  the  groups  previously  studied. 
The  barium  compound  will  always  be  found  at  one  end  of  the 
scale,  the  calcium  compound  at  the  other,  and  the  strontium 
compound  interposed  between  the  two.  The  hydrate  and  the 
carbonate  of  calcium  are  both  readily  destroyed  by  heat, 
while  the  corresponding  strontium  compounds  are  decomposed 
with  difficulty,  and  the  barium  compounds  only  at  exceedingly 
high  temperatures.  The  solubility  of  the  oxides  diminishes 
as  we  pass  from  baryta  to  lime,  while  that  of  the  sulphates 
and  carbonates  follows  the  inverse  order.  The  specific  grav- 
ities of  the  metals  are,  Ca=  1.6,  Sr=  2.6,  Ba  =  4 ;  and  their 
atomic  weights  are  40,  87.5,  and  137,  respectively,  that  of 
strontium  being  nearly  the  mean  of  the  other  two.  The 
specific  gravities  of  their  carbonates  and  sulphates  are  as 
follows:  — CaCO3  (arragonite)  =2.95,  SrCO3  =  3.6,  BaCO3= 
4.33;  CaSO4  =  2.33,  SrSO4=3.89,  and  BaSO4  =  4.4.  It 
should  be  observed,  that  in  all  these  cases,  the  specific  grav- 
ities of  the  strontium  compounds  approximate  closely  to  the 
mean  of  the  specific  gravities  of  the  corresponding  barium 
and  calcium  compounds.  The  same  remark  applies  also  to 
the  specific  gravities  of  the  three  metals. 

LEAD. 
570.   Almost  all  the  lead  which  is  employed  in  the  arts,  is 


488  METALLIC    LEAD. 

extracted  from  sulphide  of  lead,  PbS,  the  mineral  galena. 
This  substance  is  tolerably  abundant  in  many  localities,  and 
is  often  associated  with  sulphate  of  barium,  fluor-spar,  quartz, 
and  other  commojMtfinerals  ;  it  almost  always  contains  a  small 
proportion  lOflsulphide  of  silver.  In  order  to  obtain  metallic 
lead  from  galena,  this  mineral  is  mixed  with  a  small  quantity 
of  lime,  and  then  roasted  at  a  dull  red  heat  in  the  flame  of  a 
reverberatory  furnace.  A  portion  of  the  sulphur  burns  off  as 
sulphurous  acid.  Some  oxide  of  lead,  and  more  or  less  sul- 
phate of  lead  is  farmed,  while  much  of  the  ore  remains  unde- 
composed.  After  a  time, ~lhe  roasting  process  is  interrupted, 
air  is  excluded  from  the  furnace,  the  oxide,  sulphate,  and  sul- 
phide of  lead  are  thoroughly  mixed  together,  and  the  heat  of 
the  furnace  is  suddenly  raised.  The  undecomposed  sulphide 
of  lead  then  reacts  upon  the  oxide  and  sulphate,  sulphurous 
acid  is  given  off,  and  metallic  lead  produced.  The  reactions 
may  be  thus  formulated  :  — 

I.   2PbO-f-PbS  =  3Pb  +  SO2 
II.   PbSO4  +  PbS  =  2Pb  +  2SO2 

The  lime  is  added  for  the  purpose  of  forming  a  fusible  slag 
with  any  silicious  matter  which  may  be  present  in  the  ore. 

Lead  is  a  remarkably  soft  metal,  of  bluish-white  color ;  it 
can  be  readily  cut  with  a  knife,  and  may  even  be  indented  with 
the  finger-nail ;  it  soils  paper  upon  which  it  is  rubbed.  Its 
specific  gravity  is  11.4,  and  its  atomic  weight  207.  It  may  be 
drawn  into  wire,  and  beaten  into  sheets,  though,  as  contrasted 
with  most  of  the  other  metals,  it  has  but  little  tenacity.  In 
comparison  with  other  metals,  it  is  a  rather  poor  conductor 
of  heat  and  electricity.  It  melts  at  about  325°,  and  contracts 
considerably  in  passing  from  the  liquid  to  the  solid  condition. 
Its  specific  heat  is  0.0314.  Solid  lead  expands  greatly  when 
heated,  though  the  heat  be  not  carried  near  to  the  melting 
point,  and  the  expanded  metal  does'  not  return  again  to  its 
original  dimensions  when  cooled.  Melted  lead  begins  to  emit 
vapors  at  a  red  heat,  and  at  very  high  temperatures  the  metal 
may  even  be  distilled.  Lead  may  be  obtained  crystallized  in 
octahedrons,  by  slowly  cooling  the  molten  metal. 


OXIDES    OF    LEAD.  489 

The  ready  crystallization  of  lead  furnishes  A  very  simple 
method  of  separating  this  metal  from  the  silver,  with  which 
crude  lead  is  almost  always  contaminated  as  it  comes  from  the 
smelting  furnaces.  When  melted  argentiferous  lead  is  al- 
lowed to  cool  slowly,  and  is  at  the  same  time  briskly  stirred, 
a  quantity  of  solid  crystalline  grains  separate  out  after  a 
while,  and  sink  beneath  the  liquid  metal,  whence  they  may  be 
dipped  out  in  cullenders.  These  crystals  are  composed  of 
lead,  nearly  free  from  silver,  while  all  but  a  trace  of  the  silver 
contained  in'  the  original  lead,  is  left  in  that  portion  of  the 
metal  which  has  not  yet  solidified;  in  a  word,  the  alloy  of 
lead  and  silver  melts  at  a  lower  temperature  than  pure  lead. 
By  methodically  remelting  and  recrystallizing  the  lead  crys- 
tals on  the  one  hand,  and  the  silver  alloy  on  the  other,  it  has 
been  found  profitable  to  extract  the  silver  from  lead  so  poor 
that  it  contained  less  than  one  thousandth  part  its  weight  of 
the  precious  metal. 

When  in  thick  masses,  such  as  the  common  sheets  and  pipes 
of  commerce,  lead  is  scarcely  at  all  acted  upon  by  cold  sul- 
phuric acid,  and  is  but  slowly  corroded  by  chlorhydric  acid. 
Both  these  acids  form,  by  uniting  with  lead,  difficultly  soluble 
salts  ;  and  so  soon  as  a  layer  of  the  salt  has  once  been  de- 
posited upon  the  surface  of  the  metal,  the  latter  is  thereby 
protected  from  further  corrosion.  By  hot,  concentrated  sul- 
phuric acid,  however,  lead  is  dissolved  rather  easily.  The 
best  solvent  of  metallic  lead  is  diluted  nitric  acid ;  strong 
nitric  acid  will  not  dissolve  it  readily,  since  nitrate  of  lead  is 
well  nigh  insoluble  in  concentrated  nitric  acid. 

571.  Oxides  of  Lead.  —  When  a  compact  piece  of  metallic 
lead  is  freshly  cut  it  exhibits  considerable  lustre,  and  this 
lustre  may  be  preserved  unimpaired  by  keeping  the  lead  in 
perfectly  dry  air,  or  beneath  the  surface  of  pure  water  free 
from  air  (§  47).  But  by  exposure  to  ordinary  air  the  brilliant 
surface  soon  tarnishes  in  consequence  of  the  formation  of  a 
thin  coating  of  suboxide  of  lead  ;  this  incrustation  protects 
the  metal  beneath  from  further  oxidation.  Finely  divided 
lead,  on  the  contrary,  soon  changes  completely  to  suboxide 


488  METALLIC    LEAD. 

extracted  from  sulphide  of  lead,  PbS,  the  mineral  galena. 
This  substance  is  tolerably  abundant  in  man}*-  localities,  and 
is  often  associated  wijk  sulphate  of  barium,  fluor-spar,  quartz, 
and  other  commoji-^mnerals  ;  it  almost  always  contains  a  small 
proportion  /edTsulphide  of  silver.  In  order  to  obtain  metallic 
lead  from  galena,  this  mineral  is  mixed  with  a  small  quantity 
of  lime,  and  then  roasted  at  a  dull  red  heat  in  the  flame  of  a 
reverberatory  furnace.  A  portion  of  the  sulphur  burns  off  as 
sulphurous  acid.  Some  oxide  of  lead,  and  more  or  less  sul- 
phate of  lead  is  formed,  while  much  of  the  ore  remains  unde- 
composed.  After  a  time,~lhe  roasting  process  is  interrupted, 
air  is  excluded  from  the  furnace,  the  oxide,  sulphate,  and  sul- 
phide of  lead  are  thoroughly  mixed  together,  and  the  heat  of 
the  furnace  is  suddenly  raised.  The  undecomposed  sulphide 
of  lead  then  reacts  upon  the  oxide  and  sulphate,  sulphurous 
acid  is  given  off,  and  metallic  lead  produced.  The  reactions 
may  be  thus  formulated  :  — 


II.   PbSO4  +  PbS  =  2Pb  +  2SO2 

The  lime  is  added  for  the  purpose  of  forming  a  fusible  slag 
with  any  silicious  matter  which  may  be  present  in  the  ore. 

Lead  is  a  remarkably  soft  metal,  of  bluish-white  color  ;  it 
can  be  readily  cut  with  a  knife,  and  may  even  be  indented  with 
the  finger-nail  ;  it  soils  paper  upon  which  it  is  rubbed.  Its 
specific  gravity  is  11.4,  and  its  atomic  weight  207.  It  may  be 
drawn  into  wire,  and  beaten  into  sheets,  though,  as  contrasted 
with  most  of  the  other  metals,  it  has  but  little  tenacity.  In 
comparison  with  other  metals,  it  is  a  rather  poor  conductor 
of  heat  and  electricity.  It  melts  at  about  325°,  and  contracts 
considerably  in  passing  from  the  liquid  to  the  solid  condition. 
Its  specific  heat  is  0.0314.  Solid  lead  expands  greatly  when 
heated,  though  the  heat  be  not  carried  near  to  the  melting 
point,  and  the  expanded  metal  does'  not  return  again  to  its 
original  dimensions  when  cooled.  Melted  lead  begins  to  emit 
vapors  at  a  red  heat,  and  at  very  high  temperatures  the  metal 
may  even  be  distilled.  Lead  may  be  obtained  crystallized  in 
octahedrons,  by  slowly  cooling  the  molten  metal. 


OXIDES    OF    LEAD.  489 

The  ready  crystallization  of  lead  furnishes  a  very  simple 
method  of  separating  this  metal  from  the  silver,  with  which 
crude  lead  is  almost  always  contaminated  as  it  comes  from  the 
smelting  furnaces.  When  melted  argentiferous  lead  is  al- 
lowed to  cool  slowly,  and  is  at  the  same  time  briskly  stirred, 
a  quantity  of  solid  crystalline  grains  separate  out  after  a 
while,  and  sink  beneath  the  liquid  metal,  whence  they  may  be 
clipped  out  in  cullenders.  These  crystals  are  composed  of 
lead,  nearly  free  from  silver,  while  all  but  a  trace  of  the  silver 
contained  in'  the  original  lead,  is  left  in  that  portion  of  the 
metal  which  has  not  yet  solidified ;  in  a  word,  the  alloy  of 
lead  and  silver  melts  at  a  lower  temperature  than  pure  lead. 
By  methodicall}'  remelting  and  recrystallizing  the  lead  crys- 
tals on  the  one  hand,  and  the  silver  alloy  on  the  other,  it  has 
been  found  profitable  to  extract  the  silver  from  lead  so  poor 
that  it  contained  less  than  one  thousandth  part  its  weight  of 
the  precious  metal. 

When  in  thick  masses,  such  as  the  common  sheets  and  pipes 
of  commerce,  lead  is  scarcely  at  all  acted  upon  by  cold  sul- 
phuric acid,  and  is  but  slowly  corroded  by  chlorhydric  acid. 
Both  these  acids  form,  by  uniting  with  lead,  difficultly  soluble 
salts ;  and  so  soon  as  a  layer  of  the  salt  has  once  been  de- 
posited upon  the  surface  of  the  metal,  the  latter  is  thereby 
protected  from  further  corrosion.  By  hot,  concentrated  sul- 
phuric acid,  however,  lead  is  dissolved  rather  easily.  The 
best  solvent  of  metallic  lead  is  diluted  nitric  'acid  ;  strong 
nitric  acid  will  not  dissolve  it  readily,  since  nitrate  of  lead  is 
well  nigh  insoluble  in  concentrated  nitric  acid. 

571.  Oxides  of  Lead.  —  When  a  compact  piece  of  metallic 
lead  is  freshly  cut  it  exhibits  considerable  lustre,  and  this 
lustre  may  be  preserved  unimpaired  by  keeping  the  lead  in 
perfectly  dry  air,  or  beneath  the  surface  of  pure  water  free 
from  air  (§  47).  But  by  exposure  to  ordinary  air  the  brilliant 
surface  soon  tarnishes  in  consequence  of  the  formation  of  a 
thin  coating  of  suboxide  of  lead  ;  this  incrustation  protects 
the  metal  beneath  from  further  oxidation.  Finely  divided 
lead,  on  the  contrary,  soon  changes  completely  to  suboxide 

32 


490  CORROSION    OF    LEAD. 

when  exposed  to  the  air.  If  the  metal  be  fine  enough,  it  will 
oxidize  instantaneously  with  evolution  of  light  and  heat,  and 
formation  of  yellow  protoxide  of  lead. 

Exp.  300. — Prepare  a  small  quantity  of  tartrate  of  lead,  as  fol- 
lows :  Dissolve  0.25  grm.  of  common  sugar  of  lead  (acetate  of 
lead)  in  8  or  10  c.  c.  of  water,  also  dissolve  0.1  grm.  of  tartaric 
acid  in  4  or  5  c.  c.  of  water,  and  mix  the  two  solutions.  Collect 
upon  a  filter  the  white  precipitate  of  tartrate  of  lead  which  will  be 
formed,  wash  it  with  water,  then  unfold  the  filter  and  spread  it  out 
with  its  contents  to  dry  in  the  air,  or  better,  at  a  gentle  heat  upon  a 
ring  of  the  iron  stand,  high  above  a  single  gas  flame. 

Fill  an  ignition  tube  one-third  full  of  the  dry  tartrate  of  lead,  and 
heat  it  upon  a  sand-bath  so  long  as  any  fumes  escape,  then  cork  the 
tube  tightly  and  set  it  aside  to  cool.  Holding  the  cooled  tube  high 
in  the  air,  sprinkle  its  contents  upon  a  plate,  and  observe  that  the 
black  powder  takes  fire  spontaneously,  and  burns  with  a  red  flash. 
The  composition  of  tartrate  of  lead  may  be  represented  by  the 
formula  C4H4Pb2O6 ;  on  being  heated  this  substance  gives  off  water 
and  carbonic  oxide,  as  may  be  seen  by  lighting  the  fumes  which 
escape  from  the  tube,  and  there  is  left  as  a  residue  an  intimate  mix- 
ture of  carbon  and  of  metallic  lead,  so  finely  divided  that  it  in- 
flames in  ordinary  air.  A  spontaneously  inflammable  mixture  such 
as  this  is  called  a  pyropJiorus. 

572.  Lead  is  far  more  readily  oxidized  by  the  continued 
action  of  air  and  water  than  by  ordinary  moist  air.  When 
exposed  to  the  simultaneous  or  alternate  action  of  these 
agents,  a  coating  of  the  white  hydrated  protoxide  of  lead  is 
rapidly  formed  ;  but  as  this  compound  is  somewhat  soluble  in 
water,  it  is  continually  dissolved  away  and  affords  little  or 
no  protection  to  the  lead  beneath.  The  corrosive  action  of 
water  upon  lead  is  modified  very  materially  by  the  presence 
of  small  quantities  of  various  saline  substances.  Water  con- 
taining traces  of  nitrates,  nitrites,  and  chlorides,  corrodes 
lead  more  rapidly  than  pure  water,  while  the  corrosive  action 
of  pure  water  appears  to  be  diminished  by  the  presence  of 
sulphates,  phosphates,  and  carbonates,  oxide  of  lead  being 
scarcely  at  all  soluble  in  water  which  contains  these  salts  in 
solution.  Water  containing  a  solution  of  carbonate  of  cal- 
cium in  carbonic  acid,  such  as  is  frequently  met  with  in 


LITHARGE.  491 

nature,  has  been  found  to  have  remarkably  little  action  upon 
lead  ;  in  such  water  a  coating  of  insoluble,  or  nearly  insoluble 
carbonate  of  lead  is  formed  upon  the  metal,  which  protects  it 
from  further  action.  But,  on  the  other  hand,  water  which  con- 
tains much  free  carbonic  acid  dissolves  away  the  protective 
coating  and  exposes  fresh-  surfaces  of  lead  to  corrosion.  As 
a  general  rule,  the  acids,  even  when  very  dilute,  greatly  accel- 
erate the  oxidation  of  lead  in  the  air  ;  and  the  same  remark  is 
true  of  organic  substances  which,  by  their  decay,  and  of  met- 
als which,  by  galvanic  action,  hasten  the  corrosion. 

Since  solutions  of  lead  are  poisonous,  and  since  the  metal 
is  employed  to  an  enormous  extent  for  cisterns  and  conduits, 
a  knowledge  of  the  action  of  water  upon  lead  is  very  impor- 
tant in  a  sanitary  point  of  view.  The  question  has  conse- 
quently engaged  the  attention  of  many  chemists,  and  has  been 
much  discussed.  It  has  been  proved  by  numberless  experi- 
ments that  the  action  of  natural  waters  upon  lead  is  so  gen- 
eral that  it  is  rare  to  find  any  sample  of  water  which  has  been 
kept  in  a  leaden  cistern,  wholly  free  from  traces  of  that  metal. 
The  opinion  of  most  chemists  is  at  this  time  (1867)  decidedly 
adverse  to  the  use  of  leaden  water-pipes  in  houses,  in  spite  of 
the  fact  that  the  metal  is  nowadays  employed  for  this  purpose 
upon  every  hand  with  apparent  impunity. 

When  lead  is  melted  in  the  air  it  oxidizes  readily,  with 
formation  at  first  of  gray  suboxide,  and  afterwards  of  the  yel- 
low protoxide. 

573.  Suboxide  of  Lead  (Pb2O)  may  be  prepared  in  a  state 
of  purity  by  cautiously  heating  oxalate.of  lead  at  a  temper- 
ature not   exceeding  300°  in  a  retort  from  which  air  is  ex- 
cluded, so  long  as  any  gas  is  evolved :  — 

2PbC  A  =  Pb20  +  CO  +  3C02. 

After  the  retort  has  become  cold  suboxide  of  lead  will  be 
found' in  it  as  a  black  velvety  powder.  It  is  decomposed  by 
acids,  with  formation  of  salts  of  protoxide  of  lead,  and  sepa- 
ration of  metallic  lead. 

574.  Protoxide  of  Lead  (PbO),  commonly  called  litharge, 
may  be  obtained  as  a  lemon-yellow  powder  by  gently  igniting 


492  CUPELLATION. 

nitrate,  carbonate,  or  oxalate  of  lead  upon  an  iron  plate,  or 
in  an  open  porcelain  crucible.  The  oxide  fuses  at  a  red  heat, 
and  when  melted  in  vessels  of  porcelain  or  earthen-ware  it 
rapidly  destroys  them  by  combining  with  their  silica  ;  an  easily 
fusible  slag  or  glass  composed  of  double  silicates  of  lead, 
aluminum,  iron,  etc.,  being  formed.  Silicate  of  lead  is  an 
actual  constituent  of  the  easily  fusible  variety  of  glass  known 
as  flint  glass  (see  §  492,  and  Appendix,  §  3). 

In  the  arts,  litharge  is  prepared  upon  the  large  scale  by 
heating  metallic  lead  in  a  current  of  air ;  the  color  and  tex- 
ture of  the  product  varies  considerably  according  to  the  tem- 
perature and  other  conditions  at  which  the  litharge  has  been 
prepared. 

Exp.  301.  —  Heat  a  small  fragment  of  lead  upon  charcoal  in  the 
oxidizing  flame  of  the  blowpipe,  and  observe  the  gray  film  of  sub- 
oxide  which  forms  at  first,  and  the  yellow  incrustation  of  litharge 
which  is  obtained  subsequently.  The  litharge  may  be  melted  if  a 
strong,  hot  flame  be  thrown  upon  it. 

This  property  of  lead  of  rapidly  oxidizing  when  heated  in  the 
air,  taken  in  connection  with  the  easy  fusibility  of  the  oxide,  is  the 
basis  of  the  common  method  of  separating  lead  and  silver  in  the 
large  way  known  as  cupellation.  The  iridescent  film  of  litharge 
continually  formed  upon  the  surface  of  the  molten  metal  as  inces- 
santly flows  off,  exposing  new  surfaces  of  the  metal  to  the  action 
of  the  air.  The  silver,  on  the  other  hand,  undergoes  little  or  no 
change,  and  when  the  lead  has  been  completely  burned  the  silver 
appears  in  all  its  brilliant  whiteness. 

The  cupel  is  a  shallow  cup  or  basin,  composed  either  of  marl  or 
of  a  mixture  of  bone  ash  and  wood  ashes,  firmly  compacted  and 
beaten  to  a  smooth  surface,  which  may  be  placed  either  in  the 
muffle  of  an  assay  furnace,  upon  the  hearth  of  a  reverberatory,  or 
in  any  position  where  it  can  be  strongly  heated  at  the  same  time 
that  a  free  current  of  air  plays  over  its  surface.  A  charge  of  ar- 
gentiferous lead  having  been  melted  upon  the  cupel,  new  portions 
of  the  lead  are  added  as  fast  as  the  melted  litharge  flows  off  from 
the  convex  surface  of  the  metal  and  makes  room  for  these*  addi- 
tions, until  an  alloy  very  rich  in  silver  has  been  obtained.  This 
alloy  is  then  cupelled  until  the  last  traces  of  lead  have  been  re- 
moved, and  the  silver  is  left  pure  and  glistening.  In  cupelling 
upon  the  small  scale,  for  purposes  of  assaying,  the  cupel  is  made 


PEROXIDE    OF    LEAD.  493 

of  bone  ash  of  such  quality  that  the  litharge  may  be  absorbed  into 
the  substance  of  the- cupel,  and  not  flow  off  through  gutters  upon 
its  edge,  as  is  the  case  in  large  metallurgical  operations. 

575.  Protoxide  of  lead  unites  readily  with  acids,  and  forms 
many  important  salts.     When  in  the  state  of  powder,  it  even 
absorbs  a  certain  amount  of  carbonic  acid  from  the  air  ;  hence, 
the  powdered  litharge  of  commerce  always  contains  more  or 
less    carbonate  of  lead,   and    therefore    effervesces   on   being 
treated  with  acids,  as  has  been  seen  in  Exp.  42.     As  a  gen- 
eral rule,  it  is  far  better  to  prepare  the  salts  of  lead,  by  dis- 
solving the  protoxide  in   acids,  than  to  treat  the  metal  itself 
with  acids.     Protoxide  of  lead  has  a  remarkable  tendency  to 
form  basic  salts  ;  thus  besides  the  normal  nitrate  (PbO,N2O5) 
there  is  a  dinitrate  (2PbO,N2O5),  a  trinitrate  (3PbO,N205), 
a  tetranitrate  (4PbO,N2O5),  and  a  hexanitrate  (6PbO,N2O5). 

Though  a  strong  base  as  regards  the  acids,  protoxide  of  lead 
behaves  like  an  acid  towards  the  alkalies  and  alkaline  earths. 
For  example,  it  dissolves  readily  in  soda  or  potash  lye,  with 
formation  of  plumbite  of  sodium,  or  of  potassium,  as  the  case 
may  be.  The  term  plumbite,  like  the  symbol  of  lead,  Pb,  is 
derived  from  plumbum,  the  Latin  name  of  the  metal. 

576.  Peroxide  of  Lead  (Pb  O2),  may  be  prepared  by  oxidiz- 
ing the  protoxide,  —  for  example,  by  passing  a  current  of  chlo- 
rine gas  through  water  in  which  protoxide  of  lead  is  kept  sus- 
pended by  agitation,  or  as  follows  :  — 

Exp.  302. —  Place  8  or  10  grms.  of  very  finely  powdered  sugar 
of  lead  in  a  capacious  porcelain  dish,  cover  the  salt  with  a  filtered 
solution  of  bleaching  powder  (§  563),  heat  the  solution  to  boiling, 
and  maintain  it  at  this  temperature,  until  the  escaping  vapors  smell 
strongly  of  acetic  acid ;  then  pour  the  contents  of  the  dish  upon  a 
filter,  and  wash  with  water  the  dark  brown  powder  of  peroxide  of 
read,  which  has  been  formed. 

Peroxide  of  lead  may  be  easily  obtained  also,  by  digesting  red 
lead  with  dilute  nitric  acid,  as  will  appear  in  the  following  para- 
graph. 

Peroxide  of  lead  is  a  powerful  oxidizing  agent ;  it  readily 
gives  up  oxygen  to  many  organic  substances,  even  at  the  or- 
dinary temperature  of  the  air.  On  being  heated  to  redness, 


494  RED    LEAD. 

it  loses  half  its  oxygen,  and  is  converted  into  the  protox- 
ide. 

Exp.  303.  —  In  a  small  porcelain  mortar,  rub  together  a  mixture 
of  1  grm.  of  oxalic  acid,  and  1  grm.  of  peroxide  of  lead.  De- 
composition will  occur,  aqueous  vapor  and  carbonic  acid  will  be 
given  off,  and  carbonate  of  lead,  PbCO3  will  be  left  as  a  residue. 
When  the  peroxide  is  thus  mixed  with  one-eighth  its  weight  of 
sugar,  or  with  one-sixth  its  weight  of  tartaric  acid,  so  much  heat 
is  developed,  that  the  mass  in  the  mortar  glows. 

Peroxide  of  lead  is  decomposed  by  chlorhydric  acid,  with 
liberation  of  chlorine,  and  formation  of  normal  chloride  of 
lead  :  — 

PbO2  +  4  HC1  =  PbCl2  +  2H2O  +  2  Cl, 

and  by  hot  sulphuric  acid,  with  evolution  of  oxygen.  It  com- 
bines with  sulphurous  acid  readily,  and  is  often  employed  in 
the  laboratory  as  an  absorbent  of  this  gas  ;  and  it  is  note- 
worthy, that  the  product  of  this  combination  is  sulphate  of 
lead  :  — 

PbO2  +  SO2  =  PbSO4. 

It  is  indifferent,  therefore,  whether  we  put  together  peroxide 
of  lead  and  sulphurous  acid,  or  protoxide  of  lead  and  sulphu- 
ric acid,  the  product  will,  in  either  case,  be  common  sulph- 
ate of  lead,  for, 


As  a  rule,  peroxide  of  lead  does  not  readily  enter  into  com- 
bination with  acids,  though  compounds  of  it,  with  acetic, 
phosphoric,  arsenic  acids,  etc.,  have  been  obtained.  With 
strong  bases,  however,  it  combines  readily,  forming  salts 
known  as  plum^ates. 

577.  Red  Lead  or  Minium.  —  WThen  protoxide  of  lead  is 
kept  at  a  low  red  heat  for  some  time,  in  contact  with  air,  it 
gradually  absorbs  one  or  two  per  cent,  of  oxygen,  and  acquires 
a  brilliant  red  color.  The  product  of  this  oxidation  is  exten- 
sively used  as  a  pigment  and  in  the  manufacture  of  some  kinds 
of  glass  ware  ?  it  may  be  regarded  as  a  compound  of  PbO  and 
PbO2  in  varying  proportions.  By  digesting  it  for  some  time 
in  dilute  nitric  acid,  the  protoxide  of  lead  may  all  be  dissolved 


SULPHIDES    OF    LEAD.  495 

out  and  converted  into  nitrate  of  lead,  while  the  peroxide  of 
lead  is  left  as  a  residue  :  — 

2PbO  ;  Pb02  +  2H,N206  -  2PbN2O6  +  2H20  +  Pb02. 

Exp.  304. —  Heat  in  an  iron  spoon,  4  grms.  of  litharge  and  1 
grm.  of  chlorate  of  potassium,  and  observe  that  the  color  of  the 
mixture  soon  changes  from  yellow  to  red.  Throw  the  cooled  pro- 
duct upon  a  filter,  wash  it  with  water,  then  dry  it  and  compare  its 
color  with  that  of  the  original  litharge.  In  this  experiment,  the  red 
lead  could  be  obtained  as  well  by  simply  heating  litharge  without 
admixture  in  the  air  for  many  hours,  at  ^temperature  just  below  its 
melting  point ;  time  alone  is  gained  by  employing  chlorate  of  potas- 
sium as  the  source  of  oxygen.  For  commercial  purposes,  red  lead 
is  obtained  by  heating  metallic  lead  in  reverberatory  furnaces,  or 
when  a  very  pure  article  is  needed,  by  heating  carbonate  of  lead. 

When  heated  strongly,  red  lead  is  resolved  into  protoxide  of  lead 
and  free  oxygen.  Oxygen  gas  may  be  prepared  from  it  in  the  same 
manner  as  from  oxide  of  mercury  (Exp.  5),  though  at  a  higher 
temperature. 

578.  Sesquioxide  of  Lead  (Pb203),  is  recognized  as  a  dis- 
tinct oxide  by  some  chemists,  but  is  more  generally  regarded 
as  a  compound  of  the  proto  and  peroxides,  PbO,»Pb02,  —  a 
plnmbate  of  lead. 

579.  Sulphides  of  Lead.     There  are  several  of  these  com- 
pounds, but  the  protosnlphide,  PbS,  is   the  only  one  whose 
composition  is  accurately  known.     This  is  found  native  as  the 
mineral  galena  (§  570)  and  may  readily  be  prepared  either  by 
melting  together  lead  and  sulphur  in  atomic  proportions  or  by 
treating  the  solution  of  any  lead  salt  with  sulphuric  acid  (§  209). 
The  native  mineral,  like  the  compound  obtained  artificially  by 
waj*  of  fusion,  is  of  a  leaden  gray  color  of  7.5  specific  gravity, 
but  the  precipitate  which  forms  when  sulphydric  acid  is  added 
to  the  solution  of  a  lead  salt,  is  black,  or  brown,  or  even  red, 
if  the  solution  be  dilute.     On  account  of  the  deep  color,  as 
well  as  the  insolubility  of  this  precipitate,  sulphydric  acid  is 
often  made  use  of  as  a  means  of  detecting  lead  ;  the  test  is,  in 
fact,   so   delicate   that   solutions  containing  only  a  hundred 
thousandth  of  their  weight  of  metallic  lead,   will  assume  a 
brown  color  on  being  charged  with  sulphuretted  hydrogen. 


496  CHLORIDE    OF    LEAD. 

Exp.  305. — Dissolve  quarter  of  a  gramme  of  sugar  of  lead  in  4 
litres  of  water,  add  to  the  solution  a  few  drops  of  nitric  acid  so  that 
it  shall  exhibit  a  faint  acid  reaction  with  litmus  paper,  pass  into  the 
solution  a  current  of  sulphydric  acid  gas,  until  the  solution  smells 
strongly  of  it,  and  observe  the  brown  color  imparted  to  the  fluid 
after  some  time. 

In  testing  for  the  presence  of  lead  in  excessively  dilute  solutions, 
such,  for  example,  as  water  drawn  from  leaden  pipes,  it  is  well  to 
evaporate  the  liquid  to  a  small  bulk»in  a  porcelain  dish,  to  acidulate 
the  concentrated  liquor  very  slightly  with  nitric  acid,  and  then  to 
transfer  it  to  a  beaker  glass.  The  liquid  should  then  be  saturated 
with  sulphuretted  hydrogen  gas,  the  beaker  covered  with  a  glass 
plate,  and  left  to  stand  during  several  hours  in  a  moderately  warm 
room.  If  lead  be  present,  it  will  be  indicated  after  a  while  either 
by  the  brown  coloration  of  the  liquid,  or  by  the  actual  separation  of 
a  black  powder  at  the  bottom  or  upon  the  sides  of  the  glass. 

580.  Sulphide  of  lead  is  volatile  at  high  temperatures,  and  is 
often  found  in  the  cracks  and  upon ,  the  walls  of  smelting 
furnaces,  in  the  form  of  crystals,  which  have  been  deposited 
by  sublimation.     By  virtue  of  the  volatility  of  its  sulphide, 
lead  may  be  transported  to  very  considerable  distances  from 
furnaceg  where  ores  containing  galena  are  roasted  or  reduced. 
It  has  been  found,  moreover,  that  growing  plants  are  capable 
of  taking  up  the  lead  thus  deposited,  and  of  assimilating  a  cer- 
tain portion  of  it  in  their  tissues.     Comparative  experiments 
made  at  very  high  temperatures  have  shown  that  galena  may 
lose  as  much  as  3.7  per  cent,  of  its  weight  by  volatilization, 
while  metallic  lead,  exposed  to  the  same  conditions,  loses  less 
than  0.1  per  cent. 

Sulphide  of  lead,  like  the  sulphides  of  the  alkali  metals 
and  those  of  the  alkaline  earthy  metals,  acts  as  a  sulphur 
base ;  with  sulphantiinonic  acid,  for  example,  it  unites  to  form 
a  salt  3PbS,SbS5  analogous  to  that  formed  by  the  union  of 
antimonic  acid  and  oxide  of  lead,  3PbO,SbO5. 

581.  Chloride  of  Lead  (PbCl2).    Metallic  lead  is  but  slowly 
acted  upon  by  chlorine,  or  by  chlorhydric  acid,  though  hot 
chlorhydric  acid   dissolves    a    little  of  it  with  formation  of 
chloride  of  lead  and  evolution  of  hydrogen,  even  when   out 
of  contact  with  the  air  ;  the  chloride  may,  however,  be  readily 


SALTS.  OF    LEAD.  497 

prepared  by  digesting  oxide  or  carbonate  of  lead  in  chlorhy- 
dric  acid,  or  by  mixing  the  solution  of  almost  any  lead  salt 
with  chlorhydric  acid,  or  with  a  solution  of  some  soluble 
chloride :  — 

PbN206  +  2NaCl  =  PbCl2  +  Na2N2O6. 

Chloride  of  lead  is  but  sparingly  soluble  in  cold  water, 
and  is  still  less  soluble  in  water  acidulated  with  chlorhydric 
acid,  hence  it  may  readily  be  precipitated  as  above  described, 
and  collected  upon  a  filter.  In  hot  water,  however,  it  dissolves 
rather  easily  and  it  is  somewhat  soluble  in  concentrated  acid 
also. 

Exp.  306. — Boil  together,  in  a  small  flask,  1  grm.  of  litharge,  14 
grms.  of  strong  chlorhydric  acid,  and  14  grms.  of  water,  during 
15  or  20  minutes.  Pour  the  mixture  upon  a  filter  supported  in  a 
funnel,  which  has  been  gently  warmed  by  holding  it  over  the  flame 
of  the  gas  lamp,  and  collect  the  clear  filtrate  in  a  warm  bottle.  As 
the  solution  cools,  lustrous,  needle-shaped  crystals  of  chloride  of 
lead  will  form  in  it. 

Exp.  307.  — Pour  off  the  cold  supernatant  liquor  from  the  crys- 
tals of  chloride  of  lead,  obtained  in  Exp.  306,  place  the  crystals 
upon  a  fragment  of  porcelain,  dry  them  at  a  gentle  heat,  and 
finally  heat  them  more  strongly.  It  will  be  found  that  the  crystals 
melt  very  easily,  and  that  on  cooling  they  solidify  to  a  soft  translu- 
cent horny  mass,  whence  the  old  name  of  this  substance  horn-lead. 

582.  The  compounds  of  lead  with  iodine,  bromine,  and 
fluorine,  are  analogous  to  chloride  of  lead.  The  iodide  is 
remarkable  on  account  of  its  beautiful  yellow  color,  which  may 
readily  be  shown  b}r  adding  a  drop  or  two  of  a  solution  of 
iodide  of  potassium  to  a  small  quantity  of  a  solution  of  nitrate 
of  lead. 

Of  the  numerous  other  salts  of  lead  little  need  here  be 
said  :  —  the  nitrate  and  tartrate  have  already  been  prepared 
(Exps.  42,  300),  —  we  have  obtained  the  sulphate  also  as  a 
white,  nearly  insoluble  powder  by  adding  water  to  concentrated 
sulphuric  acid,  and  it  may  be  had  in  any  quantity  by  mixing 
the  solution  of  a  lead  salt  with  dilute  sulphuric  acid,  or  with 
the  solution  of  a  soluble  sulphate.  Acetate  of  lead,  one  of 
the  most  important  of  the  lead  salts,  and  the  one  most  readily 

33 


498  WHITE    LEAD. 

to  be  procured  in  commerce  in  a  state  of  purity,  is  prepared 
by  dissolving  oxide  of  lead  directly  in  acetic  acid,  such  as  is 
obtained  in  the  distillation  of  wood  (§  380),  or  indirectly  by 
moistening  plates  of  metallic  lead  with  vinegar  in  vessels 
open  to  the  air.  It  crystallizes  readily,  is  easily  soluble  in 
water,  and  has  a  sweet,  astringent  taste,  whence  the  name, 
sugar  of  lead.  Like  the  other  lead  salts,  it  is  highly  poisonous. 
It  is  employed  for  many  purposes  in  the  arts,  and  is  in  particu- 
lar much  used  in  medicine.  Carbonate  of  lead  (PbCO3)  or 
rather  compounds  of  carbonate  of  lead  and  of  Ivydrate  of  lead 
in  varying  proportions,  are  used  to  an  enormous  extent  as  a 
white  paint,  under  the  general  name  of  white  lead.  The  com- 
position of  this  substance  may  usually  be  expressed  by  a  for- 
mula lying  within  the  limits  PbCO3 ;  PbH2O2  upon  the  one 
hand,  and  3PbCO3 ;  PbH2O2  on  the  other.  As  contrasted 
with  the  other  white  pigments  it  possesses  remarkable  covering 
power  and  durability,  and  is  consequently  much  esteemed 
in  spite  of  its  high  cost,  of  the  injurious  influence  which 
it  exerts  upon  the  health  of  workmen  who  have  to  do  with 
it,  and  of  the  fact  that  it  is  blackened  by  sulphuretted 
hydrogen.  White  lead  is  often  adulterated  with  sulphate  of  ba- 
rium, with  oxide  of  zinc,  and  with  gypsum.  It  is  usually  pre- 
pared by  bringing  carbonic  acid,  obtained  from  decaying  vege- 
table matter,  or  from  the  combustion  of  fuel,  into  contact 
with  basic  acetate  of  lead  ;  the  latter  being  prepared  in  this 
case  either  by  mixing  litharge  and  vinegar  to  the  consistence 
of  a  paste,  or  by  exposing  rolls  of  sheet  lead  to  the  simulta- 
neous action  of  vapors  of  vinegar  and  air,  or  by  actually  dissolv- 
ing an  excess  of  litharge  in  vinegar.  Sometimes  the  carbonic 
acid  is  made  to  act  upon  the  subacetate  at  the  very  moment 
when  it  is  being  formed,  while  at  other  times  the  acetate  is 
prepared  by  itself,  and  subsequently  treated  with  carbonic 
acid. 

583.  Silicate  of  lead  is  of  interest  from  being  an  important 
ingredient  of  flint  glass  ;  a  certain  proportion  of  it  renders 
glass  lustrous  and  very  beautiful.  Such  glass  is,  however, 
soft,  easily  fusible,  and  incapable  of  bearing  sudden  changes 


MAGNESIUM.  499 

of  temperature  ;  it  is,  moreover,  rather  easily  acted  upon  by 
alkalies,  acids,  and  other  chemical  agents,  and  is  hence  com- 
paratively useless  in  the  chemical  laboratory. 

584.  In  many  points  of  chemical  behavior  the  compounds 
of  lead  resemble  more  or  less  clearly  the  corresponding  com- 
pounds of  barium,  strontium,  and  calcium.  Its  compounds 
are  moreover  isomorphous  with  those  of  the  metals  in  ques- 
tion, and  its  atom,  like  the  atoms  of  these  metals,  is  bivalent. 
Lead  is,  therefore,  classed  as  a  member  of  the  calcium  group, 
though,  as  in  the  case  with  fluorine  in  the  chlorine  group,  it 
differs  in  some  respects  from  the  other  members  of  the  family. 
The  specific  quantity  of  lead  is  11.4,  and  its  atomic  weight 
207.  The  specific  gravity  of  carbonate  of  lead  is  6.5,  and 
that  of  sulphate  of  lead  is  6.2. 


CHAPTER    XXIX. 
MAGNESIUM,  ZINC,  CADMIUM. 

MAGNESIUM. 

585.  This  metal,  or  rather  its  oxide,  was  formerly  classed 
with  the  group  which  comprises  the  alkaline  earths,  but  it  is 
now  known  to  be  more  closely  connected  with  zinc  and  cad- 
mium than  with  any  other  of  the  elements.     It  is  found  widely 
diffused,  and  rather  abundantly,  in  nature.     The  bitter  taste 
of  sea-water  and  of  some   mineral  waters  is  due  to  the  pres- 
ence of  magnesium  salts,  while  silicate  of  magnesium  and  car- 
bonate of  magnesium  are  contained  in  a  variety  of  minerals, 
and  in  such  common  rocks  as  dolomite,  serpentine,  soapstone, 
and  talc. 

586.  Metallic  magnesium  may  be  prepared  by  heating  anhy- 
drous chloride  of  magnesium  with  sodium  in  a  crucible  of  por- 
celain or  platinum,  and  subsequently  dissolving  out  in  cold 


500  OXIDE    OF    MAGNESIUM. 

water  the  chloride  of  sodium  which  results  from  the  reaction. 
Magnesium  is  a  lustrous  metal,  as  white  as  tin ;  its  specific 
gravity  is  1.75  and  its  atomic  weight  24.  It  does  not  tarnish 
in  dry  air,  though  in  damp  air  it  soon  becomes  covered  with  a 
film  of  hydrate  of  magnesium.  It  melts  at  a  low  red  heat,  and 
volatilizes  at  higher  temperatures  ;  it  may  be  readily  distilled 
at  a  bright  red  heat.  When  heated  strongly  in  the  air  it  takes 
fire  and  burns  with  a  bluish-white  light  of  great  brilliancy  and 
high  actinic  power.  The  metal  is  employed  by  photographers 
for  illuminating  caverns  and  other  places  into  which  sunlight 
cannot  penetrate,  and  in  cloudy  weather  it  is  even  used  by 
them  as  a  substitute  for  daylight.  The  metal  can  be  pressed 
into  wire  or  into  thin  ribbons,  and  a  considerable  quantity  of 
it  is  now  used  in  both  these  forms  for  purposes  of  illumination, 
as  above  stated.  Magnesium  lanterns  are  much  used  in  the- 
atres for  illuminating  scenery  and  tableaux.  The  white  light 
has  the  advantage  of  showing  colors  just  as  they  look  by  day- 
light. For  scenic  effects  the  light  may  be  modified  by  trans- 
mission through  colored  glass.  Magnesium  is  only  slowly 
acted  upon  by  cold  water,  but  is  rapidly  oxydized  by  hot 
water  and  by  water -acidulated  with  almost  any  acid ;  oxide 
of  magnesium  is  formed  and  hydrogen  set  free. 

587.  Oxide  of  Magnesium  (MgO).  There  is  but  one  com- 
pound of  magnesium  and  ox}^gen ;  it  is  obtained  as  a  white 
amorphous  powder  when  magnesium  is  burnt  in  the  air,  or 
when  carbonate,  chloride,  or  nitrate  of  magnesium  is  ignited. 

Exp.  308.  —  Roll  10  or  12  c.  m.  of  magnesium  wire  or  thin  ribbon 
into  a  coil  around  a  small  pencil ;  withdraw  the  pencil  and  place  in 
its  stead  a  piece  of  iron  wire  or  a  knitting-needle  ;  holding  this  wire 
horizontally,  apply  a  lighted  match  to  the  end  of  the  magnesium 
coil ;  the  magnesium  will  burn  to  the  white  oxide  which  coheres  in 
an  imperfect  coil,  clinging  to  the  iron  wire.  A  portion  of  the  ox- 
ide goes  off  as  white  smoke.  The  magnesium  wire  for  this  experi- 
ment may  be  procured  at  toy  shops  as  well  as  of  dealers  in  fine 
chemicals. 

The  oxide  is  tasteless  and  odorless  ;  it  is  soluble  to  a  very 
slight  extent  in  water,  and  the  solution  has  an  alkaline  reac- 
tion. -The  specific  gravity  of  the  solid  oxide,  or  magnesia,  as 


HYDRAULIC    MAGNESIA.  501 

it  is  often  called,  varies  from  3. 07  to  3.2  as  ordinarily  prepared, 
but  on  being  very  strongly  ignited  it  becomes  denser,  and  sam- 
ples have  been  prepared  in  this  way  of  specific  gravity  as  high 
as  3.61. 

The  light,  powdery  oxide  of  magnesium,  known  as  "cal- 
cined magnesia,"  which  is  prepared  by  gentle  but  prolonged 
ignition  of  the  hydrated  carbonate,  differs  materially  in  sev- 
eral particulars  from  the  more  compact  oxide  obtained  by  cal- 
cining nitrate  or  chloride  of  magnesium  at  high  temperatures, 
or  by  intensely  heating  the  powdery  oxide.  Common  calcined 
magnesia  is,  for  example,  readily  soluble  in  acids,  but  after 
the  oxide  has  been  exposed  to  very  high  temperatures  it  dis- 
solves but  slowly  even  in  the  strongest  acids.  Similar  dif- 
ferences between  the  products  obtained  at  high  and  at  low 
temperatures  are  met  with  among  the  oxides  of  almost  all  the 
metals  hereafter  to  be  studied. 

588.  A  compact  variety  of  oxide  of  magnesium,  obtained 
by  heating  the  nitrate  or  chloride  to  bright  redness,  but  no 
higher,  exhibits  remarkable  hydraulic  properties.  On  being 
wet  it  quickly  combines  with  a  portion  of  water,  and  is  con- 
verted into  a  crystallized  hydrate  of  compact  texture,  harder 
than  marble,  and  of  great  durability. 

A  mixture  of  equal  parts  of  the  hydraulic  magnesia  and  of 
chalk,  or  powdered  marble,  made  into  paste  with  water,  yields 
a  slightl}7  plastic  mass,  which  admits  of  being  readily  pressed 
into  any  desired  shape ;  if  the  moulded  material  be  then 
placed  in  water  it  will  become,  after  some  time,  extremely 
hard  and  compact  (§  591). 

Oxide  of  magnesium  is  altogether  infusible  at  temperatures 
short  of  that  of  the  oxyhydrogen  flame.  Very  excellent  cru- 
cibles for  scientific  purposes  are  prepared  by  compressing 
oxide  of  magnesium  into  suitable  forms.  These  crucibles 
undergo  far  less  change  in  the  air  than  those  made  from  lime ; 
and  like  the  lime  crucibles  they  do  not  unite  with  oxide  of 
iron  and  the  other  metallic  oxides  to  form  the  fusible  slags  or 
glasses,  which  are  so  annoying  in  the  ordinary  crucibles,  of 
which  silicic  acid  is  an  essential  component. 


502  EPSOM    SALTS. 

589.  Chloride  of  Magnesium  (MgCl2)  is  found  in  sea-water 
and  in  many  saline  springs.     It  is  formed  when  magnesium 
is  burnt  in  chlorine  gas  ;  when  a  current  of  chlorine  is  passed 
over  a  red  hot  mixture  of  charcoal  and  oxide  of  magnesium  ; 
and,  in  combination  with  water,  by  dissolving  oxide  of  mag- 
nesium in  chlorhydric  acid.     It  is  remarkable  that  the  hy- 
drated  chloride  last  mentioned  cannot  be  made  anhydrous  by 
evaporation  and  ignition  without  some  decomposition  of  the 
chloride ;   oxide  of  magnesium  is  formed  and  chlorine  goes 
off  in  combination  with  hydrogen  as  chlorhydric  acid. 

590.  Sulphate  of  Magnesium   (MgSO4),  or  rather  the  hy- 
drated  compound  (MgSO4 -(-  7H2O),  is  largely  employed  as  a 
medicament  under  the  name  of  Epsom  salts.     It  is  obtained 
not  only  from  the  mineral  spring  at  Epsom,  in  England,  and 
from  various  other  springs,  but  is  also  prepared  from  sea- 
water,  and  by  dissolving  the  minerals  serpentine  (silicate  of 
magnesium),  magnesite  (carbonate  of  magnesium),  and  dolo- 
mite (carbonate  of  magnesium  and  of  calcium),  in  sulphuric 
acid.     Hydrated  sulphate  of  magnesium  is  a  colorless,  crys- 
talline salt,  readily  soluble  in  water,  and  possessing  the  pe- 
culiar bitter  taste  common  to  most  of  the  soluble  magnesium 
compounds.    It  is  often  employed  in  laboratories  as  the  source 
from  which  to  prepare  other  magnesium  salts. 

591.  Carbonate  of  Magnesium  (MgCO3)  is  found  as  a  min- 
eral in  nature,  and  with  due  care  may  be  prepared  artificially. 
As  met  with  in  commerce,  however,  —  the  magnesia  alba  of 
the  shops,  prepared  by  mixing  hot  solutions  of  sulphate  of 
magnesium  and  carbonate  of  sodium,  —  it  is  mixed  with  vary- 
ing proportions  of  hydrate  of  magnesium.     This  compound 
is  employed  as  a  medicament.     A  compound  of  carbonate  of 
magnesium   and   carbonate  of  calcium  occurs   abundantly  in 
nature,  as  the  mineral  dolomite,  constituting  extensive  beds 
in  various  regions.     Dolomite  is  much  more  slowty  soluble  in 
acids  than  true  limestone,  but  when  heated  with  a  dilute  acid 
it  effervesces  readily.     When  burnt,  at  temperatures  so  low 
that  the  carbonic  acid  shall  be  expelled  only  from  the  magne- 
sium salt,  while  the  carbonate  of  ^calcium  remains  unaltered, 


ZINC.  503 

dolomite  affords  a  hydraulic  cement,  preferable  in  many  re- 
spects to  ordinary  lime.  The  product  of  the  calcination 
"sets"  rapidly  under  water,  and  is  converted  into  a  hard, 
compact  stone  (§  588).  Citrate  of  magnesium,  a  preparation 
made  from  carbonate  of  magnesium  and  citric  acid,  is  also 
largely  emplo3red  as  a  medicament. 

Of  the  other  salts  of  magnesium,  none  are  of  sufficient  im- 
portance to  be  described  in  this  manual.  Most  of  them  arc 
easily  soluble  in  water ;  hence  the  insoluble  double  phosphate 
of  magnesium  and  of  ammonium  (MgNH4PO4  -f-  6H2O)  ob- 
tained by  adding  ordinary  diphosphate  of  sodium  to  a  mix- 
ture of  ammonia  water  and  any  magnesium  salt  is  of  impor- 
tance to  the  analyst,  since  by  means  of  it  magnesium  may  be 
separated  from  its  solutions.  It  should  be  observed  that  the 
ready  solubility  of  sulphate  of  magnesium  is  in  marked  con- 
trast with  the  insolubility  of  the  sulphates  of  the  alkaline 
earthy  group  of  metals. 

ZINC. 

592.  Ores  of  zinc  occur  in  considerable  abundance  in  sev 
eral  localities.  The  metal  is  extracted  from  the  carbonate, 
oxide,  silicate,  and  sulphide.  The  carbonate  and  sulphide  are 
first  roasted  in  order  to  convert  them  into  oxides,  and  the 
oxide  is  then  reduced  by  means  of  hot  charcoal,  in  earthen 
retorts  or  in  crucibles  provided  with  iron  delivery  tubes. 
Since  metallic  zinc  is  volatile  at  high  temperatures,  it  distils 
over  from  the  retorts  as  fast  as  it  is  formed  and  is  condensed 
in  receivers. 

Zinc  is  a  bluish-white  metal  of  crystalline  texture,  brittle 
at  the  ordinary  temperature,  and  also  when  heated  above  200°, 
but  at  a  temperature  of  about  130°  or  140°  it  may  easily  be 
rolled  out  or  hammered  into  sheets.  The  metal  melts  at  425° 
and  boils  at  a  bright  red  heat ;  in  presence  of  air  the  red-hot 
metal  takes  fire  and  burns  with  a  brilliant  bluish-white  light 
and  formation  of  a  dense  cloud  of  white  oxide  of  zinc. 

Exp.  309. —  Melt  200  or  300  grms.  of  metallic  zinc  in  a  small 
Hessian  crucible,  or  in  an  iron  ladle,  placed  in  an  anthracite  fire. 


504  GRANULATED    ZINC. 

Remove  the  crucible  from  the  fire  by  means  of  appropriate  tongs 
(Appendix,  §  27),  and  pour  its  contents  in  a  very  fine  stream  into  a 
pailful  of  cold  water,  taking  care  to  hold  the  crucible  at  a  distance 
of  5  or  6  feet  above  the  pail.  Replace  the  empty  crucible  in  the 
fire,  in  order  that  it  may  be  ready  for  Exp.  310. 

The  small,  thin  pieces  of  zinc  which  will  be  found  in  the  pail 
when  the  water  is  poured  away,  are  known  as  granulated  or  feath- 
ered zinc.  This  process  of  granulation  may  be  conveniently  applied 
to  any  of  the  other  easily  fusible  metals,  such  as  bismuth,  lead,  or 
tin,  when  they  are  required  in  a  finely  divided  condition. 

Granulated  zinc  is  much  used  in  chemical  laboratories,  for  a  va- 
riety of  purposes,  but  particularly  for  preparing  hydrogen  (§50). 
In  order  that  it  may  be  fit  for  this  purpose,  it  is  best  to  heat  the 
melted  metal  nearly  to  redness  before  pouring  it  into  the  water,  for 
it  has  been  noticed  that  when  zinc  is  melted  at  the  lowest  possible 
temperature  and  then  immediately  poured  into  water,  the  granules 
obtained  are  but  slowly  acted  upon  by  dilute  sulphuric  acid,  while 
another  portion  of  the  same  metal,  heated  nearly  to  redness,  and 
then  granulated,  is  readily  soluble  in  the  acid.  If  the  hot  metal 
be  poured  upon  a  warm  iron  plate,  it  will  be  found  to  be  still 
more  readily  soluble  in  acids  than  that  which  has  been  suddenly 
cooled  by  the  water. 

Exp.  310.  — Dry  20  grms.  of  the  granulated  zinc  of  Exp.  309, 
and  mix  it  intimately  in  a  mortar  with  40  grms.  of  crude  saltpetre  ; 
remove  the  empty  crucible  of  Exp.  309  from  the  fire,  and  place  it 
in  such  position  that  any  fumes  which  may  subsequently  be  evolved 
from  it  shall  be  drawn  into  the  chimne}7.  By  means  of  a  spoon  or 
ladle,  project  into  the  red-hot  crucible  the  mixture  of  zinc  and  salt- 
petre, taking  care  to  stand  away  as  far  as  possible  from  the  cruci- 
ble. The  metal  will  burn  fiercely,  at  the  expense  of  the  oxygen  in 
the  saltpetre,  for  the  most  part,  though  a  portion  of  it  will  be  vol- 
atilized by  the  intense  heat  of  combustion,  and  converted  into  oxide 
of  zinc  in  the  air.  The  residue  in  the  crucible  is  a  soluble  com- 
pound of  oxide  of  zinc  and  potash,  known  as  zincate  of  potassium. 

If  a  strip  of  thin  sheet  zinc  be  held  in  the  flame  of  the  gas  lamp, 
it  can  readily  be  burned  to  oxide.  The  experiment  succeeds  best 
with  zinc  leaf,  which  instantly  burns  with  a  vivid  flame  and  forma- 
tion of  floating  flocks  of  the  white  oxide.  In  oxygen  gas,  zinc 
burns  with  peculiar  brilliancy. 

Zinc  is  not  much  acted  upon  either  by  moist  or  dry  air,  at 
the  ordinary  temperature  ;  but  a  fresh,  bright  surface  of  it 


THE  GALVANIC  CURRENT.  505 

when  exposed  in  a  moist  atmosphere,  soon  tarnishes  and  be- 
comes covered  with  a  thin  film  of  basic  carbonate  of  zinc, 
which  adheres  closely  to  the  metal,  and  protects  it  from  fur- 
ther change.  Owing  to  this  durability,  the  metal  is  much  used 
in  the  form  of  sheets.  Sheet  iron  and  iron  wire  also,  are  often 
covered  with  a  protecting  coating  of  zinc,  and  are  then  said 
—  most  improperly — to  be  galvanized.  The  specific  gravity 
of  zinc  varies  from  6.8  to  7.3  ;  its  atomic  weight  is  65. 

593.  Zinc  is  readily  attacked  and  dissolved  by  acids,  with 
evolution  of  hydrogen  in  most  instances.  The  chemical  action 
of  dilute  acids  upon  zinc  is  a  very  common  source  of  that  pe- 
culiar mode  of  force  called  a  galvanic  current.  There  are  few, 
if  any,  chemical  reactions  which  cannot  be  made  to  produce 
electricity,  and  in  general,  the  more  powerful  the  chemical  ac- 
tion, the  more  powerful  is  the  electrical  action  which  results. 

Exp.  311.  —  Solder  a  piece  of  stout  copper  wire  to  one  end  of  a 
strip  of  sheet  zinc,  4  c.  m.wide  by  10  c.  m.  long.  The  soldering  will  be 
readily  effected  by  rubbing  the  zinc  and  the  wire,  in  the  vicinity  of 
the'  proposed  place  of  contact,  with  a  strong  solution  of  chloride 
of  zinc,  before  applying  the  melted  solder.  In  the  same  way,  solder 
a  similar  wire  to  a  like  strip  of  bright  sheet  copper.  Place  the  strips 
of  zinc  and  copper  in  a  tumbler  filled  with  water,  acidulated  with 
l-12tli  to  l-10th  its  volume  of  sulphuric  acid,  in  such  a  way  that  the 
two  strips  shall  not  touch  each  other  either  within  or  without  the 
liquid.  So  long  as  the  wires  coming  from  the  strips  of  metal  do 
not  touch  each  other,  the  copper  remains  quiescent,  while  the  zinc 
is  attached,  and  bubbles  of  gas  rise  from  its  surface  ;  but  if  the  two 
copper  wires  are  brought  into  close  contact,  by  means  of  a  binding- 
screw,  or  by  the  application  of  solder,  the  following  phenomena 
occur :  1st.  Minute  bubbles  of  hydrogen  gas  will  be  evolved  from 
the  surface  of  the  copper  plate.  2d.  The  zinc  dissolves  more -rapidly 
than  before,  and  at  the  close  of  the  experiment,  sulphate  of  zinc 
may  be  recovered  from  the  liquid  in  the  beaker.  3d.  This  transfer 
of  the  hydrogen  from  the  zinc  to  the  copper  instantly  ceases,  if  the 
contact  between  the  wires  is  destroyed.  4th.  If  the  two  wires  be 
connected  with  the  two  ends  of  the  coil  of  wire  which  surrounds  the 
magnetic  needle  of  the  common  galvanometer,  the  deflection  of  the 
suspended  needle  will  demonstrate  the  fact  that  an  electric  current 
is  passing  through  tlfe  wires  from  one  plate  of  metal  to  the 
other. 

34 


506  THE    LEAD    TREE. 

This  conversion  of  chemical  force  into  electrical  force  is  a 
striking  illustration  of  the  doctrine  that  all  physical  forces  are 
correlated. '  The  preceding  experiment  well  illustrates  the 
principle  on  which  a  large  class  of  batteries  employed  in  tel- 
egraphing and  in  electro-metallurgy  are  constructed  and 
worked,  except  that  the  corrosion  of  the  zinc  is  generally  hin- 
dered by  coating  it  with  mercury.  Artificial  products,  like 
metals,  acids,  and  saline  solutions,  are  used  to  supply  all  the 
chemical  force  which  is  immediately  converted  into  and  util- 
ized as  electrical  force  in  the  useful  arts.  We  have  not  yet 
succeeded  in  realizing  as  electricity  any  considerable  propor- 
tion of  the  prodigious  chemical  force  which  is  incessantly  ac- 
tive in  the  common  processes  of  combustion. 

594.  Zinc  dissolves  in  hot  solutions  of  the  caustic  alkalies 
as  well  as  in  acids  ;  hydrogen  is  given  off  and  a  zincate  of  the 
alkali  formed  :  — 

Zn  +  2NaHO  =  Na2ZnO2  -f  2H. 

When  immersed  in  the  solution  of  a  lead  salt,  such  as  the 
nitrate  or  acetate,  zinc  dissolves  and  lead  is  deposited  in  the 
metallic  state :  — 

PbN2O6  +  Zn  =  ZnN2O6  +  Pb. 

Exp.  312. — Dissolve  10  grms.  of  acetate  of  lead  in  250  c.  c.  of 
water,  add  a  few  drops  of  acetic  acid  in  order  to  dissolve  the  cloudy 
precipitate  of  carbonate  of  lead,  which  is  formed  from  the  carbonic 
acid  in  the  water,  pour  the  solution  into  a  wide  mouthed  bottle  and 
suspend  in  it  from  the  cork  a  strip  of  sheet  zinc.  The  mine  will 
soon  be  covered  with  a  brilliant  coating  of  crystalline  spangles  of 
metallic  lead,  and  this  crystalline  vegetation,  as  it  were,  will  shoot 
out  or  grow  even  as  far  as  the  sides  of  the  bottle.  In  the  course  of 
24  hours  all  the  lead  will  have  been  deposited  from  the  solution  and 
the  latter  will  contain  nothing  but  acetate  of  zinc.  Under  the  con- 
ditions of  this  experiment*  and  as  a  general  rule,  zinc  is,  chemically 
speaking,  a  stronger  or  more  basic  element  than  lead  ;  it  is  capable 
of  displacing  lead  from  its  compounds.  The  growth  of  lead,  wit- 
nessed in  this  experiment,  is  frequently  spoken  of  as  the  lead-tree; 
the  experiment  is  often  performed  in  chemical  laboratories  for  the 
sake  of  the  chemically  pure  leaxl  which  it  finishes. 

Many  other  metals  besides  lead  maybe  thus  thrown  down  by 


OXIDE    OF    ZINC.  507 

zinc,  and  the  zinc  may  itself  be  replaced  by  other  metallic  precipi- 
tants.  The  whole  series  of  experiments  of  which  the  one  here  indi- 
cated may  be  taken  as  the  type,  is  interesting  as  illustrating  the 
general  law  of  the  replacement  of  metals  one  by  another  in  atomic 
proportions,  and  from  the  fact  that  by  means  of  these  experiments, 
the  atomic  weight  of  various  metals  may  readily  be  determined. 
For  example,  if  in  the  foregoing  experiment  the  piece  of  zinc  be 
weighed  before  and  after  its  immersion  in  the  acetate  of  lead,  and 
if  the  precipitated  lead  be  also  weighed,  it  will  be  found  that  the 
weight  of  lead  obtained  is  to  the  weight  of  zinc  dissolved,  very 
nearly  as  207  is  to  65,  the  atomic  weights  of  lead  and  zinc  respec- 
tively. The  atom  of  zinc  dissolved  has  replaced  in  the  solution  the 
atom  of  lead  which  was  precipitated.  By  the  exercise  of  care  in 
the  manipulation,  by  employing  boiled  water  free  from  carbonic 
acid  so  that  the  addition  of  acetic  acid  to  the  lead  salt  shall  be  unnec- 
essary, and  by  finally  drying  the  lead  in  an  atmosphere  of  hydrogen, 
a  close  approximation  to  the  numbers  above  given  can  be  obtained. 

595.  Oxide  of  Zinc  (ZnO).     Like  magnesium,  zinc  forms 
but  a  single  compound  with  oxygen.     This  compound  may  be 
readily  obtained  by  burning  the  metal,  or  by  igniting  carbon- 
ate or  hydrate  of  zinc.     As  thus  prepared,  oxide  of  zinc  is  an 
insoluble,  white,  amorphous  powder,  which,  under  the  name  of 
zinc  white,  has  of  late  years  been  largely  employed  as  a  white 
paint.     It  lacks  the  opacity  or  covering  power  of  white  lead 
(§582),  but  on  the  other  hand  has  no  injurious  action  upon  the 
health  of  the  workmen  and  does  not  blacken  or  become  dis- 
colored when  exposed  to  the  fumes  of  sulphydric  acid.     When 
heated  in  a  crucible,  oxide  of  zinc  exhibits  a  3^ellow  color,  but 
it  becomes  white  again  on  cooling.     The  oxide  dissolves  easily 
in  acids,  with  formation  of  salts  of  zinc. 

596.  Chloride  of  Zinc  (ZnCl2),  obtained  by  dissolving  me- 
tallic zinc  in  chlorhydric  acid,  is  a  compound  readily  soluble 
in  water  ;  it  is  somewhat  extensively  employed  for  preserving 
timber,  and  as  a  disinfecting  fluid.     It  is  used  also  by  tinmen 
as  a  wash  to  cleanse  the  surfaces  of  tin-plate  before  soldering. 

597.  Sulphate  of  Zinc  (ZnSO4),  is  one  of  the  commonest 
of  the  zinc  salts.     The  hydrated  compound,  ZnSO4+ 7H2O, 
known  as  white  vitriol,  is  used  to  a  certain  extent  in  medicine, 


508  CADMIUM. 

and  for  other  purposes  in  the  arts.  The  action  of  carbon 
upon  sulphate  of  zinc  differs  somewhat  from  its  action  upon 
the  sulphates  previously  studied.  When  a  dry  mixture  of  the 
sulphate  of  zinc  and  charcoal  is  heated  to  dull  redness,  car- 
bonic and  sulphurous  acids  are  evolved  in  the  proportion  of 
two  volumes  of  the  former  to  one  of  the  latter  gas,  and  pure 
oxide  of.  zinc  remains  :  — 

2ZnSO4  +  C  =  2ZnO  +  2SO2  +  CO2. 

It  would  be  quite  possible  to  obtain  metallic  zinc  from  the 
sulphate  in  one  operation  by  employing  an  excess  of  carbon, 
heating  the  mixture  gently  at  first  until  the  sulphuric  acid  had 
all  been  decomposed,  and  then  urging  the  fire  in  order  to  ob- 
tain the  temperature  requisite  for  the  reduction  of  the  oxide 
of  zinc  and  volatilization  of  the  metal.  But  if  the  mixture 
of  sulphate  of  zinc  and  charcoal  be  quickly  raised  to  a  high 
temperature,  then  sulphide  of  the  metal  is  formed  and  carbonic 
acid  set  free  :  — 

ZnSO4  +  4C  =  ZnS  +  4CO. 

Zinc  forms  several  valuable  alloys  ;  brass  is  an  alloy  of  zinc 
and  copper,  and  German  silver  is  a  brass  whitened  by  the  ad- 
mixture of  a  small  proportion  of  nickel. 

CADMIUM. 

598.  Cadmium  is  a  comparatively  rare  metal,  found  associ- 
ated with  zinc  in  nature  ;  it  is  remarkably  similar  to  zinc  in 
its  chemical  relations.  In  the  process  of  obtaining  zinc  from 
its  ores,  the  small  proportion  of  cadmium  which  these  ores 
contain  comes  over  with  the  first  products  of  the  distillation, 
since  cadmium  is  more  readily  volatile  than  zinc. 

Cadmium  may  be  prepared  either  from  this  early  distillate  or 
from  the  residues  obtained  in  the  preparation  of  chloride  of  zinc 
for  manufacturing  purposes,  by  dissolving  metallic  zinc  in  chlorhy- 
dric  acid.  These  residues  always  contain  a  quantity  of  lead,  which 
next  to  cadmium  is  the  commonest  impurity  of  commercial  zinc, 
and  if  care  has  been  taken  to  keep  an  excess  of  metallic  zinc  in  the 
dissolving  vat,  they  will  contain  also  all  the  cadmium  with  which 
the  zinc  was  contaminated. 


PROPERTIES    OF    CADMIUM.  509 

From  either  of  these  sources,  cadmium  salts  may  be  prepared  by 
dissolving  the  crude  materials  in  dilute  nitric  acid,  separating  the 
lead  by  means  of  sulphuric  acid  and  throwing  down  the  cadmium 
with  sulphuretted  hydrogen.  Sulphide  of  cadmium  is  a  bright  yel- 
low powder,  insoluble  in  dilute  acids,  while  sulphide  of  zinc  is 
readily  soluble  in  acids.  Once  isolated,  the  sulphide  of  cadmium 
may  be  dissolved  in  boiling,  concentrated  chlorhydric  acid,  from  the 
solution  of  chloride  of  cadmium  thus  obtained,  carbonate  of  cadmium 
may  be  precipitated,  and  from  the  carbonate  any  of  the  other  cad- 
mium compounds  can  readily  be  prepared. 

Metallic  cadmium  is  of  a  white  color  tinged  with  blue ;  it  is 
lustrous  and  takes  a  fine  polish,  but  gradually  tarnishes  upon 
the  surface  when  exposed  to  the  air.  Its  specific  gravity  va- 
ries from  8.6  to  8.7.  It  melts  and  volatilizes  at  temperatures 
below  redness.  Heated  in  the  air  it  takes  fire  and  burns  to  a 
brown  oxide. 

When  combined  with  other  metals  such  as  lead  or  tin,  cad- 
mium forms  alloys  of  remarkably  easy  fusibility ;  in  this  re- 
spect it  far  surpasses  bismuth,  which  has  long  been  used  in  the 
preparation  of  fusible  metal  (§  359). 

599.  Cadmium  is  a  volatile  substance,  and  the  specific 
gravity  of  its  vapor  has  been  experimentally  determined  to  be 
56.85  ;  the  weight  of  a  unit-volume  of  the  vapor  is  56.85  times 
greater  than  the  weight  of  the  same  volume  of  hydrogen.  Now 
we  have  seen  that  the  specific  gravity  of  the  elementary  gases 
and  of  the  vapors  of  the  elements  included  in  the  chlorine  and 
sulphur  groups,  are  the  same  as  the  atomic  weights  of  these 
elements.  On  the  other  hand,  the  specific  gravities  of  the 
vapors  of  phosphorus  and  arsenic  were  twice  the  atomic 
weights  of  these  elements.  Cadmium  presents  still  a  new  re- 
lation between  the  least  combining  weight  and  the  unit-volume 
weight,  for  the  specific  gravity  of  its  vapor,  56.85,  is  about 
one-half  of  112,  its  accepted  atomic  weight.  The  significance 
of  this  fact  may  be  illustrated  from  its  chloride  ;  cadmium  is 
bivalent  and  forms  the  chloride  CdCl2  containing,  as  experi- 
ment has  proved,  112.24  parts,  by  weight,  of  cadmium  to  71 
parts  of  chlorine  ;  if  the  unit-volume  weight  of  cadmium  were 


510  THE    MAGNESIUM    GROUP. 

the  same  as  its  atomic  weight,  two  unit-volumes  of  chloride 
of  cadmium  would  contain  one  volume  of  cadmium  and  two 
volumes  of  chlorine  ;  but  were  it  possible,  by  experiment,  to 
resolve  the  vapor  of  chloride  of  cadmium  into  its  component 
vapors,  it  would  be  found  that  two  volumes  of  cadmium  were 
therein  combined  with  two  volumes  of  chlorine. 

The  atom  of  cadmium  when  converted  into  vapor  occupies 
twice  as  much  space  as  the  atom  of  oxj^gen,  or  hydrogen,  or 
chlorine  does,  and  accordingly  the  product  volumes  of  its  com- 
pounds are  packed  with  one  volume  more  than  the  product- 
volumes  of  the  corresponding  compounds  of  oxj'gen  or  any 
member  of  the  sulphur  group.  Whether  the  bivalent  metals 
in  general  resemble  cadmium  on  the  one  hand  or  oxygen  on 
the  other,  in  regard  to  the  relation  between  their  vapor-densi- 
ties and  their  atomic  weights,  is  a  point  on  which  experiment 
has  thus  far  thrown  but  little  light.  Mercury  resembles  cad- 
mium, but  it  is  certainly  possible  that  these  two  elements  con- 
stitute an  exception  to  some  general  rule  hereafter  to  be 
proved  ;  a  rule,  for  example,  like  that  which  many  chemists  are 
inclined  to  accept  in  advance  of  proof,  namely,  that  the  com- 
bining weights  and  the  unit-volume  weights  of  the  elements 
are  normally  identical. 

Cadmium  is  so  soft  that  paper  may  be  marked  with  it,  but  it 
is  flexible,  malleable,  and  ductile.  In  dilute  chlorhydric  and 
sulphuric  acids,  it  dissolves  with  evolution  of  hydrogen,  though 
less  readily  than  zinc.  Its  best  solvent  is  nitric  acid.  It 
does  not  dissolve  in  the  caustic  alkalies. 

600.  From  the  foregoing  it  is  apparent  that  the  members  of 
the  group  of  metals  now  under  consideration,  resemble  one 
another  with  respect  to  volatility,  and  several  other  of  their 
physical  properties,  besides  being  very  closely  related  in  most 
of  their  chemical  characters.  The  order  of  progression  fs  from 
magnesium  to  cadmium,  zinc  and  its  compounds  occupying 
always  an  intermediate  position.  The  specific  gravities  of  the 
three  metals  are:  Mg=1.75,  Zn  =  7.1,  Cd  =8.6  ;  and  their 
atomic  weights  are:  Mg  =  24,  Zn  =  65,  Cd  =  112.  Magne- 
sium volatilizes  at  a  bright  red  heat,  cadmium  at  a  low  red 


ALUMINUM. 


heat,  and  zinc  at  temperatures  between  these  extremes.  Cad- 
mium is  very  fusible,  melting  at  about  360°,  zinc  melts  at  425°, 
and  magnesium  at  a  moderate  red  heat.  All  of  these  metals 
are  bivalent ;  each  forms  but  one  oxide,  sulphide,  and  chloride. 


CHAPTER    XXX. 

ALUMINUM,  GLUCINUM,  CHROMIUM,  MANGANESE,  IRON,  COBALT, 
NICKEL,  and  URANIUM. 

ALUMINUM. 

601.  NEXT  to  oxygen  and  silicon,  aluminum  is  perhaps  the 
most  abundant  element  upon  the  earth's  surface.  It  is  the 
most  abundant  of  all  the  metals,  as  much  as  a  twelfth  of  the 
solid  crust  of  the  globe  being  composed  of  it.  It  occurs  in 
enormous  quantities  in  combination  with  oxygen  and  silicon, 
in  all  the  so-called  primitive  rocks,  and  for  that  matter,  in 
most  rocks  and  soils.  It  is  contained  in  clay,  marl,  and  slate, 
as  well  as  in  feldspar,  mica,  and  many  other  common  min- 
erals. 

Oxide  of  aluminum,  chloride  of  aluminum,  and  many  salts 
of  the  metal  may  readily  be  prepared  artificially  from  the  na- 
tive minerals ;  they  have  long  been  known  to  chemists,  and 
made  use  of  in  the  arts ;  but  the  rnetal  itself  is  less  readily 
obtainable.  It  is  but  a  few  years'  since  metallic  aluminum 
has  been  prepared  upon  a  manufacturing  scale.  The  metal  is 
nowadays  prepared  by  heating  metallic  sodium  either  with 
chloride  or  fluoride  of  aluminum,  or  with  a  double  chloride  or 
fluoride  of  aluminum  and  sodium.  -  It  is  bluish-white  metal, 
of  remarkable  lightness.  Its  specific  gravity,  2.56,  is  about 
the  same  as  that  of  porcelain,  and  only  about  a  quarter  of 
that  of  silver.  The  metal  is  malleable,  ductile,  and  tenacious, 


512  ALLOYS    OF   ALUMINUM. 

and  may  be  beaten  into  thin  sheets,  like  gold  and  silver,  and 
drawn  into  fine  wire.  It  melts  at  a  temperature  lying  between 
the  melting  points  of  zinc  and  silver,  but  is  not  volatile.  It 
conducts  electricity  much  better  than  iron,  and  heat  even  bet- 
ter than  silver ;  after  having  been  heated,  it  cools  very  slowly. 
It  is  remarkably  sonorous,  a  bar  of  it  suspended  by  a  wire 
rings  with  a  clear  musical  note  on  being  struck. 

In  the  air  aluminum  undergoes  no  alteration  even  at  a 
strong  red  heat ;  it  may  be  melted  in  open  crucibles  without 
oxidation,  and  readily  cast  into  any  desired  form.  It  is  not 
^acted  upon  by  water  at  temperatures  short  of  a  white  heat,  so 
long  as  it  is  in  the  ordinary  compact  condition.  Sulphydric 
acid  has  no  action  upon  it.  Nitric  acid,  whether  dilute  or 
concentrated,  has  no  action  upon  aluminum  at  the  ordinary 
temperature,  butvwhen  boiling,  dissolves  the  metal  slowly.  Cold 
dilute  sulphuric  acid  has  scarcely  any  action  upon  it ;  but  it  is 
easily  soluble  in  chlorhydric  acid,  either  dilute  or  concentrated, 
at  all  temperatures.  It  is  soluble  also  in  aqueous  solutions 
of  caustic  potash,  soda,  or  ammonia.  The  vegetable  acids, 
such  as  acetic  and  tartaric  acids,  exert  no  perceptible  action 
upon  it.  Although  soluble  with  evolution  of  hydrogen  in 
aqueous  solutions  of  the  fixed  caustic  alkalies,  aluminum  is 
not  acted  upon  by  fused  hydrate  of  sodium,  or  hydrate  of  po- 
tassium, nor  is  it  even  attacked  by  fused  nitrate  of  potassium, 
except  at  temperatures  high  enough  to  decompose  the  nitre  so 
completely  that  it  gives  off  nitrogen  ;  when  this  limit  is  reached 
the  aluminum  is  immediately  oxidized  with  incandescence. 

602.  Aluminum  unites  readily  with  many  of  the  metals  to 
form  alloys,  among  which  that  of  copper  and  aluminum,  called 
aluminum  bronze,  promises  to  be  of  especial  importance.     Alu- 
minum-bronze, composed  of  90  parts  copper  and  10  parts  alu- 
minum is  exceedingly  hard,  very  malleable,  as  tenacious  as 
steel,  of  a  beautiful   golden   color,  and   susceptible   of  being 
highly  polished. 

603.  By  uniting  with  the  non-metallic  elements,  aluminum 
forms  only  one  class  of  compounds,  of  which  the  oxide  A12O3 
may  be  taken   as  the  type.     The   atom  Al  is  trivalent,  or,  in 


OXIDE    OF    ALUMINUM.  513 

other  words,  it  is  equivalent  to  three  atoms  of  hydrogen,  and 
of  the  same  value  as  one  and  a  half  atoms  of  oxygen.  Since 
it  would  be  inconvenient  to  employ  fractional  expressions  in 
writing  chemical  formulae,  as  well  as  illogical  to  speak  of  half 
atoms,  it  is  customary  to  write  the  formula  of  oxide  of  alumi- 
num A12O3  as  above,  and  not  AlO^,  as  might  perhaps  at  first 
sight  seem  best.  For  the  sake  of  consistency,  the  formula  of 
the  chloride  is  in  like  manner  written  A12C16  and  not  A1C13. 
Since  it  contains  1£  times  as  much  oxygen  as  aluminum,  the 
oxide  is  often  called  a  Sesqui  (one  and  a  half)  oxide. 

If  no  other  element  analogous  to  aluminum  were  known,  if 
this  metal  were  not  intimately  related  to  glucinum,  iron,  chro- 
mium, and  the  other  metals  to  be  considered  in  the  present 
chapter,  chemists  might  possibly  have  taken  the  atomic  weight 
of  aluminum  at  two-thirds  of  its  present  value,  namely,  at 
18.3  instead  of  27.4.  The  formula  of  oxide  of  aluminum 
would  then  have  been  written  A12O,  and  that  of  chloride  of 
aluminum  A1C1,  corresponding  respectively  with  the  formulae 
of  the  oxides  and  chlorides  of  the  alkali  metals.  But  as  will 
appear  directly  from  the  study  of  the  other  members  of  the 
aluminum  family  of  metals,  and  particularly  from  the  isomor- 
phism of  their  various  compounds,  the  atomic  weight  27.4, 
and  the  formulae  first  given,  must  be  regarded  as  the  most 
probable.  The  atomic  weight  18.3,  and  the  formulae  derived 
from  it  are  inadmissible,  since  there  is  no  analogy  between 
the  chemical  properties  of  aluminum,  whether  simple  or  com- 
pounded, and  those  of  the  alkali  metals. 

604.  Oxide  of  Aluminum  ( A12O3) ,  commonly  called  Alumi- 
na, is  found  crystallized  in  nature  as  the  mineral  corundum. 
The  sapphire  and  the  ruby  are  also  composed  of  this  oxide,  to- 
gether with  a  little  oxide  of  iron.  It  may  be  prepared  artificial- 
ly by  oxidizing  the  metal,  or  by  igniting  the  hydrate,  or  almost 
any  oxygen  salt  of  aluminum.  Though  unalterable  in  oxygen 
so  long  as  it  is  compact,  powdered  aluminum  and  aluminum- 
leaf  burn  brightly  when  heated  to  redness  in  the  air,  with  for- 
mation of  oxide  of  aluminum.  It  has  been  found  by  careful 
experiments  that  53.3  parts  of  the  metal  unite  with  46.69  parts 


514  HYDRATE    OF    ALUMINUM. 

of  oxygen  to  form  100  parts  of  the  oxide.  Now,  since  oxide 
of  aluminum  is  isomorphous  with  certain  oxides  of  iron  and 
of  chromium,  which  are  known  to  be  sesquioxides,  and  is  ca- 
pable of  replacing  these  oxides  in  any  proportion  in  their 
compounds  (§252)  it  is  inferred  that  oxide  of  aluminum  is 
likewise  a  sesquioxide.  Upon  this  view  the  atomic  weight  of 
aluminum  is  directly  derived  from  the  foregoing  experimental 
data  by  the  equation  :  — 

46.69       :        53.3         =         48  :  54.8. 

wt.  of  3  atoms          wt.  of  2  atoms 
of  oxygen.  of  aluminum. 

605.  Hydrate  of  Aluminum  (AlglleOe),  may  be  obtained  as 
a  gelatinous,  flocculent  precipitate,  by  adding  ammonia  water 
to  the  solution  of  an  aluminum  salt,  such  as  common  alum. 
When  dried  at  a  moderate  heat  it  forms  a  soft,  friable  mass, 
which  adheres  strongly  to  the  tongue  like  clay  ;  when  dried 
still  more  thoroughly,  it  forms  a  hard,  yellowish,  translucent, 
horn-like  substance  ;  and  at  a  red  heat  gives  off  all  its  water. 
The  volume  of  the  original  precipitate  contracts  to  an  enor- 
mous extent  during  the  operation  of  drying;  the  bulk  of  the 
final  anhydrous  oxide  is  exceedingly  small  as  compared  with 
that  of  the  moist  hydrate  from  which  it  has  been  derived. 

606.  Anhydrous  alumina  may  be  melted  in  the  flame  of  the 
oxyhydrogen  blowpipe.     It  is  neither  decomposable  by  heat 
alone  nor  can  it  be  reduced  by  carbon,  or  any  of  the  more 
common  deoxidizing  agents.     At  a  white  heat,  potassium  de- 
composes it  partially,  and  an  alloy  of  aluminum  and  potassium 
is   formed.      Oxide  of  aluminum  is   insoluble   in   water  and 
after  having  been  strongty  heated  it  is  scarcely  at  all  acted 
upon  by   acids,  excepting   concentrated,  boiling  chlorhydric 
and  nitric  acids.     The  crystallized  native  oxide  is  insoluble 
in  all  acids.     The  anhydrous  oxide  is  insoluble  in  solutions 
of  the  caustic  alkalies,  but  dissolves  readily  in  water  after 
having  been  fused  at  a  red  heat  with  either  hydrate  or  car- 
bonate of  sodium  or  of  potassium.     Hydrate  of  aluminum,  on 
the  contrary,  though  insoluble  in  water,  dissolves  easily  in 
acids  and  in  solutions  of  the  fixed  caustic  alkalies.     Alumina 


ALUMINATES.  515 

is  in  fact  capable  of  acting  not  only  as  a  strong  base,  forming 
well  denned  salts  by  uniting  with  acids,  but  it  plays  the  part 
of  an  acid  as  well  (compare  §  350),  and  combines  with  the 
alkalies  and  with  other  metallic  oxides  to  form  salts  known 
as  aluminates.  Aluminate  of  potassium  (K20,A12O3)  and 
aluminate  of  sodium  (Na2O,Al2O3)  are  substances  somewhat 
extensively  used  in  the  arts  ;  the  mineral  spinelle  is  an  alum- 
inate of  magnesium  MgO,Al2O3  and  a  native  aluminate  of 
zinc  ZnO,Al2O3  is  called  gahnite  by  mineralogists. 

Exp.  313.  —  Heat  a  small  fragment  of  alum  with  water  in  a  test 
tube  until  it  has  completely  dissolved,  pour  half  .the  'solution  into 
another  tube,  and  add  to  it,  drop  by  drop,  ammonia  water,  until  the 
odor  of  ammonia  persists  after  the  mixture  has  been  thoroughly 
shaken.  Hydrate  of  aluminum  will  be  precipitated  in  accordance 
with  the  reaction  :  — 

A123SO4  +  6(NH4)HO  =  A12O3,3H2O  +  3(NH4)2S04. 

Pour  two  or  three  drops  of  the  moist  hydrate  of  aluminum  into 
another  test  tube  and  cover  them  with  ammonia  water ;  no  clear 
solution  will  be  obtained,  for  hydrate  of  aluminum  is  but  little  solu- 
ble in  ammonia  water. 

Pour  two  or  three  drops  of  the  moist  hydrate  of  aluminum  into 
still  another  test  tube,  and  cover  them  with  a  solution  of  hydrate  of 
sodium;  the  precipitate  will  dissolve  immediately;  aluminate  of 
sodium  is  formed,  and  this  salt  is  easily  soluble. 

Exp.  314. —  Take  another  portion  of  the  clear  solution  of  alum 
prepared  in  Exp.  313,  and  add  to  it,  drop  b}^  drop,  a  dilute  solution 
of  caustic  soda.  A  precipitate  will  soon  fall,  as  in  experiment  313, 
and  if  no  excess  of  hydrate  of  sodium  were  added,  over  and  above 
that  necessary  to  form  sulphate  of  sodium  with  the  sulphuric  acid 
of  the  alum,  this  precipitate  would  remain  undissolved,  but  on  add- 
ing  more  of  the  soda  solution  the  precipitate  dissolves  at  once, 
with  formation  of  aluminate  of  sodium. 

607.  Hydrate  of  aluminum  combines  readily  with  many 
vegetable  coloring  matters,  forming  compounds  which  are  in- 
soluble in  water.  The  fibre  of  cotton,  when  impregnated  with 
alumina,  can  be  made  to  retain  colors  which  the  cotton  itself 
has  no  power  to  hold,  hence  the  use  of  aluminum  salts  as  mor- 
dants in  dyeing. 

Exp.  olo.  — Boil  a  few  crushed  granules  of  cochineal  in  water 


516  CHLORIDE  OF  ALUMINUM. 

until  a  considerable  portion  of  their  coloring  matter  has  been  ex- 
tracted ;  add  to  the  filtered  solution  an  equal  bulk  of  a  solution  of 
alum,  and  to  the  mixture  add  ammonia  water.  A  colored  precipi- 
tate, consisting  of  hydrate  of  aluminum  and  of  the  coloring  matter  of 
the  cochineal,  will  be  thrown  down  ;  it  is  the  substance  called  car- 
mine-lake. Similar  precipitates  may  be  prepared  by  substituting 
almost  any  other  organic  coloring  matter  for  the  cochineal  of  this 
experiment.  Precipitates  thus  formed  by  the  union  of  a  metallic 
oxide  and  a  coloring  matter  are  all  classed  as  lakes. 

608.  Chloride  of  Aluminum  (A12C16),  may  be  prepared,  in 
/  the  same  way  that  the  chlorides  of  boron  and  silicon  are  pre- 
pared (§§  449,  470),  by  passing  chlorine  over  a  heated  mix- 
Hure  of  alumina  and  carbon.  It  is  formed  also  when  hot, 
..finely  divided  aluminum  is  brought  in  contact  with  chlorine 
gas.  Hydrated  chloride  of  aluminum  (A12C16 ;  12H2O),  can  be 
made  very  easily  by  dissolving  hydrated  oxide  of  aluminum  in 
chlorhydric  acid,  but  the  anhydrous  chloride  cannot  be  prepared 
by  heating  tbis  hydrate,  since  a  great  part  of  the  chlorine  is  ex- 
pelled from  it,  together  with  the  water,  at  a  low  heat.  When 
obtained  in  the  dry  way,  however,  chloride  of  aluminum  is 
readily  volatile.  The  anhydrous  chloride,  prepared  by  the  re- 
action, 

A1203  +  3C  +  6C1  =  A12C16  +  SCO, 

previously  described,  is  found  condensed  in  the  cold  portions 
of  the  tube  in  which  the  materials  have  been  heated,  apart  from 
the  residue  of  undecomposed  alumina  and  carbon.  It  occurs 
either  as  »  flocculent  powder,  or  as  a  transparent  wax-like 
mass  of  crystalline  texture.  It  is  colorless  when  pure,  very 
deliquescent  and  soluble  in  water.  When  large  masses  of  it 
are  heated  to  dull  redness,  a  portion  of  it  liquefies,  but  at  tem- 
peratures near  the  melting  point  it  volatilizes  rapidly.  Un- 
like oxide  of  aluminum,  it  may  be  readily  decomposed  by  . 
sodium  and  potassium  at  a  heat  below  redness,  metallic  alum- 
inum being  set  free. 

Chloride  of  aluminum  combines  readily  with  several  of  the 
other  metallic  chlorides,  forming  compounds  analogous  to  the 
chloride  of  aluminum  and  sodium  (2NaCl,  A12C16),  from  which 
metallic  aluminum  is  commonly  manufactured. 


SULPHATE     OF    ALUMINUM.  517 

609.  Sulphate  of  Aluminum  (A123SO4),  is  a  salt  largely 
employed  in  the  arts.  It  is  commonly  prepared  nowadays  by 
acting  upon  hot  roasted  clay  with  sulphuric  acid.  Clay  is  a 
silicate  of  aluminum  not  very  easily  attacked  by  acids  so  long 
as  it  remains  in  the  native  plastic  condition,  but  after  having 
been  exposed  for  some  time  to  a  dull  red  heat  it  readily  yields 
its  alumina  to  acids.  A  solid  mixture  of  sulphate  of  aluminum 
and  free  silicic  acid  obtained  as  the  product  of  this  reaction  is 
known  in  commerce  as  alum-cake.  By  lixiviating  alum-cake, 
sulphate  of  aluminum  may  readily  be  obtained  in  solution  ; 
and  from  this  solution  the  salt  crystallizes  as  a  hydrate,  the 
composition  of  which  may  be  represented  by  the  formula, 
A123SO4+18H2O. 

Until  a  comparatively  recent  period,  sulphate  of  aluminum 
has  been  sent  into  commerce  neither  in  its  free  state  nor  mixed 
with  silica,  but  in  combination  with  sulphate  of  potassium 
in  the  form  of  alum.  Common  alum  is  a  hydrated  double 
sulphate  of  aluminum  and  of  potassium,  the  composition  of 
which  may  be  represented  by  the  formula,  Al2K24SO4-f-  24H2O. 

It  crystallizes  very  easily  in  large,  compact,  well  denned 
octohedrons,  belonging  to  the  first  or  regular  system.  The 
crystalline  character  of  alum  is  important,  since  it  is  solely 
on  account  of  this  character  that  the  salt  has  come  into  such 
general  use.  Neither  the  sulphate  of  potassium  nor  the  water 
in  alum  plays  any  useful  part  in  the  reactions  for  which  this 
salt  is  commonly  employed.  Since  100  parts  of  alum  contain 
only  62  parts  of  sulphate  of  aluminum,  it  follows  that  38  per 
cent,  of  the  compound  is  to  all  chemical  intents  and  purposes, 
simply  inert  matter,  which  has  to  be  transported  and  manipu- 
lated for  the  sake  of  the  62  per  cent,  of  real  sulphate  of  alumi- 
jtium.  The  reason  why  this  waste  of  labor  and  loss  of  the  po- 
tassium salt,  is  tolerated  is  two-fold  :  —  Until  a  comparatively 
recent  period,  sulphate  of  aluminum  could  be  more  readily 
purified  by  crystallization  in  alum  than  in  any  other  way.  At 
the  present  time,  when  it  is  eas'y  to  obtain  pure  sulphate  of 
aluminum  from  responsible  manufacturers,  alum  is  still  pre- 
pared because  its  clean,  sharply-defined  crystals  afford  a  val- 


518  ALUM. 

uable  criterion  of  purity.  So  long  as  it  is  left  in  the  condition 
of  crystals,  alum  cannot  be  adulterated  with  any  foreign  sub- 
stance. 

Potash-alum  is  still  the  common  alum  of  the  American  mar- 
ket, but  in  Europe,  ammonia-alum  (A12(NH4)2  4SO4 ;  24H2O) 
is  at  present  largely  employed  ;  it  is  there  prepared  by  adding 
sulphate  of  ammonium  obtained  from  the  ammoniacal  liquor 
of  the  gas-works  to  sulphate  of  aluminum,  resulting  from  the 
action  of  sulphuric  acid  upon  clay.  Ammonia-alum  crystal- 
lizes almost  as  easily  as  potash-alum,  but  it  is  remarkable  that 
the  corresponding  double  sulphate  of  aluminum  and  of  sodium 
(soda-alum)  is  easily  soluble  in  water,  and  crystallizes  with 
comparative  difficulty.  Hence  it  has  never  come  into  com- 
merce. 

Sulphate  of  aluminum  is  employed  as  the  source  of  the  va- 
rious compounds  of  aluminum  used  in  dyeing,  calico-printing, 
and  paper-making.  Acetate  of  aluminum,  for  example,  is 
largely  employed  by  dyers,  particularly  for  the  red  colors  ob- 
tained from  madder,  under  the  name  of  red  liquor. 

Exp.  316. —  Dissolve  3  grams,  of  sugar  of  lead  in  4  c.  c.  of  hot 
water,  also  dissolve  4  grams,  of  common  alum  in  6  c.  c.  of  hot  water ; 
mix  the  hot  solutions  and  filter  off  the  insoluble  sulphate  of  lead 
which  is  formed.  The  solution  obtained  consists  of  basic  acetate  of 
aluminum,  together  with  some  sulphate  of  aluminum,  and  all  the 
sulphate  of  potassium  of  the  original  alum.  Such  solutions  are 
preferred  in  practice  to  those  containing  normal  acetate  of  alumi- 
num, to  prepare  which  a  much  larger  proportion  of  acetate  of  lead 
would  be  required  than  has  been  given  above. 

Exp.  317.  Soak  a  small  piece  of  cotton  cloth  in  the  solution  of 
acetate  of  aluminum  prepared  in  Exp.  316,  and  another  piece  of 
similar  cloth  of  equal  size  in  pure  water.  Hang  up  both  pieces  to 
"age,"  best  in  a  moist  and  warm  atmosphere,  for  a  day  or  two.. 
During  the  process  of  ageing,  a  portion  of  the  acetic  acid  escapes 
from  the  salt  on  the  cloth,  and  there  is  left  within  and  upon  the 
fibres  of  the  cloth  a  quantity  of  hydrate  of  aluminum,  or  at  least,  a 
mixture  of  highly  basic  acetatp  and  sulphate  of  aluminum.  This 
deposit  is  the  true  mordant.  When  cloth  impregnated  with  it  is 
soaked  in  a  solution  of  coloring  matter,  the  coloring  matter  unites 
with  the  alumina  precisely  as  in  Exp.  315,  and  is  thereby  firmly 


SILICATES    OF    ALUMINUM.  519 

attached  to  the  cloth.  It  should  be  remembered  that  several  other 
oxides,  besides  the  oxide  of  aluminum,  are  capable  of  acting  as 
mordants ;  the  sesquioxides  of  iron  and  of  chromium  for  example, 
as  well  as  the  binoxide  of  tin,  are  largely  used  as  mordants. 

Exp.  318.  Place  a  quantity  of  a  solution  of  extract  of  logwood  in 
two  small  evaporating  dishes,  heat  the  liquor  to  40  °  or  50  °,  then 
place  the  mordanted  cloth  in  one  dish,  the  unmordanted  cloth  in 
the  other,  and  boil  the  liquor  in  both  dishes.  Continue  to  boil  dur- 
ing 10  or  15  minutes,  then  take  out  the  pieces  of  cloth  and  wash  them 
thoroughly  in  water.  It  will  appear  that  the  coloring  matter  re- 
mains firmly  attached  to  the  mordanted  cloth,  while  the  cloth  which 
has  received  no  mordant  can  readily  be  washed  clean  or  nearly  clean. 

610.  Silicates  of  Aluminum.  Of  all  the  aluminum  com- 
pounds the  silicates  are  by  far  the  most  important.  Clay  in  all 
its  varieties  is  a  hydrated  silicate  of  aluminum,  usually  mixed 
with  an  excess  of  silica,  besides  other  impurities  derived  from 
the  rocks  from  whose  decomposition  the  clay  itself  has  been 
formed.  The  purer  kinds  of  clay,  such  as  kaolin  or  porcelain 
cla}r,  are  products  of  the  decomposition  of  feldspar,  a  mineral 
composed  of  silicon,  aluminum,  potassium,  and  oxygen,  in  the 
proportions  A12O3,  K20, 6SiO2.  When  exposed  to  the  atmos- 
phere, many  varieties  of  feldspar  gradually  decompose,  an  al- 
kaline silicate  is  washed  away,  and  silicate  of  aluminum 
remains.  Clay  is  remarkable  on  account  of  its  plasticity  when 
moist,  of  the  facility  with  which  it  is  converted  into  stone-like 
masses  when  strongly  heated,  and  of  its  infusibility  when  pure. 

Earthenware,  bricks,  and  ordinary  pottery  are  made  from 
common  clay,  b}^  mixing  the  clay  with  water  enough  to  form  a 
plastic  paste,  which  is  then  moulded  in-to  any  desired  form, 
dried  and  intensely  ignited.  The  porous  ware  resulting  from 
this  operation  may  be  glazed,  and  so  made  impermeable  to 
liquids  by  coating  it  over  with  some  fusible  substance,  such 
for  example  as  a  mixture  of  litharge  and  clay,  and  again  heat- 
ing it  so  intensely  that  the  coating  shall  melt  to  a  glass,  which 
either  fills  up  the  pores  of  the  clay,  or  at  the  least  stops  their 
openings.  Porcelain  proper,  ancf  the  better  kinds  of  stone- 
ware, are  made  from  the  purest  varieties  of  clay,  and  are  glazed 
with  feldspar.  Common  stoneware,  such  as  is  used  for  jugs, 


520  EARTHEN-WARE. 

beer-bottles,  and  the  like,  is  covered  with  the  so-called  salt- 
glaze  :  —  Moist  chloride  of  sodium  is  thrown  into  the  kiln  in 
which  the  ware  is  baking,  and  being  volatilized  by  the  intense 
heat,  comes  in  contact  with  the  hot  stoneware  ;  decomposition 
ensues  ;  the  water  and  the  chloride  of  sodium  are  both  decom- 
posed, silicate  of  sodium  is  formed,  and  by  mixing  with  the 
silicate  of  aluminum,  forms  a  smooth,  hard  glaze  upon  the 
surface  of  the  ware. 

For  all  vessels  which  are  to  be  employed  for  chemical  or 
culinary  purposes  the  hard  and  durable  salt  glaze  is  very 
much  to  be  preferred  to  the  lead  glaze  prepared  from  litharge 
and  clay,  for  the  lead  glaze  is  readily  acted  upon  by  many 
chemical  agents,  and  is  liable  to  impart  its  poisonous  proper- 
ties to  articles  of  food  which  have  been  left  in  contact  with 
it. 

Fire-bricks,  crucibles,  and  similar  refractory  articles  fitted 
to  support  very  high  temperatures  without  undergoing  fusion, 
are  prepared  from  pure  varieties  of  clay,  free  from  iron,  lime, 
or  magnesia,  but  containing  an  unusually  large  proportion  of 
silica.  Some  varieties  of  fire-clay  contain  as  much  silica  as 
is  represented  by  the  formula  Al2O3,6SiO2,  while  the  composi- 
tion of  many  of  the  common  clays  may  be  approximately  re- 
presented by  the  formula  Al2O3,3SiO2,  or  better  by  the  formula 
Al2O3,2SiO2.  In  the  manufacture  of  fire-bricks  and  of  many 
varieties  of  potters'  ware,  it  is  usual  to  incorporate  with  the 
original  clay  a  certain  proportion  of  foreign  matter  which 
shall  prevent  the  moulded  article  from  shrinking  too  much  as 
it  dries,  and  from  cracking.  In  fire-bricks  the  coarse  powder 
obtained  by  pulverizing  old  fire-bricks  is  employed  for  this 
purpose  ;  in  Hessian  crucibles  it  is  very  easy  to  detect  numer- 
ous grains  of  quartz  sand,  and,  in  general,  finely  powdered 
flint  or  quartz,  as  well  as  previously  baked  clay,  is  used  for 
the  same  purpose  in  many  varieties  of  pottery. 

611.  Silicate  of  aluminum  is  moreover  a  very  important 
ingredient  of  the  common  hydraulic  cement  employed  to  re- 
place lime  mortar  in  constructions  exposed  to  the  action  of 
water.  It  has  been  found  that  by  carefully  burning  some  va- 


HYDRAULIC    CEMENT.  521 

rieties  of  impure  limestone,  containing  from  10  or  12  to  30  or 
35  per  cent,  of  clay,  and  mixing  the  product  with  water,  there 
is  obtained,  in  place  of  ordinary  mortar,  a  cement  capable  of 
"  setting"  or  hardening,  even  under  w^ater,  to  a  compact  stone. 
Hydraulic  cements  may  readily  be  prepared  artificially  by 
mixing  with  quick-lime  a  suitable  proportion  of  roasted  clay, 
or  by  heating  mixtures  of  clay  and  limestone  ;  in  fact,  some  of 
the  best  cements  now  in  use  are  artificial.  A  porous  volcanic 
stone  called  puzzuolana,  from  the  vicinity  of  Naples,  consist- 
ing of  silicates  of  aluminum,  calcium,  and  sodium,  was  much 
used  by  the  Romans  to  the  same  end.  When  powdered  and 
mixed  with  ordinary  lime  the  puzzuolana  yields  an  excellent 
hydraulic  mortar.  In  many  Roman  ruins  it  may  be  seen 
to-day  far  more  perfectly  preserved  than  the  bricks  which  it 
cements. 

When  treated  with  water  Irydraulic  limes  simply  absorb  the 
water  and  form  a  slightly  plastic  paste  without  greatly  in- 
creasing in  bulk ;  they  do  not  slake  or  evolve  much  heat  like 
ordinary  quick-lime.  The  moist  paste  soon  begins  to  set,  and 
is  then  ready  for  application ;  in  order  that  the  cement  may 
harden  properly  under  water  it  should  not  be  submerged  be- 
fore it  has  begun  to  set ;  it  should  in  any  event  be  kept  moist 
until  it  has  become  hard,  otherwise  it  is  liable  to  remain  loose 
and  porous. 

The  solidification  of  hydraulic  limes  appears  to  depend  upon 
the  formation  of  insoluble  hydrated  compounds  of  lime  with 
silicic  acid  and  alumina.  Cements  which  contain  from  25  to 
35  per  cent,  of  clay  solidify  in  the  course  of  a  few  hours,  but. 
those  in  which  the  proportion  of  clay  is  no  more  than  10  or  12 
per  cent.,  become  hard  only  after  the  lapse  of  several  weeks. 

A  mixture  of  hydraulic  cement  with  coarse  gravel  consti- 
tutes the  material  known  as  concrete,  employed  for  the  founda- 
tions of  buildings,  and  by  the  Ancients  for  walls  which  have 
proved  to  be  of  great  durability  ;  this  mixture  soon  concretes 
or  hardens  into  a  firm  mass,  well  nigh  impermeable  to  water. 
The  influence  of  magnesia  in  the  preparation  of  hydraulic 
mortars  has  already  been  indicated  in  §588. 

36 


522  CHROMIUM. 


GLUCINUM. 

612.  GLUCINUM  is  a  rather  rare  metal,  found,  together  with 
aluminum,  in  the  emerald,  in  beryl,  and  a  few  other  minerals. 
It  closely  resembles  aluminum  in  its  physical  properties,  and 
forms  compounds  analogous  in  composition  to  those  of  alu- 
minum, and  of  similar  chemical  deportment.    Like  aluminum, 
metallic  glucinum  may  be  reduced  from  its  chloride  by  means 
of  sodium  or  potassium.     There  is  but  a  single  oxide  of  glu- 
cinum G12O3,  and  a  single  chloride,  G12C16.      The  salts  of  glu- 
cinum have  a  sweet  taste,  whence  the  name  from  a  Greek  word 
meaning  sweet.    The  atomic  weight  of  glucinum  is  14,  and  its 
specific  gravity  2.1. 

CHROMIUM. 

613.  CHROMIUM  is  nowhere  found  in  very  large  quantities, 
nor  is  it  very  widely  disseminated  in  small  portions  like  iodine 
and  fluorine,  but  it  is  nevertheless  found  in  sufficient  abun- 
dance to  admit  of  its  compounds  being  rather  extensively  em- 
ployed in  the  arts.    The  chief  ore  of  chromium  is  a  compound 
of  oxide   of  chromium   and   oxide  of  iron    (FeCr2O4)    called 
chrome  iron-ore.     Metallic  chromium  may  be  reduced  from  its 
oxide   by  means  of  intensely  heated  charcoal,  and  from  its 
chloride  by  means  of  sodium,  potassium,  magnesium,  or  zinc, 
but  it  has  as  yet  been  little  studied.     Its  specific  gravity  is 
about  7,  and  its  atomic  weight  52.5. 

614.  Oxides  of  Chromium.     Chromium  forms  three  well- 
defined  oxides  :  —  a  protoxide  CrO  ;  a  sesquioxide  Cr2O3 ;  and 
a  teroxide  CrO3  called  chromic  acid.     Besides  these  there  is  a 
compound  of  the  protoxide   and   sesquioxide   (Cr3O4=  CrO, 
Cr2O3),  another  of  the  sesquioxide  and  teroxide  (Cr2O3,  CrO3  = 
3CrO2),  and    an  ill-defined  compound  containing   more  oxy- 
gen than  chromic  acid,  usually  spoken  of  as  perchromic  acid 
(Cr2O7).     Both  the  protoxide  and  the  sesquioxide  are  bases, 
corresponding  respectively  to  oxide  of  magnesium  and  oxide 
of  aluminum,  but  the  protoxide  and  all  its  compounds  rapidly 
absorb  oxygen  from  the  air,  with  formation  of  the  sesquioxide. 
On  account  of  this  instability,  they  are  rarely  prepared,  and 


SULPHATE    OF    CHROMIUM.  523 

are  mainly  interesting  from  their  analogy  with  the  protoxides  of 
manganese,  iron,  cobalt,  and  nickel,  hereafter  to  be  studied. 
The  sesqoioxide,  on  the  other  hand,  is  a  stable  compound, 
closely  resembling  oxide  of  aluminum.  Chromic  acid  is  a 
strong,  well-characterized  acid,  which  combines  with  bases  to 
form  a  great  number  of  salts. 

The  most  important  of  the  chromium  compounds  is  the  bi- 
chromate of  potassium  ;  this  salt  is  readily  procurable  in  com- 
merce, and  is  the  source  from  which  all  the  other  compounds 
of  chromium  are  commonly  derived.  Bichromate  of  potas- 
sium is  itself  prepared  by  heating  finely  powdered  chrome-iron 
ore  with  carbonate  of  potassium  and  nitre  in  a  reverberatory 
furnace.  The  sesquioxide  of  chromium  of  the  ore  is  oxidized 
and  converted  into  the  teroxide,  chromic  acid,  which  displaces 
the  carbonic  acid  of  the  carbonate  of  potassium. 

615.  Sesquioxide  of  Chromium  (Cr203).    Compounds  of  this 
oxide  are,  next  to  those  of  chromic  acid,  more  commonly  met 
with  than  any  other  of  the   chromium    salts.     As  has  been 
stated  already,  compounds  of  the  protoxide  must  be  regarded 
merely  as  chemical  curiosities.     By  adding  ammonia-water  to 
the  solution  of  a  salt  containing  the  sesquioxide,  a  bulky,  green 
precipitate  of  hydrated  sesquioxide  of  chromium  is  thrown 
down,  which  when  collected  and  ignited,  leaves  the  anhydrous 
oxide  as  a  bright  green  powder,  unchangeable  at  the  highest 
furnace  heat.     It  is  employed  in  the  decoration  of  porcelain, 
and  is  a  valued  pigment  much  used  in  painting,  under  the  name 
chrome  green. 

616.  Chlorides   of  Chromium.     There   are   two   of  these . 
compounds,  the  protochloride  (CrCl2)  and  the  sesquichloride 
(Cr2Cl6).     The  latter  compound  is  the  more  important,  and  is 
the  substance  usually  meant  when  chloride  of  chromium  is 
spoken  of.     Hydrated  sesquichloride  of  chromium,  obtained  by 
dissolving  the  hydrated  sesquioxide  in  chlorhydric  acid,  is  the 
chloride  most  commonly  met  with. 

617.  Sulphate  of  Chromium  (Cr2O3,  3S03)  is  sometimes 
prepared  in  the  pure  state,  but  like  sulphate  of  aluminum  it 
ordinarily  occurs  in  combination  with  sulphate  of  potassium 


524  CHROMIUM    SALTS. 

or  sulphate  of  ammonium,  as  a  double  salt,  called  chrome 
alum.  Chrome  alum  is  a  compound  of  a  beautiful  violet  color, 
crystallizing  in  well  defined  octahedrons  of  the  same  form  as 
the  crystals  of  ordinary  alum ;  its  composition  also  corres- 
ponds to  that  of  common  alum,  the  formula  of  the  potassium 
salt  being  K2O,SO3 ;  Cr2O3>  3SO3  -f  24H2O. 

Exp.  319.  Dissolve  15  grms.  of  powdered  bichromate  of  potas- 
sium in  100  c.  c.  of  warm  water;  cool  the  solution,  and  then  add  to 
it  25  grms.  of  concentrated  sulphuric  acid  ;  cool  the  liquor  again, 
•and  pour  it  into  a  porcelain  dish,  surrounded  with  cold  water; 
slowly  stir  into  the  mixture  6  grms.  of  alcohol,  and  set  the  whole 
aside.  In  the  course  of  24  hours,  the  bottom  of  the  dish  will  become 
covered  with  well  defined,  octahedral  crystals  of  chrome  alum. 

The  alcohol  in  this  experiment  deprives  the  chromic  acid,  of  the 
bichromate  of  potassium,  of  half  its  oxygen,  and  is  itself  converted 
for  the  most  part  into  acetic  acid  and  water. 

618.  It  is  remarkable  that  the  salts  of  sesquioxide  of  chro- 
mium, as  well  as  the  oxide  itself,  occur  in  two  isomeric  condi- 
tions. One  modification  is  known  as  the  green,  the  other  as 
the  violet  modification.  As  a  rule,  the  violet  compounds  crys- 
tallize readily,  while  the  green  compounds  do  not.  In  the 
preparation  of  chrome  alum  it  is  important  to  guard  against 
the  formation  of  a  green,  soluble  sulphate  of  chromium,  which 
does  not  ciystallize.  In  general,  if  the  solution  of  a  salt  of 
the  violet  modification  is  heated  nearly  to  boiling,  the  salt 
passes  into  the  green  modification  and  becomes  uncrystalline. 
Hydrated  sesquioxide  of  chromium,  as  obtained  by  adding  a 
caustic  alkali  to  the  cold  solution  of  a  chromium  salt  of  either 
modification,  is  readily  soluble  both  in  acids  and  in  cold  solu- 
tions of  caustic  soda  or  potash  ;  but  on  boiling  the  green,  alka- 
line solution,  all  of  the  chromium  is  precipitated  as  a  l^drate 
of  the  green  modification. 

Exp.  320. — Place  in  a  test  tube  a  few  drops  of  a  dilute  solution 
of  chrome  alum,  or  of  some  other  salt  of  sesquioxide  of  chromium  ; 
add,  drop  by  drop,  a  solution  of  hydrate  of  sodium,  until  the  precipi- 
tate which  forms  at  first  is  completely  re-dissolved.  Boil  the  clear 
solution,  and  observe  that  the  precipitate  which  forms  in  the  liquor 
is  no  longer  soluble  in  alkalies. 


CHROMIC    ACID.  525 

By  means  of  this  reaction,  oxide  of  chromium  may  readily  be 
separated  from  oxide  of  aluminum,  for,  as  has  been  seen  in  Exp. 
314,  alumina  is  readily  soluble  in  alkaline  solutions,  and  is  not  pre- 
cipitated therefrom  by  boiling. 

619.  Chromic  Add  (CrO3)  may  be  obtained  by  decompos- 
ing bichromate  of  potassium  with  sulphuric  acid. 

Exp.  321. —  Mix  40  c.  c.  of  a  cold,  saturated,  aqueous  solution  of 
bichromate  of  potassium  with  50  c.  c.  of  oil  of  vitriol,  in  a  small 
beaker  standing  in  cold  water,  and  observe  that  chromic  acid  is  de- 
posited in  crystalline  needles.  It  is  remarkable  that  in  sulphuric  acid 
of  1.55  specific  gravity,  such  as  is  obtained  in  the  foregoing  luix- 
ture,  chromic  acid  is  well  nigh  insoluble,  though  it  is  readily  soluble 
both  in  water  and  in  strong  sulphuric  acid.  Cover  the  beaker,  and 
set  it  aside  for  some  hours ;  finally,  pour  off  the  supernatant  liquor 
with  care,  scrape  out  the  chromic  acid  with  a  glass  rod,  and  place 
it  upon  a  dry,  porous  brick,  under  an  inverted  bottle,  in  order  that 
the  sulphuric  acid  which  adhered  to  it  may  be  absorbed.  Preserve 
the  dry  crystals  in  a  glass-stoppered  bottle.  Chromic  acid  deli- 
quesces rapidly  when  exposed  to  the  air.  It  is  easily  brought  to  the 
condition  of  sesquioxide  of  chromium,  both  by  heat  and  by  reduc- 
ing agents,  and  is  hence  an  oxidizing  agent  of  considerable  power. 

Exp.  322.  —  Shake  up  enough  strong  alcohol  in  a  small  bottle  to 
moisten  its  sides ;  then  throw  in  half  a  gramme  or  less  of  chromic 
acid ;  a  portion  of  the  alcohol  will  be  oxidized  so  quickly,  and  with 
evolution  of  so  much  heat,  that  the  remainder  will  take  fire  and  burn 
in  the  air. 

Several  of  the  salts  of  chromic  acid,  as  well  as  the  acid 
itself,  are  employed  as  oxidizing  agents.  A  mixture  of  bi- 
chromate of  potassium  and  of  sulphuric  acid,  for  example,  is 
employed  for  bleaching  certain  fats.  From  the  chromates, 
both  oxygen  and  chlorine  may  be  conveniently  prepared. 

Exp.  323.  —  Heat  a  mixture  of  6  grms.  of  powdered  bichromate  of 
potassium  and  9  grms.  of  concentrated  sulphuric  acid  in  a  small 
flask,  provided  with  a  delivery-tube  leading  to  the  water-pan,  and 
collect  the  oxygen  which  is  freejy  evolved :  — 

K2O,  2CrO3+4(H2O,SO3)  =  Cr2O3,  3SO3  +  K2O,  SO3  + 
4H2O  +  3O. 

Exp.  324.  —  Place  a  mixture  of  1  grm.  of  powdered  bichromate  of 
potassium  and  6  grms.  of  chlorhydric  acid  of  1.16  specific  gravity, 


526  CHROMATES. 

in  a  flask  provided  with  a  delivery-tube,  as  in  Exp.  323.  Heat  the 
flask  gently  for  a  few  seconds  until  its  contents  begin  to  react  upon 
one  another ;  then  quickly  remove  the  lamp  and  attend  to  the  col- 
lection of  the  chlorine,  which  will  continue  to  be  evolved  without 
further  heating :  — 

K2O,  2CrO3  +  14HC1  =  Cr2Cl3  +  2KC1  +  7H2O  +  9C1. 

620.  Chromates.  As  has  been  already  indicated,  bichro- 
mate of  potassium  is  the  commonest  and  the  most  important 
salt  of  chromic  acid.  It  is  the  material  from  which  most  of 
the  other  compounds  of  chromium  are  prepared,  and  is  itself 
important  in  dyeing  and  calico  printing.  It  has  of  late  years 
been  used  in  the  art  of  photolithography. 

When  a  mixture  of  gelatine  and  bichromate  of  potassium  is  ex- 
posed to  light,  the  chromic  acid  is  reduced,  and  an  insoluble  com- 
pound of  gelatine  and  sesquioxicle  of  chromium  is  formed.  In  prac- 
tice, albumenized  paper  is  covered  in  a  dark  room  with  a  mixed 
solution  of  bichromate  of  potassium  and  gelatine,  then  dried, 
pressed  smooth,  and  kept  always  in  the  dark  until  wanted  for  use. 
If  a  sheet  of  this  prepared  paper  be  placed  beneath  a  negative 
photographic  picture,  obtained  in  the  usual  way  and  exposed  to 
light  for  a  short  time,  the-  chromic  acid  will  be  reduced  in  such 
wise  that  a  positive  picture  will  be  obtained  upon  the  gelatine  paper. 
In  this  positive,  as  taken  from  the  press,  the  parts  acted  upon  by 
the  light  will  be  brown,  while  the  other  portions  of  the  sheet  retain 
their  original  yellow  color.  The  positive  is  then  washed  with 
water  in  such  manner  that  the  unchanged  portions  of  gelatine  and 
of  bichromate  are  dissolved  away,  and  an  insoluble,  clearly  defined 
impression  of  the  original  picture  is  left  upon  the  paper.  By 
means  of  pressure,  the  design  is  then  transferred  to  the  lithographic 
stone,  and  from  the  stone  any  desired  number  of  copies  may  be 
printed  upon  paper  with  ink,  in  the  usual  way. 

Besides  the  bichromate  of  potassium,  there  are  several  other 
chromates  important  in  the  arts  or  useful  to  the  analyst.  The 
normal  chromate  of  potassium  (K2O,CrO3)  is  a  yellow  salt, 
readily  obtainable  by  adding  a  molecule  of  carbonate  of  potas- 
sium to  one  of  the  bichromate  :  — 

K20,  2Cr03  +  K20,  CO2  -2(K2O,CrO3)  +  CO2. 

It  is  isomorphous  with  normal  sulphate  of  potassium 
(K2SO4) ,  chromic  acid,  like  sulphuric  acid,  being  bibasic  (§  238) . 


MANGANESE.  527 

The  salt  is  hence  easily  adulterated.  Chromate  of  barium  is 
insoluble  in  water  or  in  "acetic  acid,  chromate  of  strontium  is 
soluble  in  acetic  acid,  though  nearly  insoluble  in  water,  while 
chromate  of  calcium  is  soluble  both  in  water  and  in  acetic 
acid ;  hence,  an  easy  method  of  separating  compounds  of 
either  of  the  three  metals  from  mixtures  which  contain  all  of 
them. 

Chromate  of  lead  is  the  pigment  called  chrome-yellow ;  it 
may  easily  be  prepared  by  mixing  solutions  of  bichromate  of 
potassium  and  acetate  of  lead.  An  orange-colored  bichromate 
(2PbO,  CrO3)  may  be  obtained  by  boiling  together  yellow 
chromate  of  lead  and  slaked  lime  in  the  proportion  of  two 
molecules  of  the  former  to  one  of  the  latter.  This  process  is 
used  to  fix  a  permanent  orange  upon  calico.  A  still  more 
brilliant  color  may  be  obtained  by  fusing  one  part  of  the  yel- 
low chromate  of  lead  with  five  parts  of  nitre  ;  chromate  of  po- 
tassium and  dichromate  of  lead  are  formed,  and  the  former 
may  be  washed  away.  Chromate  of  mercury,  of  a  brick-red 
color,  may  be  precipitated  by  adding  bichromate  of  potassium 
to  nitrate  of  protoxide  of  mercury,  or,' of  an  orange-yellow  color, 
by  adding  the  potassium  salt  to  nitrate  of  dinoxide  of  mercury. 

MANGANESE. 

621.  Black  oxide  of  manganese,  such  as  has  been  employed 
in  the  preparation  of  oxygen  and  of  chlorine  (§§  14, 105),  is  a 
tolerably  abundant  mineral.  Small  quantities  of  manganese 
exist  also  in  a  great  number  of  other  minerals  and  rocks,  so 
that  the  element  is  really  very  widely  diffused  in  nature.  It 
is  often  associated  with  ores  of  iron.  By  heating  oxide  of 
manganese  very  strongly  with  charcoal,  it  may  be  reduced  to 
the  metallic  state,  though  not  readily.  The  metal  is  of  a 
grayish-white  color,  and  is  very  hard  and  brittle.  It  oxidizes 
quickly  when  exposed  to  the  atmosphere  ;  it  melts  only  at  the 
strongest  heat  of  a  blast  furnace.  The  specific  gravity  of 
manganese  is  8,  its  atomic  weight  is  55.  It  slowly  decom- 
poses water  at  the  ordinary  temperature,  and  dissolves  readily 
in  dilute  sulphuric  acid  with  evolution  of  hydrogen.  Like 


528  OXIDES    OF    MANGANESE. 

iron,  it  combines  with  carbon  and  silicon.  Metallic  manganese 
is  not  used  in  the  arts,  and  the  alloys  wnich  it  forms  with  the 
other  metals  are  of  no  commercial  importance,  except  that 
a  small  proportion  of  manganese  is  present  in  a  peculiar  kind 
of  iron  largely  used  for  making  steel. 

622.  Oxides  of  Manganese.     Six  well  defined  compounds  of 
oxygen  and  manganese  are   known ;  two  of  them  are  bases, 
two  are  acids  and  two  may  be  regarded  as  salts,  formed  by 
the  union  of  the  oxides,  one  with  the  other.     Protoxide  of 
manganese,  MnO,  is  a   powerful   base,  while  sesquioxide  of 
manganese,  Mn2O3,  is  but  a  weak  base.    Manganic  acid,  MnO;{, 
and  permanganic  acid,  Mn2O7,  are  well  characterized  as  acids, 
though  they  are  known  onty  in  combination ;  they  have  never 
been  obtained  in  the  free,  anhydrous  state.    On  the  other  hand, 
binoxide  of  manganese  (Mn2O3,MnO3  =  3MnO2)  and  the  red 
oxide  (MnO,Mn2O3  =  Mn3O4)  are  both  neutral  or  indifferent 
bodies  ;  they  exhibit  neither  acid,  nor  basic  properties. 

623.  Protoxide  of  Manganese  (MnO)  may  be  obtained   by 
heating  carbonate  of  manganese  out  of  contact  with  the  air, 
or  by  heating  cither  of  the  higher  oxides  of  manganese  to  red- 
ness, in  contact  with  charcoal   or  hydrogen.     The  protoxide 
is    itself   reduced    to    the   metallic    state   by    these    agents 
only  at  a  wrhite  heat.     It  unites  freely  with   acids  to   form 
salts   of    considerable   stabilit}^.     The   crystallized   sulphate 
MnSO4  -j-  5H2O  and  the  chloride  MnCl2  -f-  4H2O  are  commonly 
employed  in  the  laboratory.     Both  of  them  may  be  prepared 
from  the  residues  obtained  in  the  preparation  of  chlorine  and 
oxygen    (§§  105,  626).      Hydrated   protoxide  of  manganese 
may  be  precipitated  from  the  chloride  as  follows  :  — 

Exp.  325.  —  Dissolve  a  small  crystal  of  chloride  of  manganese 
in  water ;  add  to  the  solution  soda  lye  until  the  liquor  exhibits  a 
distinct  alkaline  reaction,  when  tested  with  litmus  paper.  Collect 
the  gelatinous  white  precipitate  upon  a  filter,  and  observe  that  it 
soon  becomes  brown  as  it  absorbs  oxygen  from  the  air ;  the  brown 
product  is  sesquioxide  of  manganese. 

Exp.  326.  —  Heat  a  portion  of  the  precipitated  hydrate  of  Exp.  325 
to  redness  upon  a  fragment  of  porcelain  j  it  will  slowly  absorb  ox- 
ygen, and  change  to  the  deep  brown-colored  sesquioxide. 


SESQUIOXIDE    OF    MANGANESE.  529 

Exp.  327.  —  To  a  solution  of  chloride  of  manganese  such  as  was 
prepared  in  Exp.  325  add  a  few  drops  of  sulphydrate  of  ammonium 
(§526).  A  flesh-colored  precipitate  of  sulphide  of  manganese 
(MnS)  will  fall  down.  Like  the  hydrate  above  described,  this  pre- 
cipitate soon  becomes  brown  by  exposure  to  the  air.  It  is  often 
prepared  by  the  analyst  when  testing  for  manganese. 

624.  Sesquioxide  of  Manganese  (Mn2O3)  occurs  in  nature 
in  the  minerals  braunite  and  manganite.  It  is*  prepared  arti- 
ficially by  roasting  the  protoxide  obtained  from  chlorine  resi- 
dues, and  is  itself  used  to  a  considerable  extent  in  the  prep- 
aration of  chlorine  :  — 


+  6HC1  =  2MnCl2  +  3H2O+2C1. 

It  combines  with  acids  to  form  a  series  of  unstable  salts  an- 
alogous to,  though  far  less  permanent  than,  the  sesquisalts  of 
iron.  A  solution  of  the  sesquisulphate,  for  example,  Mn2O3, 
3SO3,  is  reduced  to  the  condition  of  protosulphate  by  mere 
boiling.  In  like  manner  the  sesquichloride  Mn2Cl6  is  doubtless 
formed  when  the  protocloride  is  treated  with  cold  chlorine,  or 
the  sesquioxide  is  digested  in  cold  chlorhydric  acid  ;  but  the 
salt  is  decomposed  with  extreme  readiness  and  splits  up  into 
free  chlorine  and  the  protochloricle,  even  when  but  slightly 
heated.  In  the  preparation  of  chlorine  from  the  sesquioxide 
as  above  formulated,  there  is  no  doubt  an  intermediate  reac- 
tion, 

Mn2O3  +  6HC1  =  Mn2Cl6  +  3H2O, 
before  the  final  breaking  up  of 

Mn2Cl6  into  2MnCl2  +  2Cl. 

625.  Of  the  salts  of  the  sesquioxide,  the  double  compound 
of  sulphate  of  manganese  and  of  potassium,  known  as  man- 
ganese alum,  is  one  of  the  most  interesting  ;  it  is  of  analogous 
composition  to  ordinary  aluminum  alum,  and  is  isomorphous 
with  this  body,  as  it  is  with  the  corresponding  alums  of  iron 
and  chromium.  The  series  of  double  salts  known  as  alums, 
admirably  illustrates  the  relationship  of  the  several  members 
of  the  group  of  metals  now  under  discussion,  and  the  law  of 
isomorphism  as  well.  It  is  interesting  to  observe,  moreover, 
that  the  name  alum,  originally  applied  specifically  to  the  com- 

37 


530  ALUMS. 

pound  of  sulphate  of  aluminum  and  of  potassium,  has  with 
the  growth  of  chemical  knowledge  come  to  have  a  generic  sig- 
nification. Several  salts  are  now  classed  as  alums,  into  the 
composition  of  which  neither  aluminum  nor  potassium  enters. 
The  following  list  enumerates  some  of  the  best  known  po- 
tassium alums :  — 

Common  alum  =K2SO4,  A123SO4  +  24H2O, 
Chrome  alum  =  K2SO4,  Cr23SO4  +  24H2O, 
Manganese  alum  =  K2  SO4,  Mn23SO4  +  24H2O, 
Iron  alum  =  K2SO4,  Fe23SO4  +  24H2O. 

But  as  has  been  stated  in  §  609,  the  potassium  in  these  com- 
pounds may  be  replaced  by  any  metal  isomorphous  with  potas- 
sium. There  are  ammonium  alums  and  sodium  alums  corre- 
sponding to  each  of  the  potassium  alums  above  enumerated, 
and  there  is  evidence  that  potassium  may  be  replaced  in  these 
alums  by  the  rarer  alkali  metals.  Some  alums,  on  the  other 
hand,  are  composed  of  mixtures  in  various  proportions  of  al- 
kali metals,  and  of  the  metals  capable  of  forming  sesquioxides. 
Besides  these  true  alums,  there  are  allied  bodies  which  contain 
no  alkali  metal  whatsoever ;  such,  for  example,  are  the  fol- 
lowing :  — 

Aluminum  iron  alum  =  FeSO4,  A123SO4  +  24H2O, 
Aluminum  magnesium  alum  =  MgSO4,  Al23SO4-f-  24H2O, 
Aluminum  manganese  alum  =  MnSO4,  A123SO4  -)-  24H2O, 
but  these  affiliated  alums  do  not  crystallize  in  the  octahedral 
form  which  is  characteristic  of  the  alums  proper. 

626.  Binoxide  of  Manganese  (MflO2)  is  a  black  compound 
found  abundantly  in  nature,  and  largely  employed  in  the  arts  for 
the  purpose  of  evolving  chlorine  from  chloride  of  sodium  or 
chlorhydric  acid  (§  105),  as  well  as  for  decolorizing  glass.  It 
may  readily  be  prepared  artificially  from  the  lower  oxides  by 
the  action  of  oxidizing  agents.  By  itself,  at  the  ordinary  tem- 
perature, binoxide  of  manganese  is  an  inert  chemical  sub- 
stance, though  at  higher  temperatures  it  has  considerable 
oxidizing  power.  At  a  strong  red  heat  it  gives  off  one-third 
of.  its  oxygen  :  — 

3MnO2  =  Mn3O4  +  2O. 


MANGANIC    ACID.  531 

Formerly^  oxygen  was  often  prepared  in  chemical  labora- 
tories by  heating  the  black  oxide  of  manganese  in  iron  retorts, 
but  the  process  has  long  been  superseded  by  more  convenient 
methods.  The  oxide,  Mn3O4  =  MnO,  Mn2O3,  which  is  left  as 
a  residue  in  this  experiment,  corresponds  in  composition  with 
the  magnetic  oxide  of  iron,  an  important  ore  of  iron.  This 
oxide  is  the  most  easily  obtained  by  artificial  means  of  all  the 
oxides  of  manganese  ;  it  is  produced  when  the  protoxide  or  its 
nitrate  or  carbonate  is  strongly  heated  in  the  air,  or  when 
either  of  the  higher  oxides  is  intensely  ignited. 

Black  oxide  of  manganese  is  insoluble  in  nitric  acid,  but  is 
decomposed  by  strong  hot  chlorhydric  acid  with  formation  of 
protochloride  of  manganese,  and  of  free  chlorine,  as  has  been 
already  explained  (§  105),  and  by  hot  concentrated  sulphuric 
acid  with  evolution  of  oxygen  :  — 

MnO2+  H2SO4  =  MnSO4  +  H2O  +  O. 

Exp.  328.  —  In  a  small  glass  flask,  provided  with  a  suitable  de- 
livery-tube, heat  a  mixture  of  15  grins,  of  powdered  black  oxide  of 
manganese  and  10  grms.  of  concentrated  sulphuric  acid,  and  collect 
the  gas  over  water  in  the  usual  way. 

After  all  the  available  oxygen  has  been  obtained  in  this  experi- 
ment, and  the  flask,  together  with  its  contents,  has  been  allowed  to 
cool,  pour  15  or  20  c.  c.  of  water  into  the  flask,  boil  the  mixture, 
pour  it  upon  a  filter,  and  evaporate  the  filtrate  to  dryness,  taking 
care  to  stir  it  constantly  when  nearly  dry.  Hydrated  sulphate  of 
manganese  (MnSO4 ;  4H2O)  will  be  obtained  as  a  reddish  white 
powder. 

Exp.  329. — For  the  sake  of  comparing  the  old  process  of  mak- 
ing oxygen  with  methods  now  in  use,  charge  an  ignition  tube,  such" 
as  was  used  in  Exp.  7,  to  one  third  of  its  capacity,  with  black  oxide 
of  manganese,  connect  it  with  the  water-pan  in  the  usual  way, 
heat  it  strongly  over  the  gas  lamp,  and  observe  the  comparatively 
slow  rate  at  which  oxygen  is  evolved  from  it. 

Manganic  Acid  (MnO3)  has  not  yet  been  obtained  in  the 
free  state  ;  it  is  known  only  as  it  occurs  in  combination  with 
potash  or  some  other  base.  Of  the  manganates,  those  of  po- 
tassium, sodium,  and  barium,  are  the  best  known  ;  they  are 
isomorphous  with  the  corresponding  chromates,  sulphates,  and 


532  CHAMELEON    MINERAL. 

seleniates.  The  alkaline  inanganates  are  important  com- 
pounds to  the  analyst. 

Exp.  330.  —  Place  upon  a  piece  of  platinum  foil  as  much  dry  car- 
bonate of  sodium  as  could  be  held  upon  half  a  pea ;  mix  with  it  an 
equal  quantity  of  powdered  nitrate  of  potassium  and  a  bit  of  bin- 
oxide  of  manganese  as  large  as  the  head  of  a  small  pin.  Fuse 
the  mixture  in  the  outer  blow-pipe  flame,  and  observe  the  bluish- 
green-colored  manganate  of  sodium  which  is  produced. 

Exp.  331. — Melt  together  in  an  iron  ladle  over  an  anthracite  or 
charcoal  fire,  10  grins,  of  hydrate  of  potassium  and  7  grms.  of  chlo- 
rate of  potassium ;  stir  into  the  pasty  liquid  8  grms.  of  very  finely 
powdered  black  oxide  of  manganese,  and  maintain  the  mixture  for 
a  short  time  at  a  temperature  just  below  visible  redness,  taking 
care  to  stir  it  frequently  with  an  iron  rod.  When  the  crumbly 
mass  has  become  cold,  place  some  of  it  in  a  test  tube  with  a  small 
quantity  of  cold  water  and  shake  the  tube.  As  soon  as  the  solid 
particles  have  settled,  there  will  be  seen  a  clear,  green  liquid,  which 
is  a  solution  of  manganate  of  potassium. 

Exp.  332.  — Pour  off  half  of  the  green  solution  of  manganate  of 
potassium  into  another  short  test  tube,  and  leave  it  open  to  the  air ; 
the  green  color  of  the  solution  will  gradually  change  to  blue,  then 
to  violet,  and  to  purple,  and  finally  to  ruby  red.  The  red  color  is 
that  of  a  solution  of  permanganate  of  potassium  into  which  the  man- 
ganate is  converted  by  exposure  to  the  air.  The  intermediate  col- 
ors are  merely  mixtures  of  the  manganate  green  and  the  perman- 
ganate crimson.  On  account  of  these  remarkable  changes  of  color, 
the  name  chameleon  mineral  has  been  applied  to  manganate  of  po- 
tassium, and  by  this  term  it  is  still  commonly  known. 

Manganate  of  potassium  is  a  very  unstable  salt,  especially  when 
in  solution ;  it  may  be  readily  decomposed  in  a  great  variety  of 
ways.  It  breaks  up  into  permanganate  of  potassium  arid  binoxide 
of  manganese,  when  the  aqueous  solution  is  mixed  with  a  large 
quantity  of  water,  and  even  strong  solutions  are  rapidly  decom- 
posed in  the  same  way  by  boiling  :  — 

3K2MnO4  +  2H2O  =  K2Mn2O8  +  MnO2  +  4KHO. 

By  means  of  acids,  the  change  from  manganate  to  permangan- 
ate may  be  almost  instantaneously  effected ;  but  by  the  presence 
of  an  excess  of  alkali  the  decomposition  is  always  greatly  retarded. 

Exp.  333.  —  Add  a  few  drops  of  sulphuric  acid  to  the  remaining 
portion  of  the  solution  of  manganate  obtained  in  Exp.  331,  and 


PERMANGANIC    ACID.  533 

observe  that  a  quantity  of  the  red  permanganate  of  potassium  is 
immediately  produced. 

628.  Permanganic   Acid    (Mn2O7),   or   rather  its  hydrate 
H2Mn2O8,  may  be  obtained  in  aqueous  solution  by  decomposing 
permanganate  of  barium  with  sulphuric  acid.     The    solution 
bleaches  powerfully,  and  the  acid  is  rapidly  destroyed  by  or- 
ganic matter  and  other  reducing  agents.     Of  the  compounds 
of  this  acid,  that  with  potassium  is  by  far  the  best  known. 

Exp.  334.  — Place  300  c.c.  of  water  in  a  porcelain  dish,  heat  it  to 
boiling  and  add  to  it  by  portions  the  remainder  of  the  powdered, 
green  manganate  of  Exp.  331 ;  from  time  to  time  add  small  portions 
of  hot  water  to  replace  that  which  evaporates,  and  continue  to  boil 
until  the  green  color  of  the  solution  has  changed  to  deep  violet  red, 
and  the  manganate  of  potassium  has  all  been  changed  to  permangan- 
ate. In  case  the  manganate  contains  a  large  excess  of  free  alkali  it 
cannot  readily  be  converted  into  permanganate  by  boiling ;  it  will 
therefore  often  be  found  necessary  to  neutralize  with  nitric  acid  a 
portion  of  the  alkali,  which  is  in  excess.  As  soon  as  the  transfor- 
mation has  been  completed  pour  the  mixture  into  a  tall  bottle,  leave 
it  at  rest  until  the  binoxide  of  manganese  and  other  insoluble  matters 
have  settled  ;  then  decant  the  clear  liquor  into  a  glass-stoppered  bot- 
tle, and  preserve  it  for  use  in  subsequent  experiments.  The  insoluble 
deposit  may  be  again  boiled  with  water  and  allowed  to  settle ;  the 
clear  liquor  thus  obtained  may  be  added  to  that  previously  prepared. 

In  order  to  obtain  crystals  of  the  permanganate,  a  clear  solution 
like  that  above  described  should  be  rapidly  evaporated  to  a  small  bulk, 
then  decanted  from  the  binoxide  of  manganese,  which  is  precipita- 
ted during  the  process,  and  set  aside  to  cool.  Needle-shaped  crys- 
tals of  a  dark  purple-red  color  will  soon  be  formed ;  they  are  solu- 
ble in  16  parts  of  water  at  15°,  and  are  permanent  in  the  air.  It  is 
well  to  purify  the  first  crop  of  crystals  by  washing  them  with  a 
little  cold  water,  then  dissolving  in  the  least  possible  quantity  of 
boiling  water,  and  again  crystallizing  in  the  cold.  Neither  the 
crystals  nor  the  solution  should  ever  be  brought  in  contact  with 
paper.  Decantation  will  ordinarily  be  sufficient  in  order  to  sepa- 
rate the  crystals  from  the  mother  liquor,  but  if  nitration  be  necessary 
in  any  case,  an  asbestos  filter  should  be  employed  (Appendix  §  1.4.) 

629.  The  permanganates  are  isomorphous  with  the  perchlor- 
ates  (Exp.  69),  and  the  potassium  salts  of  the  two  acids  are 
capable  of  crystallizing  together  in  all  proportions.     These 


534  IRON. 

compound  crystals  are  red-colored  when  they  contain  much 
perchlorate  of  potassium,  but  are  black  if  they  contain  as  much 
as  half  their  weight  of  the  permanganate. 

In  the  same  way  that  perchloric  acid  is  a  more  stable  acid 
than  chloric  acid,  so  permanganic  acid  is  less  readily  decom- 
posed than  manganic  acid.  Both  manganic  acid  and  perman- 
ganic acid,  however,  give  up  oxygen  to  other  substances  with 
remarkable  facility,  and  are  much  used  as  oxidizing  agents. 
Even  a  piece  of  wood  or  paper  thrown  into  the  green  or  red 
solution  of  a  manganate  or  permanganate,  will  quickly  ab- 
stract oxygen  from  the  solution  and  destroy  its  color.  In  fil- 
tering the  colored  solutions,  paper  is  consequently  inadmissi- 
ble, as  has  been  stated  in  Exp.  334  ;  asbestos,  sand,  or  some 
other  inert  filtering  material  must  be  resorted  to.  Perman- 
ganate of  potassium  is  largely  employed  for  disinfecting  pu- 
trid water,  as  well  as  animal  or  vegetable  matters  in  a  con- 
dition of  putrefaction.  A  solution  of  it,  such  as  has  been 
prepared  in  Exp.  334,  is  of  great  use  in  volumetric  analysis, 

especially  for  testing  the  value  of  ores  of  iron. 

• 

IRON. 

630.  ALTHOUGH  iron  is  one  of  the  most  widely  diffused  and 
most  abundant  of  the  metals,  it  is  rarely  found  native  in  the  me- 
tallic state.  Meteors,  however,  fall  upon  the  earth  from  outer 
space,  which  consist  mainly  of  metallic  iron,  contaminated  with 
several  other  elements  in  small  proportions.  Minerals  contain- 
ing iron  occur  in  great  numbers  ;  and  there  are  indeed  few  natu- 
ral substances,  whether  organic  or  inorganic,  in  which  iron  is 
not  present.  It  is  found  in  the  ashes  of  most  plants,  and  in  the 
blood  of  animals.  The  natural  compounds  of  iron  which  are 
available  as  ores  of  the  metal,  are  chiefly  oxides  and  carbon- 
ates. The  most  important  varieties  of  these  ores  of  iron  are 
the  following  :  —  1.  Magnetic  iron-ore,  the  richest  of  the  ores 
of  iron,  containing  when  pure,  72.41  per  cent,  of  iron,  and  not 
infrequently  approximating  closely  to  this  composition  in  large 
masses ;  2.  Red  Hcematite,  consisting,  when  pure,  of  anhy- 
drous sesquioxide  of  iron  containing  70  per  cent,  of  iron  ;  this 


IRON-ORES.  535 

ore  often  yields  from  60  to  69  per  cent,  of  the  metal ;  3.  Spec- 
ular iron-ore,  which  is  a  crystalline  form  of  the  same  anhy- 
drous sesquioxide  of  iron ;  4.  Limonite,  or  Brown  iron-ore, 
which  consists  essentially  of  hydrated  sesquioxide  of  iron,  con- 
taining 59.89  per  cent,  of  iron  ;  }rellow  ochre  is  a  clayey  vari- 
ety of  this  very  abundant  ore ;  the  numerous  ores  classed 
under  this  head  yield  from  25  to  55  per  cent,  of  iron ;  5. 
Spathic  iron-ore,  or  Carbonate  of  iron,  which  contains  in  its 
purest  state  48.27  per  cent,  of  iron,  but  is  so  generally  con- 
taminated with  manganese,  calcium,  and  magnesium,  as  to 
yield  very  various  quantities  of  iron  ranging  from  14  to  43  per 
cent.  6.  Clay  iron-ore,  a  name  applied  to  a  mixture  of  clay 
and  carbonate  of  iron,  which  occurs  very  abundantly  in  the 
coal  measures ;  as  this  ore  is  a  mixture  in  uncertain  propor- 
tions, it  yields  various  percentages  of  iron,  ranging  from  25  to 
40  per  cent. 

From  the  richer  iron-ores,  like  the  magnetic  and  specular 
oxides,  a  very  excellent  iron  can  be  obtained,  by  simply  heat- 
ing the  broken  ore  with  charcoal  in  an  open  forge  fire,  urged 
by  a  blast.  The  ore  is  deoxidized  by  the  carbon  of  the  fuel, 
and  the  reduced  iron  is  agglomerated  into  a  pasty  lump  called 
a  "  bloom,"  while  the  earthy  impurities  contained  in  the  ore 
combine  with  a  portion  of  the  oxide  of  iron  to  form  a  fusible 
glass  or  slag.  The  spongy  bloom  is  freed  from  slag  and  ren- 
dered homogeneous  and  solid  by  hammering  while  still  red- 
hot  ;  by  reheating  and  hammering,  the  iron  is  then  converted 
into  bars  or  shaped  into  any  other  desired  form.  This  process 
is  not  economical  in  the  chemical  sense,  for  much  iron  is  lost 
in  the  slag,  and  much  fuel  is  burnt  to  waste  in  an  open  fire, 
but  when  well  conducted,  it  yields  an  admirable  quality  of  iron, 
and,  since  the  original  outlay  for  the  construction  of  a  bloom- 
ary  is  small,  and  repairs  upon  it  are  always  easy,  the  method 
has  many  advantages  in  regions  where  transportation  is  dear, 
while  rich  ores,  charcoal,  and  water-power  abound.  The  bloom- 
ary  process,  in  its  crudest  form,  is  easily  practised  by  people 
possessing  but  little  mechanical  skill  and  no  chemical  knowl- 
edge ;  it  is  undoubtedly  the  oldest  method  of  extracting  iron 
from  its  ores. 


536  THE    BLAST    FURNACE. 

631.  In  the  extraction  of  iron  from  its  common  ores,  the 
metal  is  usually  obtained,  not  pure,  but  in  a  carburetted  fu- 
sible state,  known  as  cast-iron  or  pig-iron.  The  main  features 
of  the  process  are,  first,  a  previous  calcination  or  roasting  to 
expel  water,  carbonic  acid,  sulphur  and  other  volatile  ingredi- 
ents of  the  ore  ;  secondly,  the  reduction  of  the  oxide  of  iron 
to  the  metallic  state  by  ignition  with  carbon  ;  thirdly,  the  sep- 
aration of  the  earthy  impurities  of  the  ore  by  fusion  with  other 
matters  into  a  crude  glass  or  slag ;  and  lastly,  the  carbonizing 
and  melting  of  the  reduced  iron.  With  the  purer  kinds  of  iron- 
ore,  the  preliminary  calcination  is  not  always  essential,  but 
with  the  majority  of  ores  it  is  very  desirable  ;  not  unfrequently 
all  the  drying  necessary  is  effected  in  the  upper  part  of  the 
blast-furnace  itself,  within  which  the  three  last  steps  of  the 
process  always  take  place. 

The  blast-furnace  for  iron  consists  essentially,  of  a  huge 
cylindrical  structure  of  masonry,  15  to  25  m.  in  height,  and 
5  to  6  m.  in  diameter  at  the  central  portion  of  the  cylinder, 
but  contracted  to  a  less  diameter  both  at  the  top  or  throat  and 
at  the  bottom  or  hearth.  Air  is  forced  in  at  the  bottom  of  the 
furnace  to  support  the  combustion,  and  it  has  been  found  ad- 
vantageous in  the  majority  of  cases  to  heat  this  blast  of  air  to 
about  the  melting  point  of  lead  before  it  enters  the  furnace. 
The  reduction  of  the  oxides  of  iron  being  effected  by  the  car- 
bonic oxide  resulting  from  the  combination  of  carbonic  acid 
with  hot  carbon  (Exp.  185),  it  is  not  difficult  to  calculate  the 
amount  of  carbon  and  the  amount  of  air  requisite  to  reduce  to 
metallic  state  the  iron  contained  in  a  given  weight  of  an  iron- 
ore  of  known  composition.  Thus,  the  formula  of  specular  iron- 
ore,  or  of  red  haematite,  is  Fe2O3,  and  since  the  atomic  weight 
of  iron  is  56,  these  ores  are  70  per  cent,  iron  ;  accordingly  the 
following  quantities  are  equivalent  one  to  the  other  :  — 

1          =         1.429          =  0.3214  0.4285  =         1.863. 

Iron  Fe2<)3  Requisite  weight          Weight  of  oxygen  Air. 

of  carbon  in  requisite  to  convert 
state  of  CO.  so  much  C  to  CO. 

For  every  kilogramme  of  iron  produced,  nearly  two  kilo- 


THE    BLAST   FURNACE.  537 

grammes  of  air  must  be  supplied,  and  at  least  ^  kilogramme 
of  fuel,  merely  to  accomplish  the  chemical  reaction.  The 
reduction  of  the  oxide  of  iron,  however,  is  not  alone  sufficient 
to  secure  the  metal ;  iron  ores  almost  always  contain  earthy 
admixtures,  consisting  chiefly  of  silica,  clay,  and  carbonate  of 
calcium,  and  these  substances  are  so  intimately  mixed  with 
the  reduced  metal,  that  it  is  essential  to  melt  them  before  the 
iron  can  separate  by  virtue  of  its  greater  specific  gravity. 
Any  one  of  these  substances  taken  alone  is  infusible  at  the 
temperature  of  the  furnace  ;  they  must  be  converted  into  fus- 
ible double  silicates,  and  as  it  is  rarely  the  case  that  the  nat- 
ural impurities  of  an  ore  are  present-  in  the  proportions  requisite 
for  the  formation  of  such  double  silicates,  it  is  generally  nec- 
essary to  mix  with  the  ore  a  substance  intended  to  effect  this 
result,  and  therefore  called  the  flux.  With  ores  in  which  the 
earthy  admixture  is  chiefly  calcareous,  the  flux  must  be  clay 
or  some  siliceous  material,  but  in  the  more  frequent  case  of 
ores  containing  clay  or  silica,  the  flux  will  be  limestone  or 
quicklime.  In  either  case,  a  fusible  double  silicate  of  alu- 
minum and  calcium  is  the  essential  constituent  of  the  slag. 
With  siliceous  ores  there  is  another  reason  for  the  addition  of 
lime  ;  the  double  silicate  of  aluminum  and  iron  is  very  fusible, 
and  a  considerable  quantity  of  iron  might  be  lost  in  the  slag, 
were  not  lime  enough  added  to  prevent  the  formation  of  this 
iron-containing  silicate.  Sometimes  both  calcareous  and  sili- 
ceous ores  are  within  reach  of  an  iron-furnace,  and  the  smelter, 
by  mixing  the  two  varieties  in  due  proportion,  may  avoid  the 
necessity  of  adding  a  flux. 

The  blast  furnace  is  charged  at  the  top  with  alternate  layers 
of  the  fuel,  which  may  either  be  charcoal,  anthracite,  or  coke, 
the  ore,  and  the  flux,  which  is  generally  lime  ;  these  materials  are 
constantly  supplied  at  th£  top,  and  air  is  constantly  supplied  in 
immense  quantities  at  the  bottom  of  the  furnace,  the  actual 
weight  of  the  air  forced  in  being  greater  than  the  sum  of  the 
weights  of  the  ore,  the  fuel,  and  the  flux.  Where  the  blast 
first  touches  the  ignited  fuel,  carbonic  acid  is  formed  ;  this  gas, 
rising  with  the  unused  nitrogen  through  the  furnace,  comes  in 


538  CAST-IRON. 

contact  with  white  hot  carbon,  and  is  reduced  to  carbonic 
oxide  (Exp.  185).  The  layers  of  solid  material  thrown  in  at 
the  top  of  the  furnace  gradually  sink  down,  and  as  soon  as  a 
stratum  of  ore  has  descended  sufficiently  to  be  heated  by  the 
hot  mixture  of  nitrogen  and  carbonic  oxide  it  becomes  re- 
duced to  spongy  metallic  iron,  which,  mixed  with  the  flux  and 
the  earthy  impurities  of  the  ore,  settles  down  to  hotter  parts 
of  the  furnace,  where  it  enters  into  a  fusible  combination  with 
carbon,  while  the  flux  and  earthy  impurities  melt  together  to  a 
liquid  slag.  The  liquid  carburetted  iron  settles  to  the  very 
bottom  of  the  furnace,  whence  it  is  drawn  out,  at  intervals, 
through  a  tapping-hole  which  is  stopped  with  sand  when  not  in 
use.  The  viscous  slag  flows  out  over  a  dam,  so  placed  as  to 
retain  the  iron,  but  to  prevent  the  escape  of  the  slag  which 
floats  on  the  iron,  as  fast  as  it  accumulates  in  sufficient  quan- 
tity. The  fusion  of  the  materials  in  the  lower  part  of  the 
furnace  requires  a  great  heat,  and  the  amount  consumed  in 
getting  this  high  temperature  is  much  greater  than  the  amount 
requisite  for  the  reduction  and  carbonization  of  the  metal.  As 
charcoal  is  a  much  purer  Carbon  than  coal  or  coke,  iron 
smelted  with  charcoal  is  generally  purer  than  that  smelted  with 
coal ;  but  as  charcoal  crumbles  under  great  pressure,  the  fur- 
naces in  which  charcoal  is  used  are  usually  much  smaller  than 
those  intended  for  anthracite  or  coke.  The  consumption  of 
fuel  in  smelting  1000  k.  of  iron  varies  with  the  nature 
of  the  furnace,  the  blast  and  the  fuel,  between  500  k.  and 
3000  k.  The  gases  which  issue  from  the  mouth  of  the  blast- 
furnace are  charged  with  an  enormous  heating  power,  for 
besides  being  themselves  intensely  hot  they  contain,  even 
after  having  effected  the  reduction,  a  large  proportion  of  com- 
bustible gases,  such  as  carbonic  oxide,  carburetted  hydrogen, 
and  hydrogen.  This  gaseous  mixture  takes  fjre  whenever  it 
comes  in  contact  with  the  air ;  a  part  of  its  heat  may  be  util- 
ized in  heating  the  air-blast  and  generating  steam. 

Two  distinct  varieties  of  cast-iron  exist  which  differ  in 
color,  texture,  and  fusibility ;  these  are  white  cast-iron  and 
gray  cast-iron.  White  iron  is  hard  and  brittle,  of  crys- 


IMPURITIES    OF    IRON.  539 

talline  texture  and  shining  fracture.  Gray  iron  is.  slightly 
malleable,  and  has  a  granular  texture ;  its  fracture  may  be 
either  coarse  or  fine-grained,  and  minute  particles  of  black 
graphite  are  visible  upon  the  broken  surface.  White  cast-iron 
melts  at  a  lower  temperature  than  gray, -but  does  not  bscome 
so  liquid  as  the  gray.  Gray  cast-iron,  when  rapidly  cooled, 
is  converted  into  white  iron ;  when  a  casting  is  made  in  an 
iron  mould,  the  layer  of  metal  in  contact  with  the  mould  is 
chilled  and  converted  into  the  hard  white  iron,  while  the  in- 
terior of  the  casting  will  retain  the  condition  of  the  stronger 
gray  iron.  Excellent  shot  and  shell  for  rifled  cannon  have 
lately  been  cast  on  this  plan.  The  chief  chemical  difference 
between  white  and  gray  cast-iron  consists  in  the  different 
condition  of  the  admixed  carbon.  In  white  iron  the  carbon 
seems  to  be  dissolved  in  or  combined  with  the  iron,  while  in 
gray  cast-iron,  on  the  other  hand,  the  great  part  of  the  carbon 
seems  to  be  mechanically  diffused  through  the  solid  iron,  in 
the  state  of  graphite.  The  two  varieties,  however,  shade  off 
into  each  other  through  a  great  variety  of  intermediate  mix- 
tures. White  cast-iron  rusts  much  more  slowly  than  gray 
cast-iron.  When  white  iron  is  heated  with  strong  chlorhydric 
acid  it  entirely  dissolves,  but  the  combined  carbon  enters  into 
combination  with  a  portion  of  the  nascent  hydrogen,  forming 
hydrocarbons  which  impart  a  peculiar  smell  to  the  gas  evolved. 
Gray  iron  does  not  wholly  dissolve  in  hot  chlorhydric  acid  ; 
a  residue  of  graphite  remains,  but  the  gas  evolved  has  the 
same  smell  as  the  gas  evolved  from  white  iron.  The  largest 
proportion  of  carbon  found  in  cast-iron  is  5.75  per  cent. ;  this 
large  percentage  occurs  in  a  lustrous  variety  of  white  iron 
which  contains  manganese  and  is  called  specular  iron.  In 
gray  iron  the  amount  of  carbon  varies  from  2  to  nearly  5  per 
cent. 

Silicon,  sulphur,  phosphorus,  manganese,  and  copper,  are 
very  common  impurities  in  cast-iron.  The  silicon  comes  from 
silica  deoxidized  in  the  furnace ;  its  amount  varies  from  0.1 
to  3.5  per  cent.  There  is  more  of  it  in  gray  than  in  white 
iron,  and  more  in  hot  blast  iron  than  in  cold  blast.  Sulphur 


540  WROUGHT-IRON. 

is  almost  always  present  in  cast-iron,  but  only  in  very  small 
quantity ;  its  presence  is  supposed  to  conduce  to  the  forma- 
tion of  white  iron.  The  presence  of  phosphorus  to  the  extent 
of  1  or  2  per  cent,  is  not  uncommon,  and  does  not  injure  iron 
intended  for  castings,  inasmuch  as  the  phosphorus  makes  the 
iron  more  fusible,  and  more  liquid  when  melted.  Manganese 
is  frequently  present  in  cast-iron,  as  is  not  unnatural  con- 
sidering the  common  association  of  manganese  ores  with  iron 
ores.  Cast-iron  containing  manganese  appears  to  be  espe- 
cially suitable  for  the  production  of  steel. 

The  production  of  malleable  or  "  wrought  "-iron  from  cast- 
iron,  consists  essentially  in  burning  out  the  carbon,  silicon, 
sulphur,  and  phosphorus,  which  cast-iron  contains.  This  oxi- 
dation of  the  impurities  of  cast-iron  is  effected  either  by  blow- 
ing upon  the  melted  metal  with  an  air  blast  in  a  small  char- 
coal furnace  called  a  "  finery,"  or  by  stirring  the  melted  iron 
in  a  reverberatory  furnace  in  which  the  fuel  does  not  come  in 
contact  with  the  metal,  and  into  which  air  can  be  admitted  at 
will ;  the  lattfer  process,  now  much  the  most  important  method 
of  manufacturing  wrought  iron,  is  called  "puddling."  In 
puddling,  it  is  customary  to  add  to  the  charge  of  pig-iron  a 
quantity  of  iron  scale  or  other  oxide  of  iron.  The  oxidation 
of  the  silicon,  carbon,  phosphorus,  and  other  impurities 
is  effected  partly  by  the  air  and  partly  by  the  oxide  added 
to  the  charge ;  the  carbon  burns  to  carbonic  oxide,  which 
heaves  the  seething  mass  as  it  escapes  and  burns  in  jets  of 
blue  flame  ;  the  other  impurities  form,  with  the  oxides  of  iron 
and  manganese,  a  cinder  or  slag  which  ordinarily  contains  sul- 
phur, silicic  acid,  and  phosphoric  acid.  When  the  cast-iron  is 
so  far  decarbonized  as  to  be  pasty  in  the  fire,  it  is  gathered 
into  lumps  on  the  end  of  an  iron  bar  and  carried  from  the  fur- 
nace to  a  hammer  or  squeezer  which  expresses  the  liquid  slag 
and  welds  into  a  coherent  mass  the  tenacious  iron.  The  ham- 
mered lump  may  be  reheated  and  rolled  or  forged  into  any 
desired  shape.  The  waste  of  iron  in  converting  cast  into 
malleable  iron  amounts  to  from  13  to  30  per  cent. 

Ordinary  malleable  iron    has  a  gray  color,  and  a  specific 


STEEL.  541 

gravity  of  about  7.6.  Though  less  malleable  than  gold  and 
silver,  its  malleability  is  very  great,  and  the  greater  the  purer 
the  metal,  and  the  higher  the  temperature  to  which  it  is  raised. 
At  a  red  heat,  separate  pieces  may  be  firmly  united  by  ham- 
mering or  rolling  ;  the  operation  is  called  welding.  Sulphur 
is  said  to  render  wrought-iron  brittle  or  "  red-short "  while 
hot ;  silicon  and  phosphorus  render  iron  brittle  at  the 
ordinary  temperature,  or  "  cold-short,"  in  technical  phrase- 
ology. These  common  impurities  of  cast-iron  are,  therefore, 
very  prejudicial  to  wrought-iron.  Wrought-iron  is  ham- 
mered or  rolled  while  in  a  doughy  condition,  and  the  uniform, 
close,  fibrous  texture  which  is  valued  in  malleable  iron,  de- 
pends much  upon  the  nature  of  this  mechanical  treatment,  and 
the  extent  to  which  it  is  carried.  Common  malleable  iron  still 
contains  from  0.25  to  0.5  per  cent,  of  carbon  ;  the  smaller  the 
amount  of  carbon  the  softer  the  iron.  Wrought-iron  dissolves 
almost  completely  even  in  dilute  acids,  but  the  hydrogen 
evolved  has  the  peculiar  smell  attributed  to  the  presence  of 
carbonaceous  vapor. 

Steel.  —  This  invaluble  substance  is  in  composition  interme- 
diate between  cast  and  wrought-iron,  containing  less  carbon  than 
cast-iron  but  more  than  wrought.  It  may  be  made  from  wrought- 
iron  by  heating  bars  of  iron  to  redness  for  a  week  or  more  in  con- 
tact with  powdered  charcoal  in  close  boxes  from  which  air  is 
carefully  excluded.  Though  the  iron  is  not  fused,  nor  the  car- 
bon vaporized,  yet  the  carbon  gradually  penetrates  the  iron 
and  alters  its  original  properties  ;  when  the  bars  are  withdrawn 
from  the  chests  in  which  they  were  packed,  the  metal  has  be- 
come fine  grained  in  fracture,  more  brittle,  and  more  fusible. 
The  bars  are,  however,  far  from  uniform  in  composition,  the 
outside  being  more  highly  carbonized  than  the  interior  ;  they 
are  apt  to  show  blisters  of  various  sizes  on  the  surfaces,  and 
the  steel  thus  prepared  is  called  "  blistered  "  steel.  To  obtain 
steel  of  a  uniform  quality,  it  must  be  cast  into  ingots.  This 
process  of  preparing  steel  is  called  the  "  cementation " 
process  ;  it  is  a  curious  instance  of  chemical  action  between 
solid  materials  which  are  apparently  in  a  state  of  rest.  Since 


542  THE    BESSEMER    PROCESS. 

the  materials  used  in  this  process  are  the  purest  attainable,— 
the  best  iron  and  the  best  charcoal,  —  the  steel  obtained  is  of 
the  best  quality.  Cheaper  methods  of  preparing  an  inferior 
steel  are,  however,  of  great  industrial  importance.  If  the 
u  puddling "  process  for  preparing  malleable  iron  should  be 
arrested  when  the  cast-iron  had  lost  from  one  half  to  two 
thirds  of  its  carbon,  the  product  would  be  an  impure  steel, 
impure  because  the  silicon,  phosphorus,  sulphur,  and  other 
impurities  of  the  cast-iron  would  only  be  incompletely  re- 
moved. Nevertheless,  there  are  uses  in  the  arts  for  a  steel  of 
this  quality  which  may  be  cheaply  manufactured. 

633.  A  new  and  very  rapid  method  of  preparing  cast-steel 
directly  from  cast-iron  is  that  known  as  the  Bessemer  process. 
From  two  to  six  tons  of  cast-iron,  previously  melted  in  a 
suitable  furnace,  are  poured  into  a  large  covered  crucible, 
made  of  the  most  refractory  materials,  and  swung  on  pivots  in 
such  a  manner  that  it  can  be  tipped  up  and  emptied  by  means 
of  an  Irydraulic  press.  Through  numerous  apertures  in  the 
bottom  of  the  crucible  a  blast  of  air  is  forced  up  into  the  mol- 
ten metal ;  an  intense  combustion  ensues  involving  the  car- 
bon in  the  iron  and  a  portion  of  the  metal  itself,  and 
generating  a  most  intense  heat,  which  keeps  the  mass  fluid 
in  spite  of  its  rapid  approach  to  the  condition  of  malleable 
iron.  Such  a  quantity  of  a  pure  cast-iron  is  then  added  to 
the  iron  in  the  crucible  as  is  necessary  to  give  carbon  enough 
to  convert  the  whole  mass  into  steel,  and  the  melted  steel  is 
immediately  cast  into  ingots.  Six  tons  of  cast-iron  can  thus 
be  converted  into  tolerable  steel  in  twenty  minutes.  This 
steel  is  suitable  for  the  manufacture  of  axles,  railways,  boiler- 
plates and  other  large  articles  in  which  great  strength  should 
be  combined  with  hardness.  A  pure  steel  cannot  at  present 
be  made  by  this  process,  inasmuch  as  the  combustion  in  the 
crucible  does  not  get  rid  of  the  silicon  and  phosphorus  in  the 
cast-iron  nearly  as  perfectly  as  does  the  puddling  process ; 
for  the  same  reason  the  manufacture  of  wrought-iron  by  this 
method,  though  the  original  object  of  the  invention,  has  been 
thus  far  found  impracticable.  It  deserves  mention  that  the 


OXIDES    OF    IRON.  543 

• 

nailer  who  keeps  his  nail  hot,  while  hammering  it,  by  a  care- 
fully regulated  blast  of  cold  air,  applies  the  chemical  fact 
involved  in  Bessemer's  process,  and  indeed  anticipating  Bes- 
semer's  discovery. 

634.  The  two  qualities  of  steel  which  are  of  greatest  im- 
portance are  its  hardness  and  its  elasticity.     These  qualities 
are  developed  by  quickly  cooling  the  heated  metal ;  the  del- 
icate processes  by  which  steel  tools  and  springs  are  hardened, 
tempered,   and    annealed,    are  exceedingly  curious,    but   are 
rather  physical  than  chemical  phenomena.     Many  implements 
are  sufficiently  well  made  by  converting  their  exterior  surfaces 
into  steel,  leaving  the  interior  of  cast  or  wrought  iron.     Thus 
cast-iron  tools  may  be  heated  with  oxide  of  iron  to  remove  a 
part  of  the  carbon  from  their  exterior  and  thus  coat  them,  as 
it  were,  with  steel.    Tires  for  wheels  are  well  made  of  wrought- 
iron  bars  which  have  been  superficially  converted  into  steel  by 
the  cementation  process  ;  such  tires  combine  the  toughness  of 
malleable  iron  with  the  hardness  of  steel. 

635.  Oxides  of  Iron.    There  are  several  definite  compounds 
of  iron  and  oxygen.     The  best  known  of  these  oxides  are  the 
protoxide  (FeO),  or  ferrous  oxide,  as  it  is  often  called,  and 
the  sesquioxide  (Fe2O3),  often  called  ferric  oxide,  and  some- 
times spoken  of  as  peroxide  of  iron.     There  is  another  oxide, 
ferric  acid  Fe2O6,  which  is  an  exceedingly  unstable  substance, 
known  only  as  it  exists  in  combination  with  potassium,  as  fer- 
rate of  potassium  (K2FeO4),  or  with  some  other  powerful  base. 
Besides  these  oxides,  there  are  several  compounds  of  interme- 
diate composition,  which  may  be  supposed  to  result  from  the 
union  of  ferrous  and  ferric  oxides,  in  various  proportions,  they 
are  called  collectively  ferroso-ferric  oxides ;  the  most  impor- 
tant among  them  is  the  magnetic  oxide  Fe-At  =  FeO,  Fe2O3 ; 
which  is  the  black  oxide  formed  when  iron  is  oxidized  at  high 
temperatures  in  oxygen  gas,  in  air  or  steam  (§§  9,  18,  34). 

636.  Ferrous  Oxide  or  Protoxide  of  Iron    (FeO).      This 
compound  is  not  easily  obtained  pure,  since  it  absorbs  oxygen 
from  the  air  with  great  avidity  and  thus  becomes  contaminated 
with  the  sesquioxide.     But  by  dissolving  a  ferrous  salt,  that 


544  FERRIC    OXIDE. 

is  a  salt  of  protoxide  of  iron,  in  recently  boiled  water,  and 
adding  to  the  liquid  a  solution  of  caustic  alkali,  which  has  like- 
wise been  boiled  to  expel  air,  there  will  be  precipitated  a  white 
ferrous  hydrate,  provided  the  operation  be  conducted  out  of 
contact  with  the  air.  If  this  hydrate  be  exposed  to  the  air, 
as  when  the  solution  of  a  ferrous  salt  is  mixed  with  the  alkali 
without  the  precautions  above  enumerated,  it  will  rapidly  ab- 
sorb oxygen  and  will  exhibit  various  shades  6f  light  green, 
bluish  green,  and  black,  till,  finally,  it  assumes  the  red  color 
of  hydrated  ferric  oxide  (Exp.  346).  The  anhydrous  oxide 
obtained  by  igniting  ferrous  oxalate  in  close  vessels,  absorbs 
oxygen  so  rapidly  that  it  takes  fire  when  brought  in  contact 
with  the  air. 

Hydrated  ferrous  oxide  is  readily  soluble  in  acids,  forming 
salts  known  as  the  protosalts  of  iron  or  ferrous  salts ;  many 
of  these  salts  are  of  a  pale  green  color  ;  like  the  hydrate,  they 
rapidly  suffer  decomposition  by  absorbing  oxygen  from  moist 
air. 

637.  Ferric  Oxide  (Fe2O3).  This  oxide,  called  also  red 
oxide,  sesquioxide,  or  peroxide  of  iron,  occurs  very  abundantly 
and  widely  distributed  in  nature.  Several  of  its  varieties  have 
been  already  mentioned  as  ores  of  iron  (§  630).  It  may  be 
obtained  also  by  igniting  metallic  iron  or  either  of  the  lower 
oxides  or  hydrates  in  contact  with  the  air.  For  use  in  the 
arts,  it  is  prepared  by  igniting  ferrous  sulphate  with  or  with- 
out addition  of  a  small  proportion  of  nitrate  of  potassium,  or 
by  roasting  the  native  hydrate  (yellow  ochre).  The  better 
sort,  known  as  rouge,  is  largely  employed  for  polishing  glass 
and  jewelry,  and  all  grades  of  it  are  extensively  used  as  pig- 
ments. Red  ochre  is  impure  ferric  oxide.  As  commonly  met 
with,  the  oxide  is  amorphous  and  has  a  red,  brown,  or  nearly 
black  color,  according  to  the  method  of  its  preparation.  At  a 
full  white  heat,  it  gives  off  a  portion  of  its  oxygen,  and  mag- 
netic oxide  of  iron  is  formed.  It  is  easily  reduced  to  the  me- 
tallic state  by  hydrogen  gas,  even  at  temperatures  below  red- 
ness, and  by  carbon  and  carbonic  oxide  at  a  red  heat,  as  has 
been  stated  in  §  631.  Ammonia  gas  reduces  it  also  at  a  red 
heat. 


OXIDATION    BY   IRON-RUST.  545 

Exp.  335.  —  In  the  middle  of  a  tube  of  hard  glass,  No.  3,  10  c.  c. 
long,  arid  provided  at  both  ends  with  corks  carrying  short,  straight 
delivery-tubes,  place  a  teaspoonful  of  red  ochre  or  ignited  iron-rust. 
Attach  to  one  end  of  the  tube  a  hy.drogen  generator  or  gas-holder 
provided  with  a  chloride  of  calcium  drying-tube,  and  connect  with 
the  other  end  a  U-tube.  Support  the  tube  containing  oxide  of  iron 
upon  a  ring  of  the  iron  stand,  cause  a  current  of  hydrogen  to  flow 
through  it,  immerse  the  U-tube  in  a  bottle  of  cold  water,  and  finally 
heat  the  oxide  of  iron.  The  hydrogen  will  combine  with  the  oxy- 
gen of  the  red  oxide  of  iron,  water  will  be  formed  and  will  con- 
dense in  the  U-tube,  while  finely  divided  metallic  iron  will  be  left 
behind.  After  the  reduction  has  been  completed,  allow  the  tube  to 
become  cold,  and  then  scatter  its  contents  through  the  air  upon  an 
earthen  plate.  They  will  take  fire  and  burn  again  to  the  condition 
of  red  oxide. 

Exp.  336.  —  Repeat  Exp.  335,  using  carbonic  oxide  instead  of  hy- 
drogen, the  products  will  be  iron  and  carbonic  acid  instead  of  iron 
and  water. 

638.  The  facility  with  which  red  oxide  of  iron  gives  up 
oxygen,  taken  in  connection  with  the  readiness  with  which 
metallic  iron  and  the  protoxide  take  on  oxygen,  is  a  fact  of 
great  practical  importance.  It  has  been  found  that  organic 
substances  may  be  more  rapidly  incinerated  by  heating  them 
in  the  air  in  contact  with  a  small  quantity  of  ferric  oxide  than 
in  air  alone  ;  the  oxide  of  iron  appears  to  act  as  a  carrier  of 
oxygen  as  it  is  alternately  reduced  by  the  combustible  and 
again  oxidized  by  the  air.  Even  at  ordinary  temperatures, 
and  with  the  hydrated  oxide,  the  same  reactions  are  witnessed, 
though  in  a  lesser  degree.  The  iron  nails  employed  in  the 
construction  of  ships,  bridges,  fences,  or  shoes,  actually  cor- 
rode, "  eat  up  "  or  "burn  out"  the  organic  matter  in  contact 
with  them,  by  absorbing  oxygen  from  the  air  and  transferring 
it  to  the  carbon  compound  with  which  they  are  in  contact. 
The  rotting  of  canvas  by  iron  rust,  or  of  a  fishing-line  by  the 
rusty  hook,  are  familiar  instances  of  corruption  by  rust.  * 

These  reactions  doubtless  play  an  important  part  in  the  for- 
mation of  soils,  by  the  oxidation  of  vegetable  remains. 

In  the  same  way  ferric  oxide  converts  sulphide  of  calcium 
(CaS)  into  sulphate  of  calcium  (CaSO4)  at  the  expense  of  the 


546  HYDRATE    OF    IRON. 

oxygen  of  the  air.  A  useful  cement  has  been  prepared  by 
mixing  the  residual  oxy sulphide  of  calcium  of  Leblanc's  soda 
process  with  an  equal  weight  of  the  ferric  oxide  left  as  a  re- 
sidue in  burning  iron  pyrites  for  sulphuric  acid.  Hydrated 
sesquioxide  of  iron  (Fe2O3,  3H2O)  may  readily  be  prepared  by 
adding  an  excess  of  ammonia-water  to  the  solution  of  almost 
any  ferric  salt. 

Exp.  337.  — Cover  a  teaspoonful  of  fine  iron  filings  or  small  tacks 
with  three  or  four  times  as  much  dilute  sulphuric  acid  in  a  small 
bottle  ;  wait  until  the  evolution  of  hydrogen  ceases,  then  decant  the 
clear  liquor  into  a  small  flask  or  beaker,  add  to  it  a  few  drops  of 
strong  nitric  acid,  and  heat  it  to  boiling.  The  liquor  will  soon  be 
colored  dark-brown  by  the  nitrous  fumes  resulting  from  the  decom- 
position of  the  nitric  acid,  which  are  for  a  short  time  held  dissolved 
by  the  liquid,  but  this  deep  coloration  soon  passes  away,  arid  there 
is  left  only  the  yellowish-red  color  of  the  ferric  sulphate  which  has 
been  formed.  Add  to  the  solution  ammonia-water,  until  the  odor 
of  the  latter  persists  after  agitation,  and  collect  upon  a  filter  the 
flocculent  red  precipitate  of  ferric  hydrate. 

Besides  this  normal  hydrate  (Fe2O3,  3H2O)  there  are  several 
other  ferric  hydrates  containing  smaller  proportions  of  water. 
They  are  found  in  nature,  and  may  be  obtained  by  heating  the 
normal  hydrate,  or  by  suffering  it  to  remain  for  a  long  while 
under  water,  or  by  boiling  it  for  some  time  in  water. 

639.  Generally  speaking,  hydrated  sesqnioxide  of  iron  is 
easily  soluble  in  acids,  though  some  peculiar  varieties  of  it 
dissolve  only  with  difficulty.  The  anhydrous  oxide  also  dis- 
solves in  acids,  though  less  easily  in  proportion  as  it  has  been 
more  strongly  ignited ;  its  best  solvent  is  concentrated  boiling 
chlorhydric  acid. 

By  long-continued  heating  at  300°  or  320°,  ferric  hydrate 
can  be  deprived  of  all  its  water,  and  still  be  readily  soluble  in 
acids  ;  but  when  heated  to  dull  redness,  this  powder  suddenly 
glow£  brightly  for  a  moment  and  contracts  in  bulk,  without 
either  losing  or  gaining  weight,  and  is  then  attacked  by  acids 
only  slowly  and  with  difficulty.  It  has  been  observed,  how- 
ever, that  the  ignited  oxide  may  still  be  dissolved  rather  easily 


PROPERTIES  OF  FERRIC  HYDRATE.  547 

by  a  hot  mixture  of  chlorhydric  acid  and  ferrous  chloride, 
protochloride  of  tin,  zinc,  or  some  other  reducing  agent. 

Ferric  hydrate  is  somewhat  used  as  a  mordant  in  dyeing,  and 
is  largely  employed  for  purifying  coal-gas.  As  has  been  stated 
under  arsenic,  the  recently  precipated  hydrate  acts  as  an  anti- 
dote to  arsenious  acid,  since  when  given  in  sufficient  quantity 
it  forms  a  basic  arsenite  of  iron  scarcely  at  all  acted  upon  by 
water. 

Exp.  338.  — Dissolve  half  a  gramme  of  arsenious  acid  in  40  or  50 
c.  c.  of  boiling  water.  Divide  the  solution  into  two  portions,  and 
stir  into  one  of  these  rjortions  a  considerable  quantity  of  moist  ferric 
hydrate,  such  as  was  obtained  in  Exp.  337  ;  filter  the  mixture,  acid- 
ulate the  filrate  with  chlorhydric  acid,  and  test  it  for  arsenic  by 
means  of  sulphyclric  acid  (§  ^40) . 

If  a  sufficient  quantity  of  ferric  hydrate  has  been  employed,  no 
precipitate  of  sulphide  of  arsenic- will  be  obtained  in  the  filtrate, 
though  on  adding  a  drop  of  chlorhydric  acid,  and  afterwards  sul- 
phydric  acid,  to  the  original  solution  of  arsenic,  an  abundant  yellow 
precipitate  will  be  at  once  thrown  down. 

Exp.  339.  — Fill  a  tube,  30  c.  m.  long,  with  alternate  tufts  of  cotton 
and  loose  layers  of  dry  ferric  hydrate,  pass  a  slow  current  of  sulphy- 
dric  acid  (Exp.  90),  through  the  tube,  and  observe  that  the  oxide 
gradually  becomes  black ;  it  appears  to  be  converted  into  ferrous 
sulphide,  while  water  and  sulphur  are  set  free :  — 

Fe2O3,  3H2  O+  3H2S  =  2FeS  +  S  +  6H20. 

After  a  good  part  of  the  ferric  oxide  has  become  black,  remove 
the  contents  of  the  tube  to  a  porcelain  plate,  and  leave  them  ex- 
posed to  the  air  in  a  place  where  no  harm  can  be  done  in  case  they 
take  fire.  By  the  action  of  the  air,  sesquioxide  of  iron  will  be  repro- 
duced and  sulphur  set  free  within  the  mass ;  some  sulphurous  acid 
is  given  off  at  the  same  time  and  heat  is  evolved,  as  will  readily  be 
perceived  if  the  quantity  of  material  be  large.  The  following 
equation,  though  it  does  not  fully  express  the  complete  reaction 
whfth  really  occurs,  may  still  serve  to  give  a  general  idea  of  these 
chemical  changes :  — 

2FeS  +  5O  =  FeA  +  S  +  S02. 

In  practice  the  impure  illuminating  gas  is  made  to  pass  through 
layers  of  ferric  oxide,  often  made  porous  by  an  admixture  of  saw- 
dust, and  as  soon  as  the  oxide  ceases  to  absorb  sulphuretted  hy- 
drogen, it  is  "  revivified  "  by  forcing  or  drawing  through  it  a  cur- 


548  SULPHIDES    OF    IRON. 

rent  of  fresh  air  or  by  spreading  it  in  the  air.  The  oxide  is  thus 
used  over  and  over  again  until  so  much  sulphur  has  accumulated 
within  it  as  to  interfere,  mechanically,  with  its  absorbent  power. 
The  sulphur  may  readily  be  recovered  from  this  mixture  by  distil- 
lation, or  the  spent  oxide  may  be  used  instead  of  pyrites  for  making 
sulphuric  acid,  wherever  enough  of  it  can  be  obtained  to  repay  the 
trouble  of  collecting. 

The  ferric  salts  are  usually  of  a  yellowish-brown  or  red 
color,  when  hydrated,  though  some  of  them  have  a  violet  tinge  ; 
they  are  white  when  dry.  The  normal  salts  are  usually  sol- 
uble in  water  and  deliquescent,  and  there  are  numerous  soluble 
basic  salts,  besides  other  basic  salts  which"  are  insoluble. 

In  the  ferric  salts  iron  plays  the  part  of  a  trivalent  element 
like  aluminum,  while  in  the  ferrous  salts  it  is  bivalent  like 
calcium  or  lead.  The  ferric  salts  closely  resemble  salts  of 
aluminum,  and  are  for  the  most  part  isomorphous  with  them. 
Besides  acting  as  a  base,  ferric  oxide,  like  oxide  of  aluminum, 
combines  with  several  of  the  more  powerful  bases  to  form  salts 
called  ferrites  ;  magnetic  oxide  of  iron,  for  example,  may  be 
regarded  as  the  ferrite  of  iron. 

It  is  remarkable  that  ferric  oxide  may  be  displaced  from 
many  of  its  compounds  by  the  protoxide  of  iron ;  thus,  when 
hydrated  ferrous  oxide  is  added  to  a  solution  of  ferric  sulphate, 
hydrated  sesquioxide  is  precipitated  :  — 

Fe2O3,  3SO3+  3(FeO,  H2O)  =Fe2O3,  3H2O  +  3(FeO,  SO8.) 
In  like  manner,  carbonate  of  barium  precipitates  anhydrous 
ferric  oxide  from  ferric  salts,  but  upon  ferrous  salts  it  has  no 
action :  — 

Fe2Cl6  +  3BaCO3  =  Fe2O3  +  3BaCl2  +  3CO2. 
Sulphides  of  Iron.  Tbere  are  several  sulphides  of  iron,  the 
most  important  of  which  are  the  protosulphide  (FeS)  and  Jhe 
bisulphide  (FeS2),  found  native  as  iron  pyrites.  There  is  a 
sulphide,  Fe3S4,  which  is  magnetic  like  the  oxide  to  which  it 
corresponds. 

640.  Ferrous  Sulphide  (FeS)  is  a  substance  of  great  value- 
to  the  chemist  as  the  cheapest  source  of  the  important  reagent, 
sulphydric  acid  (§210). 


PROTOSULPHIDE    OF    IRON.  549 

This  sulphide  may  be  prepared  by  igniting  pyrites  in  a  covered 
crucible,  by  rubbing  roll  brimstone  against  a  white  hot  iron  bar,  or 
by  fusing  together  sulphur  and  iron  turnings.  The  second  method 
is  to  be  recomended  if  the  student  have  ready  access  to  a  black- 
smith's forge.  The  sulphide  is  a  waste  product  in  those  chemical 
works  where  sulphate  of  lead,  obtained  from  dye-houses^  is  reduced 
to  the  metallic  state  by  fusion  with  iron  and  coal.  In  the  labora- 
tory it  may  be  prepared  as  follows  :  — 

Exp.  340.  — Heat  a  common  Hessian  crucible  to  redness  in  a  fire 
of  coke  or  anthracite,  and  project  into  it  from  an  iron  spoon  succes- 
sive small  portions  of  a  mixture  of  7  parts  of  iron  turnings  and  4 
parts  of  powdered  sulphur,  replacing  the  cover  of  the  crucible  after 
each  addition  of  the  mixture.  The  sulphur  and  iron  combine  with 
great  energy,  and  the  sulphide  formed  melts  down  to  the  liquid 
state.  Since  the  molten  sulphide  is  capable  of  dissolving  both  iron 
and  sulphur,  according  as  the  one  or  other  may  be  present  in  excess, 
it  is  impossible  to  prepare  a  pure  protosulphide  by  this  method. 
But  the  product  obtained  as  above  described,  though  of  variable 
composition,  answers  perfectly  well  for  all  ordinary  purposes. 
When  the  crucible  has  become  half- full  of  the  molten  sulphide,  re- 
move it  from  the  fire,  pour  out  its  contents  upon  a  brick  floor,  and 
if  more  of  the  sulphide  be  desired,  replace  the  crucible  in  the  fire 
and  proceed  as  before. 

Where  comparatively  large  quantities  of  the  sulphide  are  required, 
it  is  well  to  bore  a  hole  through  the  bottom  of  a  plumbago  crucible 
and  set  the  latter  upon  the  grate-bars  of  the  furnace  in  such  manner 
that  the  hole  may  remain  open  ;  fill  the  crucible  with  iron  turnings ; 
heat  it  to  redness  and  throw  in  lumps  of  sulphur  upon  the  hot  iron. 
As  fast  as  sulphide  of  iron  forms  it  will  melt  and  flow  through  the 
hole  in  the  crucible  into  the  ash-pit  below,  which  should  be  kept 
clean  to  receive  it.  In  the  preparation  of  sulphide  of  iron,  wronght- 
iron  should  always  be  employed.  From  the  filings  of  cast-iron,  but 
little  if  any  of  the  fusible  sulphide  can  be  prepared. 

The  foregoing  experiment  illustrates  the  practical  methods  of 
making  ferrous  sulphide;  but  several  other  reactions  which  pro- 
duce it  are  of  scientific  interest  (compare  Exp.  89). 

Exp.  341.  —  Arrange  a  bottle  for  generating  sulphuretted  hydro- 
gen, as  in  Exp.  90,  but  in  place  of  the  delivery  tube  in  Fig.  37 
attach  to  the  bottle  a  jet  for  burning  the  gas.  After  the  air  has 
been  completely  expelled  from  the  bottle,  light  the  sulphydric  acid 
gas  at  the  jet,  and  hold  in  its  flame  a  piece  of  fine  iron  wire  ;  the  iron 


550  IRON    PYRITES. 

will  burn  to  ferrous  sulphide,  and  if  the  wire  be  held  in  the  axis  of 
the  flame,  so  that  a  considerable  portion  of  it  shall  be  kept  red  hot, 
the  globule  of  sulphide  of  iron  formed  will  melt  and  flow  backward 
upon  the  wire  as  fast  as  the  end  of  the  latter  is  consumed. 

Exp.  342. — Mix  20  grms.  of  fine  iron  filings,  14  grins,  of  flowers 
of  sulphur,  and  7  grms.  of  water,  in  a  small  bottle,  and  heat  the 
mixture  gently  upon  a  sand  bath,  or  set  it  aside  in  a  warm  place. 
Chemical  action  will  soon  set  in,  much  heat  will  be  evolved,  and  in 
the  course  of  half  an  hour  the  mixture  will  become  black  from 
formation  of  sulphide  of  iron.  If  the  porous  black  sulphide  be 
left  exposed  to  the  air  it  will  absorb  oxygen,  and  will  be  partially 
converted  into  ferrous  sulphate. 

A  firm  packing  or  lute  for  the  joints  of  iron  vessels  is  prepared 
by  mixing  together  60  parts  of  fine  iron  filings,  1  part  of  flowers  of 
sulphur,  and  2  parts  of  powdered  chloride  of  ammonium.  The 
mixture  is  made  into  a  stiff  paste  with  water,  and  immediately  ap- 
plied to  the  iron.  It  soon  becomes  hot  and  swells  up,  and  sets  to  a 
hard  compact  mass,  while  ammonia  and  sulphuretted  hydrogen  are 
disengaged . 

Exp.  343. — Dissolve  a  small  crystal  of  ferrous  sulphate  (cop- 
peras) in  water,  and  add  to  the  liquid  a  drop  or  two  of  sulphydrate 
of  ammonium  (§  526).  Black  sulphide  of  iron  will  be  thrown 
down  (compare  Exp.  339). 

The  finely  divided  ferrous  sulphide  obtained  in  the  wet  way,  as 
in  the  last  two  experiments,  dissolves  much  more  quickly  in  acids 
than  the  compact  sulphide  obtained  by  the  way  of  fusion ;  in  con- 
tact with  acids  it  evolves  gas  so  tumultuously  that  it  would  be  in- 
convenient as  a  source  of  sulphydric  acid.  - 

The  black  earth  between  the  stones  of  the  pavements  of  cities, 
and  at  the  bottoms  of  drains  and  cesspools,  owes  its  color  to  sul- 
phide of  iron  formed  by  the  putrefaction  of  sulphuretted  compounds 
in  contact  with  ferric  oxide  contained  in  the  earth. 

641.  Bisulphide  of  Iron  (FeS2)  occurs  abundantly  in  nature 
as  the  well  known  mineral  iron  pyrites.  Two  distinct  forms  of 
it  are  met  with  ;  the  yellow  cubical  pyrites,  crystallized  in  forms 
of  the  monometric  system,  and  the  white  pyrites  or  marcasite, 
which  crystallizes  in  trimetric  forms.  A  third  variety  of  sul- 
phide of  iron  called  magnetic  pyrites  is  of  different  composition 
from  the  foregoing,  and  contains  less  sulphur  than  the  bisul- 
phide. Iron  pyrites  appear  to  have  been  sometimes  formed  in 


FERROUS    CHLORIDE.  551 

nature  by  the  deoxidation  of  sulphates,  such  as  the  sulphate 
of  calcium,  by  means  of  organic  matter  in  presence  of  chaly- 
beate waters.  The  formation  of  pyrites  has  often  been  noticed 
in  solutions  of  sulphate  of  iron  into  which  organic  matters 
have  fallen.  But  bisulphide  of  iron  may  be  readily  formed  in 
the  dry  way  also. 

The  compact  forms  of  yellow  pyrites,  whether  natural  or 
artificial,  are  permanent  in  the  air  ;  but  when  finely  divided  the 
mineral  oxidizes  rather  easily,  with  evolution  of  considerable 
heat.  White  iron  pyrites  oxidize  rapidly  in  the  air,  no  matter 
whether  they  be  compact  or  friable.  The  spontaneous  com- 
bustion of  many  kinds  of  coal  is  due  to  the  oxidation  of  iron 
pyrites  disseminated  through  the  combustible.  Alum  and 
copperas  are  often  prepared  from  pyritous  shales,  either  by 
firing  heaps  of  the  shale  artificially,  or  by  allowing  the  heaps 
to  take  fire  spontaneously  through  oxidation  of  the  pyrites, 
and  then  regulating  the  combustion  so  that  the  largest  prac- 
ticable yield  of  sulphate  of  iron  or  of  sulphate  of  aluminum 
shall  be  obtained.  So  long  as  the  temperature  of  the  burning 
pyrites  remains  comparatively  low,  ferrous  sulphate  and  sul- 
phuric acid  are  the  principal  products,  the  latter  uniting  with 
the  alumina  of  the  shale,  if  such  be  present ;  when  the  heap 
has  become  cold,  the  sulphates  can  be  separated  by  lixiviating 
the  mass  with  water.  When  pyrites  are  roasted  at  higher 
temperatures,  as  in  the  manufacture  of  sulphuric  acid,  sulphur- 
ous acid  is  given  off  and  ferric  oxide  left  as  the  principal  res- 
idue. 

When  distilled  in  close  vessels,  one  atom  of  the  sulphur  in 
iron  pyrites  is  expelled,  and  ferrous  sulphide  remains.  Sul- 
phur has  sometimes  been  prepared  in  this  way  in  a  dearth  of 
native  sulphur. 

642.  Ferrous  Chloride  (FeCl2)  may  be  obtained  by  passing 
chlorine  or  dry  chlorhydric  acid  gas  over  hot  iron ;  in  case 
chlorhydric  acid  be  employed,  hydrogen  will  be  evolved.  As 
commonly  met  with,  however,  the  chloride  is  in  the  form  of  a 
hydrate  (FeCl2  +  4H2O)  obtained  by  dissolving  metallic  iroii 
in  dilute  chlorhydric  acid.  It  crystallizes  easily,  forms  double 


552  FERROU£    SULPHATE. 

salts  by  uniting  with  many  other  chlorides,  and  may  be  de- 
prived of  its  water  without  decomposition  when  heated  care- 
fully out  of  contact  with  the  air. 

643.  Ferric    Chloride   (Fe2Cl6).      As  obtained  by  burning 
metallic  iron  in  an  excess  of  dry  chlorine,  this  compound  occurs 
in  anhydrous,  glistening  scales,  which  volatilize  easily  when 
heated.   It  dissolves  readily  in  water,  with  evolution  of  heat,  and 
deliquesces  rapidly  in  the  air.    It  hisses  when  thrown  into  water. 
Once  dissolved  in  water,  it  cannot  be  freed  from  the  water  by 
evaporation,  since  chlorhydric  acid  goes  oif  with  the  water, 
and  a  basic  compound  of  ferric  oxide  and  ferric  chloride  re- 
mains.    Hydrated  ferric  chloride  may  readily  be  obtained  by 
boiling  a  solution  of  ferrous  chloride  with  a  small  proportion 
of  nitric  acid,  or  by  passing  chlorine  gas  through  a  solution 
of  ferrous  chloride.    From  concentrated  solutions,  ferric  chlor- 
ide crystallizes  with   several  different  proportions  of  water. 
Ferric  chloride  combines  with  many  of  the  metallic  chlorides 
to  form  double  compounds,  among  which  the  ammonium  salts 
are  perhaps  the  most  stable. 

644.  Ferrous  Sulphate  (FeSO4).     A  hydrate  of  this  com- 
pound, of  composition  FeSO4-|-  7H2O,  usually  called  copperas 
or  green  vitriol  is  the  most  common  of  all  the  compounds  of 
iron.     It  may  readily  be  prepared  by  dissolving  metallic  iron 
or  protosulphide  of  iron  in  dilute  sulphuric  acid.    On  the  large 
scale  it  is  commonly  prepared  by  roasting  iron  pyrites  at  a 
gentle  heat,  or  aluminous  shales  containing  pyrites  in  the  man- 
ner already  indicated.     Sometimes,  however,  it   is   manufac- 
tured directly  from  metallic  iron  and  sulphuric  acid,  and  it  is 
obtained  as  a  secondary  product  in  certain  metallurgical  oper- 
ations where  copper  is  precipitated  by  means  of  iron,  from  a 
solution  of  copper.     The  reaction  is  analogous  to  those  em- 
ployed for  obtaining   pure   silver    (Exp.  272)  and  pure  lead 
(§  594). 

Exp.  344.  — Dissolve  5  grms.  of  common  blue  vitriol  (sulphate 
of  copper)  in  50  or  60  c.  c.  of  water,  acidulate  the  liquor  with  a 
few  drops  of  sulphuric  acid,  pour  it  into  a  bottle,  and  place  in  it  a 
rod  of  thick  iron  wire.  Copper  will  immediately  begin  to  be  pre- 


PROPERTIES  OF  FERROUS  SULPHATE.  553 

cipitated  as  a  coating  upon  the  iron,  and  in  the  course  of  an  hour  or 
two  will  be  completely  removed  from  the  solution.  The  original 
blue  color  of  the  solution  will  disappear  and  be  replaced  by  the 
faint  green  color  of  copperas,  while  a  spongy  mass  of  metallic  cop- 
per will  be  obtained  :  — 

CuSO4  +  Fe  =  FeSO4  +  Cu. 

Decant  the  solution  of  ferrous  sulphate  from  the  precipitated  cop- 
per, place  in  it  a  fragment  of  iron,  and  evaporate  it  to  a  small  bulk ; 
pour  the  concentrated  solution  into  a  wide-mouthed  phial,  cork  the 
phial  tightly,  and  set  it  aside  in  a  cool  place ;  the  liquid  will  be  con- 
verted into  a  mass  of  copperas  crystals. 

It  has  been  proposed  to  prepare  copperas  from  the  "  finery 
slag"  of  the  puddling  furnaces,  where  cast-iron  is  converted 
into  wrought-iron,  by  treating  this  slag  with  dilute  sulphuric 
acid.  The  finery  slag  consists  chiefly  o'f  basic  silicate  of  pro- 
toxide of  iron  (2FeO,  SiO2). 

645.  When  perfectly  pure,  the  crystals  of  ferrous  sulphate 
are  compact,  transparent,  and  of  a  bluish-green  color,  but  in 
dry  air  they  effloresce  and  become  covered  with  a  white  incrus- 
tation, the  color  of  which  subsequently  changes  to  rusty 
brown  through  absorption  of  oxygen.  The  common  commer- 
cial article  is  of  a  grass-green  color,  and  is  contaminated  with 
more  or  less  ferric  sulphate.  Besides  the  common  hydrate  con- 
taining 7  molecules  of  water,  there  are  hydrates  which  contain  4, 
3,  and  2  molecules  of  water  respectively.  From  all  of  these 
Irydrates  the  water  can  easily  be  expelled  by  heat,  and  if  the 
anhydrous  salt  thus  obtained  be  still  further  heated  it  will  de- 
compose ;  two  stages  in  this  decomposition  may  be  formulated 
as  follows  :  — 

I.     2FeS04  =  S02  +  Fe  A,S08. 
II.     Fe203,S03  =  Fe203  +  SO3. 

Basic  ferric  sulphate  is  at  first  formed,  while  sulphurous 
acid  is 'given  off,  and  finally  the  ferric  salt  is  itself  decom- 
posed into  anhydrous  sulphuric  acid,  and  ferric  oxide.  Upon 
this  reaction  the  preparation  of  Nordhausen  sulphuric  acid 
depends  (§  239).  Like  ferrous  hydrate,  ferrous  chloride  and 
all  the  ferrous  salts,  moist  copperas,  or  an  aqueous  solution  of 
copperas,  rapidly  absorbs  oxygen  from  the  air. 

40  ' 


554  INK. 

Exp.  345. —  Pour  a  solution  of  copperas  into  an  open  capsule  and 
leave  it  exposed  to  the  air  for  a  day  or  two ;  the  solution  will 
gradually  become  yellow  as  the  oxidation  proceeds,  and  after  a 
while  a  rusty  precipitate  of  ferric  oxide,  or  of  highly  basic  ferric 
sulphate,  will  fall.  The  oxide  of  iron  which  separates  under  these 
conditions  is  not  readily  soluble  in  dilute  acids.  It  appears  to  be 
an  isoineric  modification  of  the  easily  soluble  hydrate  which  is  pre- 
cipitated from  cold  ferric  solutions  by  alkaline  lyes.  At  all  events 
the  sulphuric  acid  of  the  copperas  is  insufficient  to  dissolve  all  of 
the  ferric  oxide  formed  during  its  oxidation.  In  most  cases  where 
a  ferrous  salt  is  to  be  converted  into  a  ferric  salt,  it  is  best  to  add  a 
certain  proportion  of  free  acid  to  the  mixture  in  order  to  prevent 
the  separation  of  the  oxide. 

A  difficultly  soluble  deposit,  similar  to  the  foregoing,  may  readily 
be  obtained  by  boiling  an  exceedingly  dilute  solution  of  almost  any 
of  the  soluble  ferric  salts'.  It  is  possible  that  these  sediments  should 
be  regarded  rather  as  highly  basic  salts  than  as  mere  hydrates. 
Their  inertness  may  perhaps  be  due  to  the  presence  of  small  pro- 
portions of  the  acids  of  the  salts  from  which  they  have  been  de- 
rived, still  held  in  chemical  combination ;  but  there  is  at  present 
less  evidence  in  favor  of  this  view  than  of  the  one  previously 
stated. 

Exp.  346.  —  To  a  teaspoonful  of  a  solution  of  copperas  add  a 
few  drops  of  soda  lye,  and  observe  that  the  hydrate  rapidly  absorbs 
oxygen,  and  changes  color  as  has  been  set  forth  in  §  636. 

Exp.  347.  —  Mix  a  few  drops  of  a 'solution  of  copperas  with  a 
drop  or  two  of  a  solution  of  tannic  acid,  such  for  example  as 
tincture  of  nutgalls,  or  of  oak-  or  hemlock-bark;  a  light  violet- 
colored  precipitate  will  be  formed  and  will  remain  suspended  in  the 
liquid ;  by  exposure  to  the  air  this  color  soon  changes  to  black. 
The  violet  precipitate  is  ferrous  tannate,  and  the  black  precipitate 
ferric  tannate ;  if  these  finely  divided  precipitates  were  produced  in 
liquids  made  slightly  viscous  by  the  addition  of  gum  or  sugar,  they 
would  remain  suspended  in  the  liquor,  which  could  then  be  used 
as  writing-ink. 

Ink  may  be  prepared  as  follows:  — Powder  separately  12  grms. 
of  nutgalls,  5  grms.  of  copperas,  and  5  grms.  of  gum- Arabic.  Boil 
the  nutgalls  two  or  three  hours  in  a  flask  with  75  c.  c.  of  water, 
taking  care  to  add  hot  water  by  small  portions,  to  supply  that  lost 
by  evaporation.  Allow  the  mixture  to  settle  and  decant  the  clear 
liquor  into  a  clean  bottle.  Dissolve  the  gum- Arabic  in  a  small 


FERRIC    SULPHATE.  555 

quantity  of  water,  and  mix  the  mucilage  thoroughly  with  the  solu- 
tion of  nutgalls.  Dissolve  the  copperas  in  another  portion  of  water, 
and  incorporate  this  solution  with  the  mixture  of  nutgalls  and  gum. 
Add  enough  water  to  make  the  volume  of  the  mixture  equal  to  100 
c.  c.  Preserve  the  ink  in  a  tight  bottle.  If  the  color  of  the  product 
be  lighter  than  is  desired,  the  liquid  may  be  left  exposed  to  the  air 
until  it  has  acquired  a  deeper  tint.  When  first  applied  to  paper, 
the  color  of  fresh  ink  is  comparatively  pale,  but  the  writing  dark- 
ens gradually  in  proportion  as  it  absorbs  oxygen . 

In  the  course  of  the  foregoing  experiments,  dip  a  small  piece  of 
cotton  cloth  in  the  solution  of  nutgalls,  and  allow  it  to  become  dry ; 
then  dip  it  in  the  solution  of  copperas  and  hang  it  up  in  damp  air  . 
Black,  insoluble  tannate  of  iron  will  be  so  firmly  precipitated  in  and 
upon  the  fibres  of  the  cloth,  that  it  cannot  be  washed  away. 

The  experiment  illustrates  one  general  method  of  dyeing,  by 
means  of  which,  blacks  and  grays  of  various  shades  may  be  ap- 
plied to  cloth  or  leather,  though  in  practice  other  astringent  dye- 
stuffs,  such  as  catechu,  cutch,  or  gambier,  are  commonly  employed 
in  place  of  nutgalls. 

Ferrous  sulphate  is  largely  employed  in  dyeing,  sometimes  direct- 
ly, as  in  the  foregoing  experiment,  but  often  as  the  source  of  other 
compounds  of  iron,  which  are  employed  as  mordants ;  ferrous  ace- 
tate, for  example,  obtained  by  decomposing  ferrous  sulphate  with 
acetate  of  calcium,  is  a  compound  much  used  by  dyers.  It  should 
be  remarked,  however,  that  acetate  of  iron  is  sometimes  made  di- 
rectly by  dissolving  scraps  of  iron  in  vinegar  or  pyroligneous  acid 
(§380). 

646.  Ferric  Sulphate  (Fe23SO4)  is  interesting,  chiefly  from 
its  analogy  with  sulphate  of  aluminum.  Like  aluminum,  it 
combines  with  the  sulphates  of  the  alkali  metals,  to  form 
well-defined  alums,  as  has  been  already  stated  (§  625).  Fer- 
ric sulphate  occurs  as  a  waste  product  in  the  mother  liquors, 
from  which  copperas  and  alum  have  crystallized.  By  drying 
these  liquors  and  igniting  them,  red  ochre  of  excellent  quality 
can  be  obtained,  in  accordance  with  the  second  reaction  of 
§  645.  Fuming  sulphuric  acid  is  commonly  manufactured 
nowadays  by  distilling  pure  ferric  sulphate,  instead  of  cop- 
peras as  formerly  at  Nordhausen.  The  ferric  salt  is  obtained 
by  dissolving  ferric  oxide  in  weak  sulphuric  acid,  and  evap- 
oratino-  the  solution  to  dryness  ;  the  residue  of  ferric  oxide 


556  FERRIC    NITRATE. 

left  after  the  ignition  of  the  sulphate  is  thus  reconverted  into 
ferric  sulphate,  and  is  used  over  and  over  again  as  often  as  it 
is  decomposed. 

647.  Ferrous  Nitrate  (FeN2O6  +  6H20)  is  a  compound  of 
considerable  scientific  interest,  which  may  readily  be  procured 
by  dissolving  ferrous  sulphide  in  cold,  dilute  nitric  acid,  or  by 
decomposing  a*  solution  of  copperas  with  an  equivalent  quan- 
tity of  nitrate  of  barium.     It  may  also  be   obtained,  mixed 
with  nitrate  of  ammonium,  by  dissolving  iron  in  cold  dilute 
nitric  acid.     The  metal  dissolves  without  evolution  of  "gas,  in 
a  manner  which  may  be  thus  formulated  :  — 

4Fe  +  10HNO8  =  4FeN2O6  +  (NH4)  NO8  +  3H2O. 
The  aqueous  solution  of  ferrous  nitrate  decomposes  readily 
when  heated,  and  in  warm  weather  changes  spontaneously  to 
a  ferric  compound. 

648.  Ferric  Nitrate  (Fe23N2O6)  maybe  obtained  inhydrated 
crystals,  containing  18  molecules  of  water,  by  dissolving  me- 
tallic iron  in  nitric  acid,  of  1.29  specific  gravity,  till  the  liquor 
has  taken  up  about  10  per  cent,  of  the  metal,  and  then  adding 
an  equal  volume  of  nitric  acid  of  specific  gravity  1.43.     The 
solution  will  deposit,  on  cooling,  rhombic  prisms  of  ferric  ni- 
trate, which  are  sometimes  colorless,  but  often  of  a  faint,  lav- 
ender-blue color.       They  are  slightly  deliquescent,  and  very 
soluble  in  water,  but  are  only  slightly  soluble  in  cold  nitric 
acid.     By  adding  nitric  acid  to  a  syrupy  solution  of  ferric  ni- 
trate, there  may  be  obtained  another  hydrate,  containing  only 
12  molecules  of  water,  crystallized  in  cubes  or  square  prisms. 
By  mixing  a  solution  of  ferric  nitrate,  or,  for  that  matter,  al- 
most any  other  of  the  normal  ferric  salts  with  recently  pre- 
cipitated ferric  hydrate,  or  by  partially  abstracting  the  acid 
of  the  salt  by  means  of  an  alkali,  deep-red  solution^  of  various 
basic  compounds  may  readily  be  obtained.     A  basic  ferric  ni- 
trate* is  employed  in  dyeing,  under  the  name  iron  mordant. 

649.  Silicates  of  Iron.     Several  native  silicates  of  iron  are 
known,  but  none  of  them  are  of  special  interest.      The  green 
tinge    of  ordinary  glass   is  due  to  the  presence  of  a  ferrous 
silicate,  and  by  increasing  the  proportion  of  the  ferrous  salt, 


CYANIDES    OF    IRON.  557 

a  deep  bottle-green  color  may  be  imparted  to  glass.  Ferric 
silicate,  on  the  other  hand,  has  comparatively  little  coloring 
power,  though  when  a  considerable  quantity  of  it  is  present, 
it  imparts  a  yellow  color  to  glass ;  it  is  sometimes  used  for 
coloring  porcelain.  To  destroy  the  green  color  of  the  ferrous 
silicate,  binoxide  of  manganese,  or  some  other  oxidizing  agent, 
is  often  added  to  glass  in  the  process  of  manufacture  ;  the  fer- 
rous silicate  is  thus  converted,  for  the  most  part,  into  ferric 
silicate,  and  a  nearly  colorless  glass  produced. 

650.  Cyanides  of  Iron.  There  is  a  ferrous  cyanide  (Fe 
(CN)2),  known  as  a  yellowish-red  precipitate,  which  takes  up 
oxygen  and  becomes  blue  when  exposed  to  the  air,  and  a  fer- 
ric cyanide  (Fe2(CN)6)  has  been  obtained  in  solution.  But  by 
far  the  best  known  of  the  cyanides  of  iron  are  certain  double 
compounds,  which  constitute  the  familiar  pigments,  known, 
collectively,  as  Prussian  blue.  Common  Prussian  blue,  for 
example  (Fe7C18N18+ 18H2O)  may  be  regarded  as  a  double 
compound  of  ferrous  and  ferric  cyanides,  3Fe(CN)2;  2(Fe2 
(CN)6)  -f-  18H2O  ;  it  may  be  prepared  as  follows  :  — 

Exp.  348. —  Add  to  an  exceedingly  dilute  solution  of  almost  any 
ferric  salt,  such,  for  example,  as  the  ferric  sulphate  of  Exp.  337,  a 
drop  of  ferrocyanide  of  potassium  (§509) .  A  beautiful  blue  precipitate 
will  form,  and  will  remain  suspended  in  the  liquor  for  a  long  while. 
Another  variety  of  Prussian  blue,  known  as  Turnbull's  blue,  may 
be  obtained  by  mixing  a  solution  of  red  prussiate  of  potash,  known 
to  chemists  as  ferricyanide  of  potassium,  with  a  solution  of  cop- 
penis  or  other  ferrous  salt. 

Since  the  yellow  prussiate  of  potash  will  give  no  blue  color- 
ation with  ferrous  salts,  and  since  the  red  prussiate  yields  no 
blue  with  ferric  salts,  it  is  evident  that  the  two  solutions  may 
be  used  as  tests  by  which  to  detect  the  presence  of  ferrous 
and  ferric  salts,  respectively,  in  any  solution. 

Exp.  349.  — Soak  a  piece  of  cotton  cloth  in  a  solution  of  ferric 
sulphate  (Exp.  337),  and  then  immerse  it  in  an' acidulated  solution 
of  yellow  prussiate  of  potash.  Prussian  blue  will  be  precipitated 
upon  the  cloth  and  will  remain  firmly  attached  to  it.  Prussian  blue 
is  largely  employed  in  dyeing  and  calico  printing  in  a  variety  of 


558  TESTING    FOR    IRON. 

Now  that  we  have  discovered  a  ready  means  of  detecting 
ferrous  and  ferric  salts  it  will  be  well  to  determine  experimen- 
tally how  easily  the  members  of  either  of  these  classes  may  be 
changed  to  salts  of  the  other  class. 

Exp.  350. —  Dissolve  4  or  5  grins,  of  iron  tacks  or  wire  in  dilute 
chlorhydric  acid  in  a  test  tube,  pouring  off  the  liquid  from  time  to 
time  as  it  becomes  nearly  saturated.  Test  a  few  drops  of  the  solu- 
tion first  with  ferro-  and  then  with  ferri-cyanide  of  potassium  in 
order  to  prove  that  it  is  pure  ferrous  chloride.  Boil  the  rest  of  the 
liquid  with  a  few  drops  of  nitric  acid  to  convert  it  to  ferric  chloride, 
and  determine  when  the  conversion  has  been  completed  by  testing 
as  before.  Finally,  divide  the  ferric  solution  into  three  portions  :  — 
through  the  first  portion  pass  sulphydric  acid  gas ;  sulphur  will  be 
deposited  and  ferrous  choride  formed, 

Fe2Cl6+  H2S  =  2FeCl2  +  2HC1  +S  ; 

to  the  second  portion  add  small  fragments  of  protochloride  of  tin, 
until  a  drop  of  the  mixture,  tested  with  the  ferrocyanide,  will  no 
longer  give  a  blue  coloration, 

Fe2Cl6  +  SnCl2  =  2FeCl2  +  SnCL, ; 

boil  the  third  portion  with  a  fragment  of  metallic  zinc,  and  deter- 
mine the  fact  of  reduction  as  before, 

Fe2Cl6  +  Zn  =  2FeCl2  +  ZnCl2 . 

By  leaving  either  of  these  reduced  solutions  in  the  air,  or  by  heating 
them  with  a  little  chlorate  or  nitrate  of  potassium,  nitric  acid,  or 
other  oxidizing  agent,  they  may  be  readily  converted  again  to  the 
condition  of  ferric  salts. 

COBALT    AND    NICKEL. 

651.  Cobalt  and  nickel  are  two  metals  remarkably  similar 
to  one  another  both  in  physical  and  chemical  properties. 
They  are  found  together  in  nature  in  the  same  ores,  in  combi- 
nation with  sulphur  and  arsenic,  and  are  both  ingredients  of 
meteoric  iron.  They  can  be  reduced  from  their  oxides  by 
charcoal  and  by  hydrogen  at  high  temperatures,  and  the  metals 
thus  obtained  can  be  melted  about  as  readily  as  pure  iron. 
Both  cobalt  and  nickel  resemble  iron  more  closely  than  any 
other  common  metal ;  they  are  very  tenacious,  hard,  and  re- 
fractory ;  like  iron  they  are  magnetic,  and  when  hot  they  may 


COBALT    AND    NICKEL.  559 

be  forged ;  they  rust  less  readily  than  iron,  but  resemble  it 
closely  in  most  of  their  chemical  properties.  The  atomic 
weights  of  cobalt  and  of  nickel  are  identical ;  the  same  num- 
ber, 58.8,  applies  to  both.  The  specific  gravities  of  the  two 
metals  also  are  equal  or  nearly  so,  varying  in  different  sam- 
ples from  8.2  to  8.9.  Cobalt  is  not  used  in  the  metallic  state, 
but  several  of  its  compounds  are  remarkable  for  the  beauty  of 
their  color,  and  find  important  applications  in  the  arts  as  pig- 
ments, especially  for  coloring  glass  and  porcelain.  A  blue 
glass  containing  silicate  of  cobalt,  obtained  by  fusing  oxide  of 
cobalt  with  ordinary  glass  is  largely  employed  under  the  name 
of  smalt  as  a  verifiable  pigment.  This  coloration  may  readily 
be  exhibited  by  adding  a  minute  particle  of  any  cobalt  com- 
pound to  a  borax  bead  (§  490)  upon  a  loop  of  platinum  wire, 
and  again  placing  the  bead  either  in  the  oxidizing  or  in  the 
reducing  flame  of  the  blow-pipe.  Nickel,  on  the  other  hand, 
is  used  in  the  metallic  state  as  an  ingredient  of  various  alloys, 
of  which  the  alloy  known  as  German  silver,  composed  of  cop- 
per, zinc,  and  nickel,  is  one  of  the  most  important.  A 
whitish  alloy,  obtained  by  adding  nickel  to  copper,  is  some- 
times employed  for  coin  of  low  denominations. 

652.  Both  cobalt  and  nickel  form  protoxides  (CoO  and 
MO),  protochlorides,  and  protoxide  salts,  like  those  of  iron, 
except  that  the  protosalts  of  cobalt  and  nickel  are  far  more 
stable  than  the  salts  of  protoxide  of  iron,  so  that  the  pro- 
toxides of  cobalt  and  nickel  must  be  regarded  as  the  principal 
oxides  of  these  metals.  L^ke'lron,  chromium,  and  the  other 
rnetals  of  the  family  now  under  discussion,  cobalt  and  nickel 
also  unite  with  oxygen  to  form  sesquioxides  (CoaO3  and  Ni2O3)  " 
and  these  sesquioxides,  or  at  least  the  sesquioxide  of  cobalt, 
combine  with  bases  to  form  salts ;  but  these  salts  and  the 
sesquioxides  themselves  are  comparative  unstable  bodies  ;  they 
are  far  more  easily  decomposed  than  compounds  of  the  pro- 
toxides of  cobalt  and  nickel,  or  than  compounds  of  the  sesqui- 
oxides of  the  other  metals  of  the  group.  Hence,  in  the  matter 
of  nomenclature,  the  salts  of  the  protoxides  of  cobalt  or  nickel 
take  precedence  of  the  salts  of  the  sesquioxides.  When,  for  ex- 


560  URANIUM. 

ample,  nitrate  of  cobalt  is  spoken  of,  nitrate  of  protoxide  of 
cobalt  is  the  substance  referred  to ;  whereas  when  nitrate  of 
iron  or  of  chromium  is  mentioned,  without  further  specifi- 
cation, we  must  infer  that  the  nitrate  of  the  sesquioxide  is  the 
substance  meant.  The  use  of  terms  in  the  loose  manner  re- 
ferred to  in  the  foregoing  examples  is  of  course  always  to  be 
deprecated  ;  but,  in  order  to  avoid  the  chance  of  being  misun- 
derstood, some  chemists  have  extended  to  all  metals  having 
two  salifiable  oxides  the  use  of  the  terminations  ous  and  £c, 
which  has  been  exemplified  under  iron  by  the  terms  ferrous 
and  ferric  oxides.  Thus  the  terms  cobaltous  and  cobaltic  ox- 
ides, and  nickelous  and  nickelic  oxides  have  been  applied  by 
some  writers  to  the  oxides  of  cobalt  and  nickel ;  and  there  is  at 
present  a  tendency  to  adopt  and  amplify  this  system  of  names, 
but  they  are  as  yet  too  little  employed  in  the  literature  of 
science  to  find  appropriate  place  in  an  elementary  manual. 

URANIUM. 

Uranium  is  a  rare  metal  found  in  but  few  localities!  It  can 
be  reduced  from  its  chloride  by  means  of  hot  potassium,  but 
not  from  its  oxide  by  means  of  hydrogen.  Metallic  uranium 
is  of  a  steel-white  color,  and  is  somewhat  malleable ;  it  does 
not  oxidize  in  air  or  in  water  at  ordinary  temperatures,  but 
burns  brilliantly  when  strongly  heated  in  air.  It  dissolves  in 
chlorhydric  or  sulphuric  acid,  with  evolution  of  hydrogen,  and, 
in  general,  is  closely  analogous  to  iron  and  manganese  in  its 
chemical  behavior.  The  atomic  weight  of  uranium  is  120  ; 
its  specific  gravity  is  18.4. 

There «are  two  principal  oxides  of  uranium,  capable  of  unit- 
ing with  acids  to  form  salts,  a  protoxide  UrO  and  a  sesqui- 
oxide Ur2O3,  and  two  other  intermediate  oxides  formed  by  the 
union  of  the  proto-  and  sesqui-oxides  in  different  proportions. 
The  sesquioxide  also  plays  the  part  of  a  weak  acid  towards 
bases.  Uranium  is  never  used  as  a  metal,  but  compounds  strong 
of  it  are  somewhat  extensively  employed  for  coloring  glass, 
and  to  a  certain  extent  in  photography  also.  Sesquioxide  of 
uranium  imparts  a  beautiful  greenish-yellow  color  to  glass, 


SESQUIOXIDE    SALTS.  561 

and  the  glass  thus  colored  is  to  a  high  degree  fluorescent ;  the 
protoxide,  on  the  other  hand,  gives  a  fine  black,  highly  es- 
teemed for  painting  porcelain. 

654.  The  salts  of  sesquioxide  of  uranium  are  remarkable 
in  that  they  constitute  an  exception  to  the  general  rule,  that 
to  form  a  normal  salt,  as  many  molecules  of  the  acid  are  re- 
quired as  there  are  atoms  of  oxygen  in  the  base  employed. 
The  normal  sulphate  of  calcium,  for  example  (§  241),  may  be 
formed  by  the  union  of  CaO  and  SO3 ,  and  the  normal  sulphate 
of  sesquioxide  of  iron  is  composed  of  Fe2O3  and  3SO3 ,  but  in 
sulphate  of  sesquioxide  of  uranium  we  find  only  Ur2O3,  SO3, 
and  analogous  formulae  express  the  composition  of  the  nitrate 
and  other  salts  of  this  oxide.  But  in  spite  of  this  peculiarity, 
uranium  has  many  properties  in  common  with  the  other  mem- 
bers of  the  sesquioxide  group  of  metals.  One  characteristic, 
for  example,  of  this  aluminum-iron  group,  which  is  shared  by 
uranium,  is  that  the  sesquioxides  are  capable  of  uniting  with 
acids,  not  only  in  the  fixed  and  definite  proportions  requisite 
for  the  normal,  crystallized  salts  already  described,  but  also 
in  very  numerous  indefinite  proportions  to  form  soluble  basic 
compounds,  incapable  of  crystallization  for  the  most  part ;  and 
solidifying  in  tough,  shining  masses  like  gum,  when  their  so- 
lutions are  allowed  to  evaporate  spontaneously  in  the  air. 
Nitrate  of  iron,  for  example,  may  be  made  as  basic  as  the 
compound  8Fe2O3 ,  N2O5 ,  and  still  be  soluble  in  water  ;  and  be- 
tween this  limit,  on  the  one  hand,  and  that  of  the  crystallized 
normal  salt  (Fe2O3 ,  3N2O5+  18H2O)  upon  the  other,  sesqui- 
oxide of  iron  and  nitric  acid  can  combine  chemically  in  every 
conceivable  proportion.  The  compounds  of  sesquioxide  of  iron 
with  other  acids,  and  the  nitrates  and  other  salts  of  the  sesqui- 
oxides of  the  other  metals  of  the  group  all  behave  in  a  similar 
way,  the  compounds  of  uranium  being  no  exception  to  the  rule. 
This  tendency  to  form  soluble,  gummy,  polybasic  sesquisalts, 
so  strikingly  exhibited  by  the  members  of  the  group  of  ele- 
ments now  under  discussion,  is  evidently  one  of  those  obscure 
manifestations  of  the  chemical  force  which  we  have  already 

41 


562  THE    SESQUIOXIDE    GROUP. 

met  with  when  discussing  the  phenomena  of  solution  (§  49), 
and  the  law  of  multiple  proportions  (§  76,  end). 

655.  The   most    important    point    of    difference    between 
uranium  and  the  other  members  of  the  sesquioxide  group  of 
elements  is  the  fact,  already  alluded  to,  that  one  molecule  of 
its  sesquioxide  unites  with  but  one  molecule  of  base  to  form 
crystallized  salts,  whereas  the  sesquioxides  of  .the  other  mem- 
bers of  the  group  all  unite  with  acids  in  the  proportion  of  one 
molecule  of  base  to  three  molecules  of  acid  to  form  their  nor- 
mal, crystallized  salts.     Since  the  alums  are  formed  by  the 
union  of  a  normal  sulphate  ot  some  metal  of  the  alkali  group 
with  a  teracid  sulphate  of  some  metal  of  the  sesquioxide  group, 
there  is  no  such  thing  as   a  uranium-alum,  because  the  ter- 
acid uranium-sulphate  is  wanting.     Sesquioxide  of  uranium, 
in  fact,  behaves  among  bases  somewhat   as   metaphosphoric 
acid  does  among  acids ;  it  stands  in  much  the  same  relation 
to  the  other  teracid  bases  of  its  class,  as  metaphosphoric  acid 
to  the  ordinary  terbasic  phosphoric  acid. 

656.  The  Sesquioxide  Group.     The  bond  of  union  between 
the  metals  included  in  this  class  is  the  fact  that  they  all  form 
salifiable  sesquioxides.      Most  of  them  form  also  salifiable 
protoxides,  and  if  we  arrange  the  metals  in  the  order  of  their 
atomic  weights, 

Gl  =  14,  Al  =  27.4,  Cr  =  52.5,  Mn  =  55,  Fe  =  56, 

Ni  =  58.8,  Co  =  58.8,  Ur  =  120,  " 

it  will  be  apparent  that  the  sesquioxides  of  the  metals  at  the 
head  of  the  list  are  the  most  stable  of  the  sesquioxides,  and  that 
the  protoxides  of  nickel  and  cobalt  are  the  most  stable  of  the 
protoxides,  while  with  manganese  and  iron  both  forms  of  oxide 
are  well  represented.  Uranium  does  not  conform  to  this  ar- 
rangement. Glucinum  and  aluminum  have  no  protoxides  at 
all,  and  the  protoxide  of  chromium  is  very  unstable.  Some  of 
the  metals  of  the  group  are  usually  bivalent,  others  trivalent, 
while  others  are  both  bi-  and  trivalent. 

The  existence  of  the  class  of  salts  called  alums  suggests 
strongly  the  existence  of  a  natural  relation  between  the  mem- 
bers of  the  alkali-group,  on  the  one  hand,  and  between  the 


ATOMIC  VOLUME  OP  ALUMS.  563 

members  of  the  sesquioxide-group,  on  the  other.  These 
highly  crystallized,  isomorphous  salts  are  all  moulded  upon 
one  pattern,  and  their  atomic  volumes  (§  252)  are  very  nearly 
equal,  as  the  following  table  will  illustrate  for  some  of  the 
alums. 

Alums.          Atomic  Weight.         Specific  Gravity.         Atomic  Volume. 
KA1S208,12H20  474.5  1.722  275.6 

NaAlS208,12H2Q.  458.4  1.641  279.2 

(NH4)A1S208,12H20  453.4  1.621  279.6 

ZCrS208,  12H20  499.6  1.845  270.7 

(NH4)CrS208,12H20  478.5  1.736  275.5 

(NH4)FeS208,12H20  482.0  1.712  281.4 

It  is  a  fact  not  unworthy  of  notice,  that  the  compounds  of 
this  group  of  metals,  with  the  exception  of  those  of  glucinum 
and  aluminum,  are  for  the  most  part  colored,  independently 
of  the  colors  of  the  substances  with  which  they  are  united. 
The  metals  of  the  sodium,  calcium,  and  magnesium  groups  pro- 
duce colorless  compounds,  unless  when  joined  with  an  acid  pos- 
sessing a  color  of  its  own.  Glucinum  and  aluminum  produce, 
in  like  manner,  colorless  compounds,  but  the  oxides,  hydrates, 
chlorides,  bromides,  iodides,  sulphides,  and  oxygen-salts  of 
chromium,  manganese,  iron,  nickel,  cobalt,  and  uranium  are 
all  more  or  less  colored  in  themselves,  and  every  color  of  the 
spectrum,  from  the  violet  at  one  extremity  to  the  red  at  the 
other,  can  be  matched  from  among  the  innumerable  tints  ex- 
hibited by  the  various  compounds  of  the  last  six  members  of 
the  sesquioxide  group. 

657.  With  the  members  of  the  group,  now  under  discussion, 
are  commonly  classed  a  number  of  rare  metals,  more  or  less 
nearly  related  to  aluminum  and  iron.  They  are  all,  however, 
of  subordinate  interest,  and  need  only  be  named  in  this  man- 
ual. The  following  is  a  list  of  these  elements,  together  with 
their  symbols  and  their  atomic  weights,  in  so  far  as  the  lat- 
ter have  been  determined.  Yttrium,  Yt  =  68 ;  Erbium, 
Er  =(  ?)  ;  Terbium,  Tb  =(  ?)  ;  Zirconium,  Zr  =  90(  ?)  ;  Norium, 
No  =(  ?)  ;  Cerium,  Ce  =  92  ;  Lanthanum,  La  =  92.8  ;  Didymi- 
um,  Di  =  95;  Thorium,  Th  =  231.5 (?). 


564  EXTRACTION    OF    COPPER. 


CHAPTER    XXXI. 

COPPER  AND  MERCURY. 

COPPER. 

658.  Though  by  no  means  one  of  the  most  abundant  met- 
als, copper  is  nevertheless  very  widely  diffused  in  nature,  and 
is  largely  employed  by  man.  Traces  of  it  exist  in  almost 
every  soil,  whence  it  is  taken  up  by  plants,  in  which  it  may 
almost  always  be  detected  by  refined  testing.  Traces  of  it 
have  repeatedly  been  found  also  in  the  various  animal  organs 
and  secretions.  Many  natural  waters  contain  minute  quanti- 
ties of  copper ;  its  presence  may  often  be  recognized  in  the 
deposit  of  oxide  of  iron,  which  separates  JTroni  chalybeate 
waters.  Since  the  metal  occurs  native  in  many  localities, 
several  of  its  valuable  properties  were  early  recognized  and 
made  use  of.  Long  before  the  discovery  of  methods  of  reduc- 
ing iron  from  its  ores,  tools  and  weapons  made  of  native  cop- 
per were  employed  by  many  barbarous  nations. 

Besides  occurring  in  the  native  state,  copper  is  found  in  a  great 
variety  of  combinations ;  the  most  common  of  its  ores,  however,  is 
the  sulphide,  or  rather  a  compound  of  sulphide  of  copper  and  sul- 
phide of  iron  in  varying  proportions,  known  as  copper  pyrites. 
The  processes  of  obtaining  copper  from  its  ores  vary  greatly,  ac- 
cording to  the  quality  of  the  ore.  The  oxides  and  carbonates  may 
be  readily  reduced  by  heating  the  ore  in  contact  with  some  carbon- 
aceous material,  and  a  flux  suitable  to  remove  the  impurities  of  the 
ore.  The  treatment  of  ores  containing  sulphur  is  far  more  com- 
plicated. Such  ores  are  roasted  in  the  first  place,  in  order  to  con- 
vert a  considerable  portion  of  the  sulphides  of  copper  and  iron  into 
oxides ;  a  proper  flux  is  then  added  to  the  roasted  ore,  and  the  whole 
is  melted  down  in  either  a  reverberatory  or  blast  furnace.  The 
oxide  of  copper  formed  by  roasting,  is  reconverted  into  sulphide, 
while  much  of  the  sulphide  of  iron,  which  had  escaped  oxidation 
before,  is  now  changed  to  oxide  and  passes  off  in  the  slag.  Sul- 


PROPERTIES    OF    COPPER.  565 

phide  of  copper,  comparatively  free  from  iron,  is  thus  obtained,  — 
in  other  words,  the  copper  ore  is  very  much  concentrated  by  the 
operation.  If  need  be,  the  concentrated  product  is  subjected  to  a 
series  of  roastings  and  meltings  similar  to  the  first,  until  it  has  been 
almost  completely  freed  from  sulphide  of  iron  and  other  impurities. 
The  pure  or  nearly  pure  sulphide  of  copper  is  then  roasted  in  a  cur- 
rent of  air,  until  a  certain  proportion  of  the  sulphide  has  been  con- 
verted into  oxide.  Finally,  the  mixture  of  sulphide  and  oxide  is 
strongly  heated  to  a  temperature  at  which  its  ingredients  react  upon 
one  another  in  such  manner  as  to  yield  sulphurous  acid  and  metallic 
copper : — 

Cu2S  +  2CuO  =  SO2  +  4Cu. 

Sometimes  copper  is  obtained  by  precipitating  it  with  iron  from 
solutions  of  its  salts,  as  has  been  shown  in  Exp.  344.  The  copper 
thus  thrown  down  by  iron  is  known  as  cement-copper,  and  is  fre- 
quently obtained  from  the  drainage-water  of  certain  mines,  in  which 
a  small  proportion  of  sulphide  of  copper  is  oxidized  by  the  air  to 
sulphate  of  copper,  and  so  carried  into  solution.  In  some  localities, 
low-grade  copper-ores  are  lixiviated  with  chlorhydric  acid,  obtained 
as  a  waste  product  from  the  manufacturers  of  soda-ash,  and  the 
copper  solution  subsequently  made  to  flow  over  fragments  of  scrap 
iron. 

The  common  method  of  assaying  copper  ores  is  another  applica- 
tion of  the  precipitation  of  copper  by  means  of  iron. 

659.  Copper  is  a  rather  hard  metal,  of  a  well-known  red 
color ;  it  is  very  tenacious,  ductile,  and  malleable.  The  spe- 
cific gravity  of  the  metal  when  free  from  air-bubbles  varies 
between  8.92  and  8.95.  Copper  melts  less  readily  than  silver, 
but  more  readily  than  gold ;  its  melting-point  has  been  esti- 
mated to  be  about  1170°.  At  an  intense  heat  it  volatil- 
izes, though  for  all  ordinary  purposes  it  may  be  regarded  as 
non-volatile.  It  is  one  of  the  best  conductors  of  heat  and 
electricity  known.  Its  specific  heat  is  0.09515.  Copper  com- 
bines with  oxygen  far  less  readily  than  iron.  Even  at  a  bright 
red  heat  it  is  not  capable  of  decomposing  water,  excepting  to 
a  very  slight  extent.  Finely  divided  copper,  however,  soon 
becomes  oxidized  on  being  exposed  to  the  air ;  thdugh,  as  is 
well  known,  solid  masses  of  the  metal  suffer  little  or  no  change, 
at  the  ordinary  temperature,  in  air  free  from  sulphydric  and 


566  ALLOYS    OF    COPPER. 

carbonic  acids.  When  strongly  heated  in  the  air,  copper 
quickly  becomes  covered  with  a  coating  of  black  oxide  of  cop- 
per (See  Exp.  12).  Metallic  copper  is  not  very  readily  acted 
upon  by  acids,  excepting  those  rich  in  oxygen.  The  weaker 
acids,  such  as  acetic  acid,  have  no  action  upon  it,  unless  air 
be  present,  in  which  event  the  metal  is  soon,  corroded ;  and 
the  same  remark  applies  to  dilute  chlorhydric  and  sulphuric 
acids.  Finely  divided  copper,  however,  slowly  dissolves  with 
evolution  of  hydrogen  in  hot,  concentrated  chlorhydric  acid ; 
and  in  hot  oil  of  vitriol  the  metal  dissolves  readily,  as  has 
been  seen  in  the  preparation  of  sulphurous  acid.  (See  Exp. 
100.) 

Copper  is  readily  soluble  in  somewhat  diluted  nitric  acid, 
such  as  is  commonly  found  in  commerce  (See  Exp.  37)  ;  but 
the  strongest  nitric  acid,  of  specific  gravity  1.52,  does  not  act 
upon  it.  When  immersed  in  such  acid  the  metal  remains 
bright,  and  no  bubbles  of  gas  arise  from  its  surface.  The 
phenomenon  is  explained  by  the  fact  that  nitrate  of  copper  is 
insoluble  in  monohydrated  nitric  acid,  though  readily  soluble 
in  water  and  in  dilate  acid.  Ammonia-water  and  many  salts, 
such  as  chloride  of  sodium  and  the  various  salts  of  ammo- 
nium, corrode  copper  rather  rapidly  when  in  contact  with  air. 
Finely  divided  copper  takes  fire  in  chlorine  gas,  and  at  a  red 
heat  the  metal  unites  directly  with  bromine,  iodine,  sulphur, 
silicon,  and  the  various  metals.  It  does  not  appear  to  unite 
directly  with  carbon  or  with  nitrogen  at  any  temperature. 

Several  of  its  compounds  with  other  metals  are  of  great 
importance  in  the  arts.  Brass  and  the  yellow-metal  used  for 
sheathing  ships  are  alloys  of  zinc  and  copper ;  bronze,  gun- 
metal,  and  bell-metal  are  alloys  of  tin  and  copper,  and  various 
compositions  are  produced  by  mixing  these  alloys  with  brass. 
German-silver  in  its  various  forms  is  an  alloy  of  nickel,  zinc, 
and  copper ;  and  copper  is  an  essential  ingredient  of  all  the 
common  coins,  implements,  and  ornaments  of  gold  and  silver. 

660.  Dinoxide  of  Copper  (Cu2O)  is  sometimes  found  in  na- 
ture as  Ruby  copper ;  it  may  readily  be  obtained  by  heating 
protoxide  of  copper  with  finely  divided  metallic  copper,  or 


DINOXIDE    OF    COPPER.  567 

other  reducing  agents ;  so,  too,  when  masses  of  metallic  cop- 
per are  gently  heated  in  the  air,  they  become  covered  with  a 
thin  film  of  the  dinoxide. 

Exp.  351.  —  Dissolve  in  a  test-tube,  a  few  drops  of  honey  or  a  bit 
of  grape  sugar  in  a  little  water.  Add  to  the  solution  two*or  three 
drops  of  a  rather  dilute  solution  of  sulphate  of  copper,  and  then 
pour  in  enough  soda-lye  to  redissolve  the  precipitate  which  is  at 
first  produced  by  the  lye. 

Slowly  heat  the  clear  blue  solution,  and  observe  that  a  yellow 
precipitate  of  hydrated  dinoxide  of  copper  soon  separates,  first  at 
the  uppermost  part  of  the  column  of  liquid,  but  soon  in  all  parts 
of  the  tube,  as  its  contents  become  sufficiently  hot.  When  the  liquor 
is  heated  to  boiling,  the  hydrated  yellow  precipitate  changes  after 
a  time  to  anhydrous  red  dinoxide. 

Most  of  the  dilute  acids  decompose  dinoxide  of  copper  with 
formation  of  salts  of  the  protoxide  and  separation  of  metallic 
copper.  But  it  dissolves  in  concentrated  chlorhydric  acid  and 
in  ammonia-water,  forming  colorless  solutions.  The  ammo- 
niacal  solution  may  be  employed  as  a  test  for  the  presence  of 
oxygen  in  any  mixture  of  gases ;  oxygen  is  immediately  ab- 
sorbed by  the  solution  and  a  compound  of  protoxide  of  copper 
and  ammonia  (Exp.  357)  of  characteristic  deep  blue  color  is 
formed.  Dinoxide  of  copper  is  employed  to  a  certain  extent 
for  coloring  glass  ruby -red. 

661.  Protoxide  of  Copper  (CuO)  maybe  prepared  by  heat- 
ing the  metal  or  the  dinoxide  in  a  current  of  air,  or  by  igniting 
carbonate,  hydrate,  or  nitrate  of  copper. 

Exp.  352.  — Bind  a  bright  copper  coin  with  wire,  in  such  manner 
that  a  strip  of  wire  8  or  10  c.  m.  long  shall  be  left  projecting  from 
the  coin';  thrust  the  free  end  of  the  wire  into  a  long  cork  or  bit  of 
wrood,  and  by  means  of  this  handle  hold  the  coin  obliquely  in  a 
small  flame  of  the  gas-lamp.  A  beautiful  play  of  iridescent  colors 
will  appear  upon  the  surface  of  the  copper,  particularly  if  it  be 
moved  to  and  fro.  Thrust  the  hot  coin  into  water,  and  observe  that 
it  is  at  this  stage  covered  with  a  coating  of  red  suboxide  of  copper. 
Replace  the  coin  in  the  lamp  and  hold  it  in  the  hot  oxidizing  portion 
of  the  flame  (Exp.  199)  ;  it  will  soon  become  black  from  the  forma- 
tion of  protoxide  of  copper.  After  a  rather  thick  coating  of  oxide 
has  been  formed,  again  quench  the  coin  in  water ;  the  black  coating 


568  PROTOXIDE    OF    COPPER. 

or  scale  of  oxide  will  fall  off,  and  beneath  it  will  be  seen  a  thin  film 
of  the  dinoxide  firmly  adhering  to  the  metal.  This  film  of  dinoxide 
is  intentionally  produced  upon  the  surfaces  of  many  copper  imple- 
ments by  the  manufacturers.  If  the  coin  were  heated  long  enough 
it  would  $11  be  converted,  first  into  the  red  dinoxide,  and  then  into 
black  protoxide  of  copper.  The  scales  which  fall  off  when  hot 
metallic  copper  is  beaten  or  rolled,  like  those  obtained  from  the  coin 
in  this  experiment,  always  consist  of  a  mixture  of  the  two  oxides. 

Exp.  353.  —  Evaporate  to  dryness  in  a  porcelain  dish  upon  a  sand- 
bath  some  of  a  solution  of  nitrate  of  copper  prepared  from  copper 
as  in  Exp.  37.  There  will  be  left  as  a  residue  a  green  basic  nitrate 
of  copper.  Place  a  small  quantity  of  this  residue  upon  a  fragment  of 
porcelain,  and  ignite  it  until  red  nitrous  fumes  are  no  longer  given 
off.  Pure  protoxide  of  copper  will  be  left  upon  the  porcelain. 

Though  no  oxygen  can  be  expelled  from  protoxide  of  cop- 
per by  mere  exposure  to  heat,  all  its  oxygen  may,  neverthe- 
less, be  removed  with  great  facility  by  means  of  reducing 
agents.  Oxide  of  copper  is,  in  fact,  one  of  the  most  conven- 
ient oxidizing  agents  in  the  chemist's  possession,  and  is 
largely  employed  to  this  end  in  the  analysis  of  organic  com- 
pounds. When  heated  with  carbonaceous  substances,  it  con- 
verts all  their  carbon  into  carbonic  acid,  and,  in  like  manner, 
hydrogen  is  immediately  oxidized  by  it  and  converted  into 
water.  Since  carbonic  acid  and  water  can  readily  be  collected 
and  weighed,  and  since  their  composition  is  accurately  known, 
the  determination  of  the  amounts  of  carbon  and  hydrogen  in 
any  substance,  through  the  agency  of  oxide  of  copper,  is  merely 
a  matter  of  mechanical  detail. 

Exp.  354.  — Repeat  Exp.  335,  with  the  exception  that  ateaspoon- 
ful  of  black  oxide  of  copper  (Exp.  553)  is  placed  in  the  tube  instead 
of  the  iron  rust.  Water  may  be  collected  in  the  U-tube  as  before, 
and  metallic  copper  will  be  left  in  the  reduction-tube.  But,  unlike 
the  easily  oxidizable  iron,  the  reduced  copper  will  not  take  fire  in 
the  air. 

662.  Protoxide  of  copper  is  soluble  in  most  acids,  with 
formation  of  salts,  which  are  blue  or  green  when  hydrated,  but 
white  when  thoroughly  dried.  From  the  solutions  of  most  of 
these  salts  hydrated  oxide  of  copper  may  be  precipitated  by 
means  of  any  of  the  strong  soluble  bases. 


SULPHIDES    OF    COPPER.  569 

Exp.  355. — Place  in  a  test-tube,  or  small  bottle,  8  or  10  c.c.  of  a 
cold  dilute  solution  of  sulphate  of  copper  (Exp.  100),  and  add  to  it 
enough  of  a  solution  of  caustic  soda  to  render  the  mixture  alkaline 
to  test  paper.  A  light  blue  precipitate  will  fall ;  hydrate  of  copper 
is  insoluble  in  water  and  in  soda  lye. 

Exp.  356.  —  Repeat  Exp.  355,  with  the  difference  that  the  solu- 
tions of  caustic  soda  and  sulphate  of  copper  are  both  heated  to 
boiling  and  are  mixed  while  hot.  Instead  of  the  blue  hydrate, 
black  protoxide  of  copper  will  now  be  thrown  down,  for  hydrate 
of  copper  readily  parts  with  its  water  when  heated,  even  if  it  be  all 
the  while  immersed  in  water;  it  does  not  again  combine  with  water 
after  it  has  become  cold.  Instead  of  mixing  boiling  solutions  of 
the  alkali  and  copper  salt,  the  moist  precipitated  hydrate  of  cop- 
per, of  Exp.  355,  might  be  changed  to  black  oxide  by  simple  boil- 
ing ;  but  the  transformation  would  be,  comparatively  speaking,  slow, 
and  the  experiment  less  striking  than  the  one  here  described. 

Exp.  357.  —  Again  repeat  Exp.  355,  but  instead  of  soda  lye  add 
to  the  copper  salt  ammonia-water,  drop  by  drop,  and  shake  the 
tube  after  each  addition  of  the  ammonia.  Hydrate  of  copper  will 
be  precipitated  as  before  in  accordance  with  the  reaction, 

CuSO4  +  2(NH4)HO  =  (NH4)2SO4  +  CuH2O2  ; 
for,  as  has  been  said,  this  hydrate  is  insoluble  in  water ;  but,  since  hy- 
drate of  copper  is  readily  soluble  in  ammonia-water,  the  precipitate 
will  redissolve  as  soon  as  more  of  this  agent  than  is  needed  to  de- 
compose the  copper  salt  is  added.  The  arnmoniacal  solution  of  cop- 
per has  a  magnificent  azure-blue  color. 

663.  The  Sulphides  of  Copper  (Cu2S)  and  (CuS)  are  inter- 
esting from  their  occurrence  as  ores,  and  from  the  reactions 
already   briefly   explained,    which   are    so   important   in   the 
industry  of  copper-smelting  (§658).     The  protosulphide,  as 
obtained  by  adding  sulphydric  acid  to  acidulated  solutions  of 
the  salts  of  copper,  is  an  important  substance  to  the  analyst ; 
it  is  a  black  powder,  insoluble  in  water,  in  dilute  acids  and 
in  alkaline  lyes. 

664.  Dichloride  of  Copper  (Cu2Cl2)  may  be  obtained  by  treat- 
ing a  mixture  of  protoxide  of  copper  and  finely  divided  me- 
tallic copper  with  concentrated  chlorhydric  acid,  or  by  boiling 
a  solution  of  the  protochloride  with  sugar,  or  some  other  re- 
ducing agent.    It  is  a  white  compound,  insoluble  in  water,  but 

42 


570  CHLORIDES    OF    COPPER. 

soluble  in  strong  chlorhydric  acid,  and  in  aqueous  solutions 
of  chloride  of  sodium,  chloride  of  potassium,  and  many  other 
chlorides. 

665.  Protochloride  of  Copper  (CuCl2)  is  formed  when  cop- 
per is  burned  in  an  excess  of  chlorine.     It  may  readily  be  pre- 
pared in  the  hydrated  condition  (CuCl2+ 2H2O)  by  dissolv- 
ing oxide,  hydrate,  or  carbonate  of  copper  in  chlorhydric  acid, 
and  evaporating  the  solution  upon  a  water  bath.     Anhydrous 
chloride  of  copper  is  brown,  but  the  hydrated  salt  forms  green, 
needle-shaped  crystals.     The  concentrated  aqueous  solution  is 
green  ;  when  diluted  with  water  it  becomes  blue,  but  turns 
green  again  on  being  boiled.    The  dry  salt  fuses  when  heated, 
and  at  a  red  heat  gives  off  half  its  chlorine  and  is  changed  to 
the  dichloride.     Chloride  of  copper  is  soluble  in  alcohol  and 
ether ;  if  some  of  the  alcoholic  solution  is  poured  upon  a  tuft 
of  cotton  and  then  ignited,  it  will  burn  with  a  green  flame, 
which  is  characteristic  of  copper. 

666.  Sulphate  of  Copper  (CuSO4)  has  been  already  obtained 
in  solution  as  a  secondary  product  in  Exp.  100.    It  may  also 
be  readily  prepared  by  dissolving  oxide  of  copper,  copper- 
scale  for  example,  in  moderately  dilute  sulphuric  acid.    Much 
of  it  was  formerly  prepared  in  this  way  for  use  in  the  arts  ; 
it  is  an  incidental  product   also   in  the  process   of  refining 
gold   and   silver,   and   in   certain    metallurgical   operations. 
As   it  crystallizes  from  aqueous  solutions,  sulphate  of  cop- 
per holds  in  chemical  combination  5  molecules  of  water,  and 
may  then  be  represented  by  the  formula  CuSO4+  5H2O.    This 
hydrated  salt,  known  as  blue  vitriol,  is  the  commonest  salt  of 
copper  ;  most  of  the  various  pigments  and  other  preparations 
of  copper,  medicinal  or  chemical,  are  made  from  it,  and  it  is 
itself  used  to  a  considerable  extent  by  dyers  and  calico-print- 
ers, and  largely  by  the  electro typers. 

Exp.  358.  —  Tie  a  piece  of  bladder  over  one  end  of  a  lamp  chim- 
ney, or  over  the  mouth  of  a  wide-mouthed  bottle  or  beaker,  off 
which  the  bottom  has  been  somewhat  evenly  broken.  -Solder  a 
piece  of  thick  copper  wire  to  a  strip  of  stout  sheet  zinc,  just  wide 
enough  to  enter  the  chimney  or  bottle,  and  a  little  longer  than  the 
bottle  is  deep.  Place  the  zinc  in  the  bottle  or  chimney,  and  sink 


\ 
SULPHATE    OF    COPPER.  571 

the  bottle  or  chimney,  with  the  closed  end  down,  in  a  beaker  or 
large  tumbler  containing  a  strong  solution  of  sulphate  of  copper. 
Fill  the  bottle  or  chimney  with  dilute  nitric  acid,  attach  to  the  cop- 
per wire  a  clean  medal  or  coin,  of  which  one  side  has  been  var- 
nished, and  the  other  rubbed  over  with  plumbago,  and  bend  the 
wire  so  that  the  medal  or  coin  may  be  immersed  in  the  sulphate  of 
copper  solution  contained  in  the  beaker  or  tumbler.  Thorough 
contact  must  be  secured  between  the  copper  wire  and  the  unvar- 
nished side  of  the  coin  or  medal.  In  a  few  hours  a  coherent  film 
of  solid,  malleable  copper  will  be  firmly  deposited  on  that  face  of 
the  coin  or  medal  which  was  not  protected  by  the  non-conducting 
varnishl  The  shell  of  copper  may  be  readily  detached  from  the 
coin  or  medal,  the  plumbago  ensuring  the  ready  separation  of  the 
two  metallic  surfaces  ;  it  is  a  perfect  reverse  of  the  object  copied. 

This  experiment  illustrates  on  a  small  scale  the  important  art  of 
electro-metallurgy.  Plating  in  gold  and  silver,  as  well  as  in  cop- 
per, is  extensively  performed  by  a  process  perfectly  analogous  to 
that  of  this  experiment.  Wood-cuts,  type,  medals,  maps,  and  en- 
gravings are  accurately  copied  by  means  of  this  deposition  of  met- 
als from  their  solutions  under  the  action  of  the  galvanic  current. 

It  is  remarkable  that  the  blue  color  of  sulphate  of  copper 
depends  upon  the  presence  of  water. 

Exp.  359.  —  Heat  1  c.  c.  of  powdered  blue  sulphate  of  copper 
upon  a  piece  of  porcelain ;  as  it  loses  its  water,  the  light-blue  pow- 
der will  turn  white.  A  drop  of  water  upon  the  anhydrous  powder 
will  restore  its  blue  color. 

If  concentrated  sulphuric  acid  be  poured  over  the  blue  crystals,* 
it  will  abstract  water  from  them,  and  a  quantity  of  the  white  anhy- 
drous salt  will  be  formed. 

Since  the  protoxides  of  iron  and  of  copper  are  isomorphous, 
or  rather  since  the  metals  iron  and  copper  are  capable  of  re- 
placing one  another  in  many  of  their  compounds,  it  is  not 
surprising  that  the  sulphates  of  iron  and  copper  should  crys- 
tallize together  to  form  a  compound  which  may  contain  almost 
any  proportion  of  either  salt.  This  isomorphous  mixture  of 
the  two  salts  was  formerly  largely  employed  in  the  arts,  and 
is  still  somewhat  used  to  meet  the  requirements  of  old  receipts 
for  dyeing.  In  a  similar  way,  sulphate  of  copper  crystallizes 
together  with  the  sulphates  of  nickel,  cobalt,  zinc,  and  mag- 
nesium. 


572  VERDIGRIS. 

667.  Nitrate  of  Copper  (CuN2O6)  may  be  obtained  crystal- 
lized, by  allowing  the  blue  solution,  obtained  in  Exp.  37,  to 
evaporate   spontaneously  in  dry  air.     It  is  interesting  as  an 
example  of  a  salt  ready  to  give  up  oxygen  on  slight  provoca- 
tion.    If  a  piece  of  tin-foil  about  20  c.  m.  square  be  twisted 
firmly  around  a  rather  large  crystal  of  nitrate  of  copper,  then 
pierced  in  several  places  with  a  needle,  and  moistened  with 
water,   or   with   a  few  drops  of  common   spirits  of  wine,  a 
powerful  re-action  will  soon  ensue.     The»tin  will  be  oxidized, 
at  the  expense  of  the  oxygen  of  the  nitrate  of  copper,  much 
heat  will  be  evolved,  and  smoke,  or  even  flame,  produced. 

668.  Acetates  of  Copper  are  formed  by  the  action  of  acetic 
acid  upon  metallic  copper,  exposed  to  the  air.    They  are  com- 
monly called  verdigris.     Purified  verdigris  is  the  normal  ace- 
tate  of  copper ;  and  common  verdigris  is  a  hydrated  basic 
acetate.     Verdigris  is  usually  prepared,  by  packing  plates  of 
copper  between  woollen  cloths  steeped  in  vinegar ;  but  some- 
times, in  wine-growing  countries,  the  refuse  of  the  wine-press 
is  suffered  to  ferment  in  contact  with  the  copper  plates.    From 
time  to  time,  the  verdigris  is  removed  from  the  surface  of  the 
copper,  the  plate  of  metal  being  again  and  again  subjected  to 
the  action  of  acetic  acid,  so  long  as  any  of  it  remains. 

In  ordinary  language,  the  term  verdigris  is  often  incorrectly 
applied  to  the  green  coating  of  carbonate  of  copper,  which 
forms  upon  copper  long  exposed  to  damp  air ;  or  to  the  rust 
formed  upon  copper,  by  the  combined  action  of  air  and  al- 
most any  acid.  A  compound  of  acetate  of  copper  and  arse- 
nite  of  copper,  constitutes  the  beautiful  and  vivid  green  color, 
known  as  Schweinfurt  green. 

MERCURY. 

669.  Small   globules   of  metallic   mercury  are   sometimes 
found  in  nature ;  but   the  principal  ore  of  this  metal  is  the 
sulphide    (HgS),  called   cinnabar.     From   this   sulphide,  the 
metal  is  readily  extracted,  by  distilling  a  mixture  of  it  and 
quick-lime,    or  iron-turnings,  in  cast-iron  retorts.     The  sul- 
phur  is  retained  by  the  lirne,  or  iron,  as   the  case  may  be, 


MERCURY.  573 

while  metallic  mercury  passes  off  in  the  state  of  vapor,  into 
receivers  containing  water,  beneath  which,  it  condenses  to  the 
liquid  state. 

A  rougher  method  of  manufacture  is,  to  heat  the  coarsely 
broken  sulphide  on  a  perforated  brick  arch,  by  a  quick  fire  of 
brush-wood ;  the  sulphur  in  the  ore  is  kindled,  and,  by  its 
combustion,  maintains  the  heat  necessary  to  continue  the  dis- 
tillation. The  liberated  mercury  is  condensed  in  wide  and 
long,  earthen  pipes,  which  slope,  first  down,  and  then  up. 

670.  At  the  ordinary  temperature  of  the  air,  mercury  is  a 
brilliant,  mobile  liquid,  of  13.6  specific  gravity,  which  vapor- 
izes slowly,  even  at  ordinary  temperatures,  and  boils  at  about 
360°.  The  vapor-density  of  mercury  does  not  coincide  with 
its  atomic  weight.  As  is  the  case  with  the  metal  cadmium 
(§  599),  the  atomic  weight  of  mercury  is  the  weight  of  two 
unit-volumes  of  its  vapor  ;  and  is,  therefore,  double  the  vapor- 
density,  instead  of  identical  with  it.  Into  the  product-volume 
of  any  compound  of  mercury,  one  more  volume  is  condensed 
than  would  be  contained  in  the  product-volume  of  a  corre- 
sponding compound  of  oxygen  or  sulphur.  The  atomic  weight 
of  mercury  is  200,  and  two  unit-volumes  of  its  vapor,  weigh 
200  times  as  much  as  one  unit-volume  of  hydrogen  ;  its  vapor- 
density  should,  theoretically,  be  100,  and  the  results  of  ex- 
periment approximate  closely  to  this  number.  (Compare 
§  259.) 

When  cooled  to — 39.4°  mercury  freezes.  In  solidifying, 
liquid  mercury  contracts  considerably,  and  there  results  a 
ductile  metal  of  tin-white  color  and  granular  fracture,  which 
may  be  cut  with  a  knife.  Perfectly  pure  mercury  undergoes 
no  change  in  air  or  in  oxygen  gas  at  the  ordinary  temperature, 
even  when  shaken  about  in  the  gas  for  a  long  while ;  but  if 
mercury,  containing  traces  of  foreign  metals,  such  for  example 
as  that  ordinarily  met  with  in  commerce,  be  exposed  to  the 
air,  a  gray  pulverulent  coating  will,  after  a  while,  appear  upon 
its  surface.  This  coating  is  composed  of  oxides  of  the  con- 
taminating metals  mixed  with  finely  divided  metallic  mercury. 
A  similar  gray  powder  of  finely  divided  mercury  may  be  ob- 


574  OXIDES    OF    MERCURY. 

tained  by  triturating  mercury  with  sulphur,  tallow,  and  a 
variety  of  other  substances,  or  simply  by  shaking  it  with 
water  or  oil  of  turpentine.  When  heated  in  the  air  to  tem- 
peratures near  its  boiling-point,  even  pure  mercury  absorbs 
oxygen,  and  is  converted  into  the  red  oxide  (§  672).  Metal- 
lic mercury  combines  directly  with  chlorine,  bromine,  iodine, 
and  sulphur. 

Chlorhydric  acid  has  no  action  upon  mercury,  not  even 
when  it  is  hot  and  concentrated.  Dilute  sulphuric  acid  has 
scarcely  any  action  upon  it,  but  hot  concentrated  sulphuric 
acid  converts  it  into  solid  sulphate  of  mercury,  while  sulphu- 
rous acid  is  evolved  (Exp.  100).  Nitric  acid,  even  when 
dilute,  dissolves  it  easily. 

Large  quantities  of  mercury  are  used  in  extracting  gold  and 
silver  from  their  ores,  for  silvering  mirrors,  and  in  the  process 
of  fire-gilding.  Preparations  of  mercury  are  employed  also 
as  medicaments,  and  for  various  purposes  in  the  useful  arts. 
The  fluidity  of  the  metal  makes  it  valuable  in  the  construction 
of  certain  philosophical  instruments,  of  which  the  thermometer 
and  barometer  are  familiar  examples. 

There  are  two  oxides  of  mercury,  a  black  dinoxide  and  the 
red  protoxide  ;  each  of  these  oxides  unites  with  acids  to  form  a 
peculiar  class  of  salts. 

671.  Dinoxide  of  Mercury  or  Mercurous  Oxide  (Hg2O)  is 
best  prepared  by  decomposing  one  of  its  salts,  calomel,  for 
example  (§  674),  by  means  of  caustic  soda.     Though  a  rather 
powerful  base  when  in  combination,  it   decomposes  readily 
when  in  the  free  state,  mere  exposure  to  light  or  to  gentle 
heat  being  sufficient  to  decompose  it  into  metallic  mercury 
and  the  red  oxide.     With  acids  it  combines  readily,  with  forma- 
tion of  salts  of  the  dinoxide  of  mercury,  or  mercurous  salts, 
as  they  are  often  called. 

672.  Protoxide  of  Mercury  or  Mercuric  Oxide  (HgO)  may  be 
prepared   by  heating   metallic  mercury  in  the  air  as  above 
described,  or  better,  by  heating  nitrate  of  mercury  at  a  tem- 
perature high  enough  to  drive  off  oxides  of  nitrogen,  but  at 
the  same  time  too  low  to  decompose  the  oxide  of  mercury ; 


RED    OXIDE    OF    MERCURY.  575 

or  again  by  precipitating  the  solution  of  a  salt  of  protoxide 
of  mercury  by  means  of  a  caustic  alkali.  As  obtained  by  the 
first  two  methods,  it  is  a  compact,  granular,  almost  crystalline, 
glistening  powder,  of  bright  brick-red  color,  but  when  pre- 
pared by  the  last  method  it  is,  when  dry,  a  soft,  light  orange- 
colored  powder.  Very  considerable  differences  in  the  chemical 
deportment  of  these  red  and  yellow  varieties  of  the  oxide 
have  been  noticed,  though  the  differences  are  hardly  so  great 
as  are  usually  found  between  the  isomeric  modifications  of 
other  substances.  The  precipitated  yellow  oxide  is,  for  exam- 
ple, more  readily  decomposed  by  heat  and  by  chlorine,  than 
the  red  oxide.  The  red  oxide,  however,  is  the  substance 
known  as  oxide  of  mercury  in  commerce  and  the  laboratory. 

Considered  as  a  source  of  oxygen,  red  oxide  of  mercury  is 
peculiarly  interesting,  since  by  means  of  it  oxygen  may  be 
derived  directly  from  the  air  (§  12)  ;  but  it  neither  affords 
the  gas  cheaply,  nor  yields  an  abundant  supply.  Since  red 
oxide  of  mercury  contains  only  a  single  atom  of  oxygen  for 
each  atom  of  mercury,  and  since,  the  atomic  weight  of  mer- 
cury (200)  is  comparatively  high,  any  given  weight  of  the 
oxide  can,  of  course,  contain  but  a  small  proportion  of  oxy- 
gen :  —  for 

216  :  16  1         :       0.074 

weight  of  weight  of  Grm.  Grm. 

a  molecule  an  atom 

of  HgO.  of  0. 

Although  it  contains  so  small  a  proportion  of  oxygen,  com- 
pared with  the  nitrates  and  chlorates  commonly  employed  for 
effecting  oxidation,  yet,  from  the  facility  with  which  it  gives 
up  its  oxygen,  mercuric  oxide  is  still  an  oxidizing  agent  of 
considerable  power.  If  a  small  portion  of  it  be  mixed  with  a 
little  sulphur,  and  then  heated,  the  mixture  will  explode  ;  so, 
too,  if  it  be  mixed  with  a  small  fragment  of  phosphorus,  and 
struck  with  a  hammer  upon  an  anvil,  a  similar  explosion  will 
ensue;  the  violent  action  depends  upon  rapid  oxidation  in 
both  cases. 

Most  acids  unite  readily  with  oxide  of  mercury  to  form 


576  CALOMEL. 

salts,  often  spoken  of  as  mercuric  salts.  Both  the  oxides  of 
mercury  are,  like  the  protoxide  of  lead  (§  575),  remarkable 
for  the  facility  with  which  they  form  basic  salts.  In  general, 
the  compounds  of  mercury  unite  with  one  another  readily  to 
form  a  great  variety  of  double  compounds  and  abnormal  basic 
salts,  such  as  the  oxychlorides  #HgO,  HgCl2  ,  and  the  chloro- 
sulphide  2HgS,HgCl2.  The  properties  of  several  of  these 
complex  substances  are  interesting,  but  none  of  them  fairly 
fall  within  the  scope  of  this  manual. 

673.  Bisulphide  of  Mercury  (Hg2S)  is  a  compound  nearly 
as  unstable  as  the  dinoxide,  but  the  proto  sulphide  (HgS)  is  a 
permanent  substance  of  considerable  importance  in  the  arts. 
Artificially   prepared    for    use   as  a    pigment,   it    is    known 
under  the  name  of  vermilion.     It  is  the  most  important  ore  of 
mercury,  as  has  been  already  stated  (§  669). 

674.  Mercurous  Chloride   (Hg  Cl)  ,  commonly  called  calo- 
mel, is  extensively  used  as  a  medicament. 

It  may  be  prepared  either  by  heating  together  a  mixture  of 
metallic  mercury  aiid  corrosive  sublimate  (§  675),  until  the  dichlo- 
ride  sublimes  :  — 


or  by  subliming  an  intimate  mixture  of  equal  parts  of  sulphate  of 
dinoxide  of  mercury  and  common  salt  :  — 

Hg2  SO,  +  2Na  Cl  =  2  Hg  Cl  +  Na2  SO4  . 

By  the  way  of  precipitation  it  may  be  made  by  mixing  together 
solutions  of  common  salt  and  nitrate  of  dinoxide  of  mercury  :  — 


In  place  of  the  corrosive  sublimate  used  in  the  first  method,  inti- 
mate mixtures  of  sulphate  of  protoxide  of  mercury  and  common 
salt,  or  of  common  salt  and  black  oxide  of  manganese  or  sulphate 
of  sesquioxide  of  iron,  may  be  substituted. 

Calomel  is  a  heavy  white  powder,  which  volatilizes  at  tem- 
peratures below  redness  without  previous  fusion. 

The  vapor-density  of  calomel  is  by  calculation,  117.75. 

Weight  of  one  atom,  or  two  unit-volumes,  of  mercury,  200.00 

"  "         "       or  one  "  chlorine,  35.5 

Weight  of  two  unit-volumes  of  mercurous  chloride,  235.5 

Weight  of  one  unit-volume  "  "  117.75 


MERCURIC    CHLORIDE.  577 

The  vapor-density  has  been  determined  by  experiment  at 
120.49.  By  slowly  cooling  its  vapor,  prismatic  crystals  of 
calomel  may  readily  be  obtained.  Unlike  metallic  mereuiy, 
calomel  does  not  volatilize  at  ordinary  temperatures.  It  is 
tasteless,  odorless,  and  as  good  as  insoluble  in  water.  Alka- 
line lyes  decompose  it  readily,  and  it  is  slightly  soluble  in 
many  saline  solutions. 

675.  Mercuric  Chloride  (Hg  C12),  better  known  by  the  name 
corrosive  sublimate,  may  be  prepared  by  burning  mercury  in 
an  excess  of  chlorine  gas,  by  dissolving  protoxide  of  mercury 
in  chlorhydric  acid,,  or  by  dissolving  metallic  mercury  in  aqua 
regia,  and  evaporating  to  dry  ness. 

In  practice,  it  is  commonly  prepared  by  sublimation,  by  carefully 
heating  an  intimate  mixture  of  sulphate  of  mercury  and  common 
salt :  — 

HgSO4  +  2  NaCl  =  HgCl2  +  Na2  SO4 . 

Another  method  is,  to  add  concentrated  chlorhydric  acid  to  a 
strong,  boiling  solution  of  nitrate  of  dinoxide  of  mercury,  as  long 
as  a  precipitate  is  formed,  and  to  subsequently  boil  this  precipitate 
with  u  quantity  of  chlorhydric  acid,  equal  to  that  used  in  prepar- 
ing it :  — 

HgNO3  +  2  HC1  =  HgCl2  +  H2O  +  N02 . 

Beautiful  crystals  of  mercuric  chloride  will  be  deposited,  as  the 
hot  solution  cools. 

676.  Mercuric  chloride  commonly  occurs,  in  commerce,  in 
translucent,  crystalline  masses  ;  but  crystals  of  it  may  readily 
be  obtained,  by  careful  sublimation,  as  well  as  by  slowly  cool- 
ing hot  solutions.     It  melts  at  about  265°,  forming  a  colorless 
liquid,  which  boils  at  293°  ;  the  fumes  are  acrid,  and,  like  the 
salt  itself,  exceeding^  poisonous. 

The  vapor-density  of  corrosive-sublimate  is,  by  calculation, 
135. 

Weight  of  one  atom,  or  two  unit-volumes  of  mercury,         .         .         .         200.00 
"        "  two  atoms  "  "         "          "   chlorine,    ....       70.00 

Weight  "    "  unit-volumes  of  mercuric  chloride,         ....         270.00 
"       "  one  unit-volume  "  «*'••;« 135. 

The  best  experiments  assign  to  the  vapor  of  the  salt,  the 
density  of  141.  Mercuric  chloride  is  rather  easily  soluble  in 

43 


578  MERCURIC    IODIDE. 

water  and  alcohol ;  with  the  alkaline  chlorides,  it  unites  to 
form  salts,  which  are  easily  soluble  in  water.  These  double 
salts  are  so  numerous  and  well-defined,  that  they  are  regarded 
as  chlorine  salts,  comparable  with  the  sulphur  salts  (§340), 
and  the  oxygen  salts.  In  this  view,  protochloride  of  mercury 
would  be  called  chloromercuric  acid,  and  its  compounds,  with 
the  alkalies,  chloromercurates.  The  compound  NaCl,  HgCl2, 
for  example,  would  be  called  chloromercurate  of  sodium  and 
the  compound  NaCl,  2  HgCl2 ,  the  bichloromercurate.  Cor- 
responding double  chlorine  salts  are  formed  by  the  union  of 
the  chlorides  of  gold  and  of  platinum  with  the  chlorides  of 
other  rnetals  and  compound  radicals,  as  willt  appear  in  the 
sequel. 

677.  Mercuric  chloride  unites  with  many  organic  substances 
to  form  compounds  insoluble  in  water  and  imputrescible.  It 
coagulates  albumen,  for  example,  and  the  more  perishable  por- 
tions of  wood  ;  hence  the  employment  of  raw  whites  of  eggs  as 
an  antidote  in  cases  of  poisoning  by  corrosive  sublimate,  and 
the  use  of  the  mercury  salt  for  preserving  wood,  —  a  purpose  for 
which  it  would,  no  doubt,  be  largely  employed  were  it  not  for 
its  high  cost.  Collections  of  dried  plants,  and  of  other  objects 
of  natural  history,  are  preserved  both  from  decay  and  from  the 
attacks  of  insects  by  brushing  over  them  a  solution  of  the 
chloride  in  alcohol.  It  is  worthy  of  mention  that  the  compound 
of  albumen  and  chloride  of  mercury,  though  insoluble  in  water, 
is  soluble  in  an  excess  of  albumen. 

Mercurous  and  mercuric  bromides,  iodides,  fluorides,  and 
cyanides  are,  in  general,  analogous  to  the  corresponding  chlo- 
rides. Mercuric  iodide  undergoes  remarkable  changes  of  color 
when  heated  or  subjected  to  friction. 

Exp.  360. — Dissolve  half  a  gramme  of  iodide  of  potassium  in  a 
small  quantity  of  water;  also  dissolve  0.4  of  a  gramme  of  corrosive 
sublimate  in  a  little  water,  and  mix  the  two  solutions.  Collect  upon 
a  filter  the  beautiful  red  precipitate  which  is  formed,  wash  it  care- 
fully with  water  and  dry  it  in  the  air.  Place  a  portion  of  the  dry 
red  powder  in  a  porcelain  capsule  ;  invert  ov«r  the  capsule  a  small 
glass  funnel,  and  heat  the  capsule  moderately  upon  a  sand  bath ; 
the  iodide  will  melt,  sublime,  and  finally  be  deposited  upon  the  cold 


NITRATES    OF    MERCURY  579 

walls  of  the  funnel  in  yellow  crystals.  On  rubbing  these  crystals 
with  a  glass  rod,  their  color  will  change  again  to  red.  Indeed,  the 
change  of  color  often  occurs  of  itself  as  the  crystals  cool,  without 
friction.  The  composition  of  the  iodide  is  neither  changed  by  the 
sublimation  nor  by  the  friction ;  the  change  of  color  is  due  to  a 
change  of  crystalline  form,  mercuric  iodide  being  dimorphous  and 
exhibiting  a  red  color  in  its  octahedral  form,  and  a  yellow  color 
when  crystallized  in  rhombic  prisms. 

The  change  of  coloration  may  be  shown  in  another  way,  by  dis- 
solving some  of  the  precipitated  iodide  in  alcohol.  The  alcoholic 
solution  is  colorless,  and  appears  to  contain  the  yellow  modification 
of  the  iodide  ;  on  pouring  it  into  water,  iodide  of  mercury  is  precipi- 
tated as  a  yellow  powder,  which  soon  changes  to  red. 

678.  Sulphates  of  Mercury.      There  is  a  sparingly  soluble 
sulphate  of  dinoxide  of  mercury  Hg2SO4 ,  a  normal  sulphate  of 
the  protoxide,  HgSO4 ,  and  a  basic  sulphate  of  the  protoxide 
of  composition  3HgO,SO3.    Normal  mercuric  sulphate  maybe 
prepared  by  dissolving  metallic  mercury  in  an  excess  of  boil- 
ing concentrated  sulphuric  acid,  and  evaporating  the  solution 
to  dryn^s.     It  is  the  material  from  which  many  other  com-  " 
pounds  of  mercury  are  derived.     It  is  decomposed  by  water  ; 
an  insoluble  trisulphate  is  thrown  down,  while  but  a  small  pro- 
portion of  mercury  remains  dissolved  in  the  dilute  sulphuric 
acid  which  is  formed. 

679.  Nitrates  of  Mercury  are  numerous.    There  are  at  least 
four  nitrates  of  the  dinoxide  and  as  many  of  the  protoxide, 
namely  the  normal  salts  and  three  basic  salts  in  either  case. 
Both  of  the  normal  salts  are  soluble  in  water  and  are  commonly 
kept  in  the  laboratory  as  examples  of  the  mercury  salts.     The 
nitrate  of  the  dinoxide  is  prepared  by  digesting  an  excess  of 
metallic  mercury  in  cold,  moderately  strong  nitric  acid.     The 
solution   should  be  kept  in  closed  bottles  containing  a  few 
globules  of  metallic  mercury.     The  nitrate  of  the  protoxide 
may  be  readily  obtained  by  dissolving  red  oxide  of  mercury  in 
an  excess  of  nitric  acid. 

680.  Amalgams.     Mercury  unites  with  most  of  the  other 
metals  forming  alloys,  many  of  which  are  pasty,   or  liquid, 
when  the  proportion  of  mercury.contained  in  them  is  large. 


\ 


580  AMALGAMS. 

I 

These  alloys  are  commonly  called  amalgams,  in  contradistinc- 
tion to  the  ordinary  solid  alloys  of  the  other  metals,  in  which 
mercury  has  no  place.  The  liquid  amalgams  are  true  solutions 
of  other  metals,  or  of  solid  amalgams,  in  the  fluid  mercury. 
The  so-called  silvering  of  mirrors  is  an  amalgam  of  tin. 

Mercury  may  be  detected  in  almost  any  soluble  salt  of  the 
element  by  introducing  into  a  solution  of  the  salt  a  piece  of 
clean  copper. 

Exp.  361.  — Place  a  drop  of  a  solution  of  either  of  the  nitrates  or 
chlorides  of  mercury  upon  a  copper  coin  and  rub  the  liquid  over  its 
surface.  A  white  coating  of  metallic  mercury  will  be  deposited 
upon  the  metal. 

681.  Copper  and  mercury  are  classed  together  partly 
because  of  certain  resemblances  between  the  two  metals,  but 
also  because  neither  of  them  falls  naturally  into  either  of  the 
other  groups  of  elements.  They  are  alike  in  that  they  are 
not  readily  acted  upon  by  air,  excepting  at  high  temperatures  ; 
that  they  do  not  decompose  water  at  any  temperature ;  and 
that  they  both  form  two  salifiable  oxides,  and  two  chlorides 
of  analogous  composition.  They  are  both  acted  upon  in  the 
same  way  by  nitric  and  by  sulphuric  acids,  the  acid  being 
reduced  to  a  lower  degree  of  oxidation,  while  the  inetal  is  dis- 
solved, as  has  been  seen  in  Exp.  100.  As  the  formula  of 
their  compounds  have  doubtless  already  suggested,  mercury 
and  copper  are  univalent,  like  the  alkali  metals,  in  the  mer- 
curous  and  cuprous  compounds,  but  bivalent  in  the  mercuric 
and  cupric  compounds. 


CHAPTER    XXXII. 

TITANIUM,   TIN. 

TITANIUM. 

682.     This  .comparatively  rare  metal  is  found  in  several 
minerals,  such  as  rutile  and  titaniferous  iron,  in  the  condition 


TITANIUM.  581 

• 

of  titanic  acid,  TiO2 .  None  of  its  compounds  are  employed  in 
the  arts,  and  the  element  itself  is  here  mentioned,  mainly  on 
account  of  the  analogies  which  it  bears  to  tin.  Titanic  acid 
is  isomorphous  with  stannic  acid  (§685),  and  resembles  it 
closely  in  its  chemical  deportment.  Sesquioxide  of  titanium, 
Ti2O3,  corresponds  to  sesquioxide  of  tin,  Sn203  (§  683), 
and  in  the  same  way  that  the  latter  may  be  regarded  as  a 
stannate  of  tin,  SnO,  SnO2,  the  titanium  compound  maybe 
considered  a  titanate  of  titanium,  TiO,  TiO2 . 

The  bisulphide  of  titanium,  and  the  bichloride,  bibromide 
and  bifluoride,  correspond  in  like  manner  to  the  tin  compounds. 

TIN. 

683.  Though  by  no  means  widely  diffused  in  nature,  and 
though  ores  of  it  occur  in  but  few  localities,  tin  is  one  of  the 
metals  which  have  longest  been  known  to  man.  The  fact 
admits,  however,  of  read}''  explanation,  for  the  specific  gravity 
of  the  ores  is  high,  and  the  metal  is  easily  reduced  from  them 
by  simple  heating  with  charcoal.  From  the  manner  of  the 
occurrence  of  many  of  these  ores,  in  the  beds  of  torrents,  it  is 
evident  that  their  great  weight  would  be  likely  to  attract 
attention,  and  that  their  behavior  towards  fire  would  soon  be 
noticed.  The  simplest  possible  metallurgical  operation,  and 
the  one  most  likely  to  suggest  itself  to  savage  man,  is  the  heat- 
ing of  a  heavy  stone  in  a  wood  fire. 

The  principal  ore  of  tin  is  the  binoxide,  called  tin-stone.  In 
order  to  extract  the  metal  from  it,  the  tin-stone  is  mixed  with 
powdered  coal,  and  heated  upon  the  hearth  of  a  reverberatory 
furnace  in  a  reducing  flame.  The  reduced  metal  melts  readily, 
and  is  then  run  out  of  the  furnace  into  iron  moulds.  Tin  is  a 
lustrous  white  metal,  soft,  malleable  and  ductile,  though  not  very 
tenacious.  Its  ductility  varies  greatly  with  the  temperature  ;  at 
100°  the  metal  may  be  drawn  into  thin  wire,  but  at  200°  it  is  very 
brittle.  When  a  bar  of  tin  is  bent,  it  emits  a  peculiar  crackling 
sound,  and  if  the  bending  be  repeated,  the  metal  becomes  de- 
cidedly warm.  These  phenomena  appear  to  depend  on  the  dis- 
turbance of  interlaced  crystals  contained  in  the  bar,  and  upon 


582  TIN. 

the  friction  of  these  crystals  one  against  the  other.  Tin  always 
exhibits  a  great  tendency  to  assume  crystalline  form  in  pass- 
ing from  the  liquid  to  the  solid  condition.  Upon  this  pecu- 
liarity is  founded  a  method  of  ornamenting  tinned  iron. 

Exp.  362.  — Heat  a  piece  of  common  tinned  iron  over  the  gas 
lamp  until  the  tin  has  melted,  thrust  the  plate  into  cold  water  in 
order  that  the  tin  may  harden  quickly,  then  remove  the  smooth 
surface  of  the  metal  by  rubbing  it  first  with  a  bit  of  paper  moistened 
with  dilute  aqua  regia,  and  then  with  paper  wet  with  soda-lye.  By 
this  treatment  there  will  soon  be  laid  bare  a  new  surface  covered 
with  beautiful  crystalline  figures,  like  frost  upon  a  window-pane. 
The  plate  should  then  be  washed  thoroughly  with  water,  dried 
quickly,  and  covered  with  some  transparent  varnish. 

The  same  crystalline  structure  can  be  brought  out,  though  less 
conspicuously,  by  removing  the  outside  polished  surface  of  almost 
any  piece  of  tin  plate  by  means  of  warm  dilute  aqua  regia,  without 
first  heating  the  plate  as  in  this  experiment. 

Tin  does  not  tarnish  in  the  air  at  ordinary  temperatures,  no 
matter  whether  the  air  be  moist  or  dry  ;  but,  when  strongly 
heated,  it  oxidizes  rapidly,  and  even  burns  with  a  brilliant 
white  light.  The  specific  gravity  of  tin  is  about  7.3  ;  its 
atomic  weight  is  118.  It  melts  at  about  230°,  —  at  a  lower  tem- 
perature than  any  of  the  other  common  metals.  At  very  high 
temperatures  it  is  slightly  volatile. 

On  account  of  its  brilliant  lustre,  and  its  power  of  resisting 
atmospheric  action,  tin  is  largely  employed  for  coating  other 
metals,  —  copper,  for  example,  as  in  ordinary  pins,  cooking 
vessels,  and  bath  tubs,  —  and  iron,  as  in  common  sheet-tin,  of 
which  the  so-called  tin  ware  is  manufactured. 

Exp.  363.  Thoroughly  clean  the  surface  of  a  copper  coin,  or 
of  a  small  piece  of  sheet-copper,  by  means  of  dilute  sulphuric  acid  ; 
place  the  copper  over  the  gas  lamp,  and  melt  upon  it  a  bit  of  tin  as 
large  as  a  pea.  Rub  the  melted  tin  over  the  copper  with  a  rag. 
It  will  not  adhere  to  the  copper,  for  although  the  latter  was  once 
carefully  cleaned,  it  afterwards  became  coated  with  oxide  of  copper 
in  such  manner  that  the  tin  could  not  come  in  contact  with  the 
metal. 

Repeat  the  experiment  as  before,  but  when  the  tin  has  melted, 
strew  over  the  copper  some  finely  powdered  chloride  of  ammonium. 


PROTOXIDE    OF    TIN.  583 

On  now  rubbing  the  tin  against  the  copper,  the  two  metals  will  ad- 
here firmly.  The  chlorine  of  the  chloride  of  ammonium  unites  with 
the  copper  of  the  oxide  of  copper  to  form  fusible,  volatile  chloride  of 
copper,  while  ammonia  and  water  are  set  free,  as  may  be  perceived 
by  the  odor.  The  excess  of  tin  should  be  wiped  off  with  a  rag,  so 
that  a  smooth  surface  may  be  left  upon  the  coin. 

When  in  contact  with  dilute  acids,  or  with  alkalies,  tin  slowly 
absorbs  oxygen  from  the  air  and  goes  into  solution.  Of  the 
strong  acids,  nitric  acid  acts  upon  it  violently,  with  formation 
of  insoluble  hydrated  binoxide  of  tin ;  a  certain  amount  of 
water  is  decomposed  as  well  as  the  nitric  acid,  in  this  reaction, 
and  some  nitrate  of  ammonium  is  formed ;  the  ammonium 
comes  from  the  union  of  tbe  nascent  hydrogen  and  nitrogen 
of  the  deoxidized  water  and  nitric  acid.  (See  §  92.)  Hot  con- 
centrated chlorhydric  acid  gradually  dissolves  tin,  and  gives 
off  hydrogen.  Boiling  concentrated  sulphuric  acid  converts 
it  into  sulphate  of  tin,  with  evolution  of  sulphurous  acid ;  but 
dilute  sulphuric  acid  has  no  action  upon  it  out  of  contact  with 
the  air.  When  heated  with  concentrated  soda  or  potash  lye, 
tin  slowly  dissolves  with  formation  of  soluble  stannate  of 
sodium  or  of  potassium,  and  evolution  of  hydrogen. 

There  are  two  prominent  oxides  of  tin,  a  protoxide  and  a 
binoxide,  besides  intermediate  oxides  compounded  of  these  two. 
The  binoxide  occurs,  moreover,  in  combination,  in  different 
isomeric  modifications. 

684.  Protoxide  of  Tin  (SnO)  may  be  obtained  as  a  black 
powder  by  heating  its  hydrate  in  an  atmosphere  of  carbonic 
acid  or  other  inert  gas.  The  hydrated  protoxide  is  prepared 
by  adding  the  solution  of  an  alkaline  carbonate  to  a  solution 
of  tin  in  chlorhydric  acid ;  the  hydrate  is  thrown  down  as  an 
insoluble  precipitate  while  carbonic  acid  escapes  :  — 

SnCl2  +  Na2C03  +  H2O  =  SnHA  +  2NaCl  +  CO2. 
The  hydrate  rapidly  absorbs  oxygen  from  the  air  when 
moist,  but  is  tolerably  permanent  when  dry.  The  anhydrous 
oxide  undergoes  no  change  in  air  at  the  ordinary  temperature  ; 
when  touched  with  a  glowing  coal,  it  takes  fire  and  burns 
vividly,  being  converted  into  the  binoxide.  The  hydrate  also 


584  B1NOXIDE    OF    TIN. 

burns  in  the  same  way,  when  lighted.  It  is  remarkable  that 
the  anhydrous  oxide  is  more  readily  soluble  in  acids  than  the 
hydrate  ;  but  in  alkaline  lyes  only  the  hydrate  is  soluble. 
Most  of  the  salts  of  protoxide  of  tin  greedily  absorb  oxygen 
from  the  air  and  from  many  oxygenated  substances.  They 
are  much  employed  as  reducing  agents. 

Exp.  364.  — To  5  or  6  c.  c.  of  a  solution  of  corrosive  sublimate 
add  a  few  drops  of  protochloride  of  tin,  and  heat  the  mixture ;  a 
gray  powder  will  separate;  it  is  metallic  mercury,  very  finely 
divided,  which  has  been  reduced  from  the  mercuric  chloride.  The 
powder  boiled  with  chlorhydric  acid  agglomerates  into  visible 
globules :  — 

HgCl2  +  SnCl2  =  SnCl4  +  Hg. 

The  protochloride  of  tin  has  abstracted  the  chlorine  from  the  chlo- 
ride of  mercury. 

685.     Binoxide  of  Tin,  or  Stannic  Acid  (SnO2).    This  oxide 
occurs  in  nature  as  the  principal  ore  of  tin,  as  has  been  stated 
in  §  683  ;  and  may  readily  be  prepared  artificially  by  roasting 
the  metal  in   a  free  current  of  air,  or  by  igniting  hydratecl 
binoxide  of  tin.     It  is  insoluble  in  water,  acids,  and  alkalies, 
and  in  general  is  not  readily  acted  upon  by  chemical  agents. 
When  fused  with  caustic  soda,  however,  it  combines  with  it  to 
form  a  soluble  compound.     Like  the  hydrated  oxides  of  phos- 
phorus and  antimony,  hydrated  binoxide  of  tin  (SnH2O3)  is 
remarkable  for  the  different  chemical  properties  it  exhibits 
when   prepared  in  different  ways.     As  obtained  by  heating 
metallic  tin  with  concentrated  nitric  acid,  it  is  almost  abso- 
lutely insoluble  in  some   acids,  and  dissolves  only  with  diffi- 
culty in  others.     The  hydrate  obtained  by  precipitation  from 
solutions  of  bichloride  of  tin  and  of  an  alkaline  carbonate,  is 
very  readily  soluble  in  acids.    By  ignition,  the  soluble  hydrate 
is  converted  into  insoluble  anhydrous  oxide  of  tin. 

The  difference  in  chemical  behavior  above  mentioned  is 
not  confined  to  the  hydrates  alone,  it  exists  as  well  in  the 
compounds  formed  by  the  union  of  these  hydrates  with  other 
substances,  both  acids  and  bases ;  hence  the  names  stannic 
acid,  applied  to  the  soluble  hydrate,  and  metastannic  acid, 
applied  to  the  insoluble  modification.  The  generic  terms 


SULPHIDES     OF    TIN.  585 

stamiate  and  metastannate  applied  to  the  compounds  of  these 
two  varieties  of  the  oxide  are  employed  precisely  as  in  the  case 
of  antimonic  and  phosphoric  acids  (§§  293,  352).  Both 
modifications  are  soluble  in  alkaline  lyes,  but  the  one  repre- 
senting metastannic  acid  is  far  less  readily  soluble  than  the 
other. 

The.  stannates  of  the  alkali  metals,  sodium  and  potassium, 
crystallize  readily  from  their  aqueous  solutions,  but  the  cor- 
responding metastannates  do  not  crystallize  ;  they  are  insol- 
uble in  saline  solutions,  and  may  be  precipitated  as  gelatinous 
masses  by  adding  almost  any  neutral  salt  of  an  alkali  metal 
to  their  aqueous  solutions.  Among  the  stannates,  that  of  tin 
(SnO,  SnO2=  Sn2O3)  is  worthy  of  mention,  since  it  is  often 
described  as  the  sesquioxide  of  tin.  Stannate  of  sodium 
is  somewhat  extensively  employed  in  the  printing  of  muslin- 
el  e-laines. 

A  good  method  of  preparing  it  is  to  boil  granulated  tin,  or 
scraps  of  tinned  iron,  in  a  solution  of  litharge  in  an  excess 
of  caustic  soda  : — 

Sn  +  Na2Pb2  O3  =  Na2  SnO3  +  2Pb. 

Or  a  mixture  of  caustic  soda,  nitrate  of  sodium,  and  metallic 
tin  may  be  melted  in  an  iron  kettle,  a  certain  portion  of  chloride 
of  sodium,  or  of  stannate  of  sodium,  from  a  previous  fusion, 
being  added  to  the  mixture  in  order  to  mitigate  the  force  of 
the  reaction. 

686.  The  Sulphides  of  Tin  (SnS  ;  Sn2S3  and  Sn  S2)  cor- 
respond to  the  oxides.  The  protosulphide  (SnS)  is  precipi- 
tated as  a  dark-brown  powder  when  sulrjhydric  acid  is  added 
.to  the  solution  of  a  salt  of  protoxide  of  tin.  The  bisulphide 
(SnS2)  when  prepared  in  the  dry  way  is  a  beautiful  yellow 
compound,  known  as  mosaic  gold;  or  bronze  powder,  and  is 
somewhat  employed  in  decorative  painting. 

Exp.  365.  — Prepare  a  quantity  of  tin  amalgam  by  heating  to- 
gether in  a  glass  flask  12  gnus,  of  granulated  tin,  and  6  grms.  of 
mercury.  Rub  the  amalgam  in  a  porcelain  mortar  together  with  7 
<rrms.  of  sulphur  and  6  grms.  of  chloride  of  ammonium  until  the 
different  ingredients  have  been  thoroughly  incorporated,  one  with 

44 


586  PROTOCHLORIDE    OF    TIN. 

the  other.  Place  the  mixture  in  a  small,  long-necked,  glass  flask, 
and  slowly  heat  it  to  low  redness  upon  a  sand  bath.  After  an  hour 
or  two  there  will  be  found  at  the  bottom  of  the  flask  a  quantity  of 
bisulphide  of  tin,  in  the  condition  of  soft,  beautiful,  golden-yellow 
powder,  of  flaky  texture,  while  in  the  neck  of  the  flask  there  will 
be  found  a  deposit  of  chloride  of  ammonium  contaminated  with 
sulphur,  sulphide  of  mercury,  and  protochloride  of  tin. 

Instead  of  the  amalgam  and  the  proportions  of  the  other  ingre- 
dients as  given  above,  there  may  be  heated  in  the  flask  an  intimate 
mixture  of  2  grms.  of  dry  protosulphide  of  tin,  half  a  grm.  of  sul- 
phur, and  1  grm.  of  chloride  of  ammonium. 

The  part  played  by  the  chloride  of  ammonium  in  these  experi- 
ments is  not  well  understood  ;  it  is  known  only  that  the  presence  of 
this  salt  promotes  the  formation  of  a  brilliant  golden-colored  product, 
though  there  is  no  evidence  that  the  chloride  either  undergoes  or 
produces  any  chemical  change.  It  is  by  no  means  improbable, 
however,  that  by  volatilizing  at  the  right  moment  the  chloride  of 
ammonium  may  so  moderate  the  heat  engendered  by  the  combina- 
tion of  the  sulphur  and  the  tin  that  the  temperature  of  the  mixture  is 
prevented  from  reaching  a  point  at  which  the  bisulphide  would  be 
decomposed. 

The  Chlorides  of  Tin  are,  perhaps,  more  important  than  any 
other  compounds  of  the  metal. 

687.  Protochloride  of  Tin  (SnCl2)  is  obtained  by  dissolv- 
ing granulated  tin  in  boiling  concentrated  chlorhydric  acicl. 
On  evaporating  the  solution,  and  allowing  it  to  crystallize, 
prismatic  hydrated  needles  are  obtained  of  the  composition 
SnCl2  -(-  2  H2O  .  These  crystals  are  largely  used  by  dyers 
and  calico-printers,  under  the  name  of  tin  salt.  Protochloride 
of  tin,  whether  in  the  condition  of  crystals,  or  in  solution, 
rapidly  absorbs  oxygen  from  the  air,  and  is  converted  into  a 
mixture  of  bichloride  of  tin,  and  an  insoluble  oxychloride. 
It  must,  therefore,  be  kept  in  tight  packages.  The  pure  salt 
is  completely  soluble  in  a  small  quantity  of  water  ;  but  when 
this  solution  is  mixed  with  a  large  quantity  of  water,  it  de- 
composes ;  a  highly  acid  solution  of  protochloride  of  tin  in 
chlorhydric  acid  remains  in  solution,  while  a  precipitate  of 
oxychloride  of  tin  (SnO,  SnCl2 -|-  2  H2O)  subsides. 

Protochloricle  of  tin  is  a  powerful  reducing  agent,  as  has  been 


BICHLORIDE    OF    TIN.  587 

shown  in  Exp.  364 ;  it  combines  readily  either  with  oxygen 
or  with  chlorine,  and  is  frequently  employed  to  remove  these 
elements  from  their  -compounds.  By  means  of  it,  the  oxides 
or  chlorides  of  arsenic,  antimony,  gold,  silver,  or  mercury, 
may  be  reduced  to  the  metallic  state  ;  the  salts  of  sesquioxide 
of  iron,  and  of  protoxide  of  copper,  may  be  reduced  to  the 
degree  of  protoxide  and  of  dinoxide  respectively,  while  acids 
such  as  chromic  and  manganic  are  reduced  to  the  condition  of 
basic  oxides.  Sulphurous  acid  is  reduced  by  it  in  such  wise  that 
a  precipitate  of  sulphide  of  tin  is  formed,  when  solutions  of 
protochloride  of  tin  and  of  sulphurous  acid  are  mixed  ;  it  con- 
verts blue  indigo  to  white  indigo,  and  is  capable  of  abstract- 
ing oxygen  from  a  host  of  other  substances. 

At  the  temperature  of  100°,  all  the  water  may  be  expelled 
from  the  crystallized  salt,  but  some  of  the  chlorhydric  acid 
is  liable  to  go  off  at  the  same  time,  so  that  it  is  not  easy,  in 
this  way,  to  obtain  the  anhydrous  sal  t^  in  a  state  of  purity. 
A  better  way  of  obtaining  the  anhydrous  salt  is  to  heat  to- 
gether equal  weights  of  finely  divided  tin,  and  corrosive  sub- 
limate :  — 

HgCl2  +  Sn  =  SnCl2  +  Hg. 

The  dry  chloride  of  tin  remains  as  a  residue,  while  metallic 
mercury  goes  off.  The  anhydrous  salt  may  itself  be  distilled 
at  a  full  red  heat. 

Protochloride  of  tin  unites  with  many  of  the  metallic  chlo- 
rides to  form  double  compounds,  which  may  be  called  chloro- 
stannites. 

688.  Bichloride  of  Tin  (SnCl4).  When  anhydrous,  this 
compound  is  a  fuming,  volatile,  colorless  liquid,  of  2.7  specific 
gravity.  It  does  not  solidify  at— 20°,  but  boils  at  115°. 
When  exposed  to  the  air  it  gradually  absorbs  water,  and  after 
a  while,  hydrated  crystals  are  formed.  When  mixed  with  about 
one-third  its  weight  of  water,  it  solidifies  to  a  mass  of  hy- 
drated crystals,  much  heat  being  at  the  same  time  evolved. 
These  crystals  are  readily  soluble  in  a  small  quantity  of 
water,  but  when  treated  with  much  water,  they  decompose ; 
hydrated  binoxide  of  tin  is  precipitated,  and  free  chlorhydric 
acid  passes  into  solution. 


588  ALLOYS    OF    TIN. 

In  order  to  prepare  anhydrous  bichloride  of  tin,  chlorine 
gas  may  be  passed  over  hot  chloride  of  tin,  or  melted  metallic 
tin  ;  or  an  intimate  mixture  of  1  part  of  tin-filings,  and  4  or  5 
parts  of  corrosive  sublimate  may  be  distilled  in  a  retort.  The 
hydrated  bichloride,  and  in  general,  all  solutions  of  the  bi- 
chloride are  obtained,  either  by  dissolving  tin  in  dilute  aqua 
regia,  by  passing  chlorine  into  solutions  of  protochloride  of 
tin,  or  by  heating  the  protochloride  with  chlorhydric  acid,  to 
which  a  little  nitric  acid  has  been  added.  The  anhydrous  salt 
may  be  prepared  from  the  hydrate,  by  distilling  the  latter 
with  concentrated  sulphuric  acid,  which  retains  the  water. 

Like  the  protochloride,  bichloride  of  tin  is  largely  employed 
in  dyeing.  It  combines  also  with  the  alkaline  and  other 
chlorides  to  form  salts,  known  as  chlorostannates.  The  sub- 
stance called  pink  salt,  in  commerce,  employed  in  the  prepa- 
ration of  pink  colors  upon  calicoes,  is  a  chlorostannate  of  am- 
monium, 2  NH4C1,  SpCl4.  There  are,  of  course,  many  other 
salts  of  tin,  but  none  of  them  are  of  sufficient  interest  to  be 
mentioned  here. 

689.  The  alloys  of  tin  are  important.     The  composition  of 
bronze,  bell-metal,   &c.,  has  been    already   mentioned    under 
copper    (§  659),    that   of   stereotype   metal   under   antimony 
(§  348),    and   that   of  tin  amalgam  under  mercury  (§  680). 
Of  the  other  alloys  of  tin,  those  formed  by  its  union  with  lead, 
are  most  remarkable.     Plumber's  solder  consists  commonly  of 
equal  parts  of  lead  and  tin,  though  some  kinds  of  it  contain 
only  one-third  their  weight  of  lead,  and  others  only  one-third 
their  weight  of  tin.     Pewter  is  composed  of  tin,  together  with 
a  small  proportion  of  lead. 

690.  With  tin  and  titanium  may  be  classed  the  two  ex- 
ceedingly rare  metals,  Columbium  (niobium)  and  Tantalum. 
The   principal   source    of  columbium   is  the   rare   American 
mineral  columbite.     Tantalum  is  procured  from  the  Scandi- 
navian mineral  tantalite.     The^  four   metals  form  binoxides 
and  volatile  chlorides  containing  four  atoms  of  chlorine,  and 
are,  therefore,  sometimes  quadrivalent.     In  this  respect  the 
tin  group  differs  from  all  the  groups  heretofore  studied,  ex- 


VANADIUM.  589 


cepting  the  group  composed   of  carbon,  boron,  and   silicon. 
Tin,  and  titanium,  have  a  like  mode  of  occurrence  in  nature. 


CHAPTER    XXXIII. 

MOLYBDENUM,  VANADIUM,  TUNGSTEN. 

MOLYBDENUM. 

691.  'This  rare  element   is   generally  found    in  nature,  in 
combination   with    sulphur,    as   bisulphide    of    molybdenum. 
This  bisulphide  is  a  mineral  closely  resembling  graphite  and 
galena  in  appearance.    The  name  molybdenum  is  derived  from 
a  Greek  word   sometimes   applied   to  galena.     Molybdenum 
is    a  white    metal  almost  as   lustrous    as    silver,  of  specific 
gravity  8.6.     Its  atomic  weight  is  96. 

There  are  three  oxides  of  molybdenum  :  a  protoxide  (MoO), 
and  a  binoxide  (MoO2),  both  acting  as  bases,  and  a  teroxide 
(MoO3),  which  is  a  strong  acid  known  as  molybdic  acid.  Molyb- 
date  of  ammonium  is  a  salt  much  valued  by  the  analyst,  since  by 
means  of  its  solution  very  small  quantities  of  phosphoric  acid 
may  be  detected  ;  a  double  compound  of  molybdate  and  phos- 
phate of  ammonium  is  deposited  as  a  yellow  crystalline  pre- 
cipitate. 

VANADIUM. 

692.  Vanadium  is   a  metal    somewhat  resembling  molyb- 
denum, on  the  one  handj  and  having  certain  analogies  with 
chromium,  upon  the  other.     Though  nowhere  found  in  large 
masses,   it    appears  to  be  rather  widely  diffused  in  nature,' 
traces  of  it  often  accompanying  the  ores  of  iron,  for  example. 
It  has  three  oxides,  a  protoxide  (VO),  and  a  binoxide  (VO2), 
which  form  salts  by  uniting  with  acids,  and  a  teroxide  (VO3), 
which  acts  as  an   acid    and   forms  salts  by  combining  with 
bases.     The  atomic  weight  of  vanadium  is  137. 


590  TUNGSTATES. 

TUNGSTEN. 

693.  The  element  tungsten  is  far  less  rare  than  the  other 
metals  of  the  group  now  under  discussion.     It  is  found  in 
considerable  quantities  in  combination  with  ox}^gen,  iron,  and 
manganese,  in  the  mineral  wolfram,  whence  the  Latin  name  of 
the  element  wolframium,  and  the  symbol  W.     The  mineral 
scheelite  also  contains  tungsten  in  combination  with  oxygen 
and  calcium.     Metallic   tungsten   may  be   reduced   from  its 
oxides  by  means  of  hydrogen  gas  at  a  bright-red  heat,  or  by 
charcoal  at  a  white  heat.     It  is  a  hard,  iron-gray  metal,  of 
specific  gravity  17.6,  and  very  refractory.     Its  atomic  weight 
is  184.     The  metal  has  been  employed  to  a  certain  extent  in 
the  preparation  of  steel ;  a  small  quantity  of  it  added  to  steel 
has  been  found  to  greatly  increase  the  hardness  of  the  steel, 
and  to  impart  to  it  other  valuable  properties. 

694.  There  are  two  oxides  of  tungsten,  —  a  binoxide  (WO2) 
which  does  not  unite  with  acids  to  form  salts,  but  acts  rather 
as  an  acid  ;  and  a  teroxide  (WO3)  called  tungstic  acid.  Tungs- 
tic  acid  by  uniting  with  bases  forms  a  large  number  of  salts, 
many  of  which  are  of  very  complex  composition.     The  most 
important  ore  of  tungsten,  wolfram,  is  a  mixture  in  varying 
proportions  of  the  tungstates  of  the  protoxides  of  iron  and  of 
manganese.     The  general  formula  of  the  mineral  may  be  writ- 
ten RO,WO3,  in  which  R  stands  for  either  iron  or  manganese, 
but  there  are  nevertheless  two  special  varieties  of  the  mineral, 
one  tending  to  correspond  with  the  formula  2(FeO,WO3)  ;  3 
(MnO,WO3),  in  which  the  proportions  of  iron  to  manganese 
are  asf  to  f ,  and  the  other  to  the  formula  4(FeO,W03)  ;  MnO, 
WO3 ,  in  which  the  relation  of  the  iron  to  the  manganese  is  as 
4.  to  -£.    The  first  variety,  richer  in  manganese,  is  of  lower  spe- 
cific gravity  than  the  variety  rich  in  iron.    But,  since  the  atomic 
weights  of  iron  and  of  manganese  are  nearly  equal,  the  propor- 
tion of  tungstic  acid  is  almost  absolutely  the  same  in  both  va- 
rieties of  the  mineral.     Wolfram  is  a  very  heavy  mineral,  its 
specific  gravity  being  as  high  as  7.3.   Indeed,  the  name  tung- 
sten is  derived  from  Swedish  words  meaning  heavy  stone. 

Tungstate  of  sodium  has  been  employed  to  a  small  extent 


GOLD.  591 

for  the  purpose  of  rendering  cotton  and  linen  uninflammable. 
If  a  weak  solution  of  the  tungstate  be  added  to  the  starch  em- 
ployed to  stiffen  light  fabrics,  the  cloth  therewith  impregnated 
may  be  exposed  to  fire  without  inflaming ;  it  will  simply  be 
slowly  charred. 

The  compounds  of  tungsten  are  remarkably  similar  to  those 
of  molybdenum.  It  resembles  both  molybdenum  and  vanadium 
in  forming  an  acid  teroxide,  a  binoxide,  and  a  volatile  ter- 
chloride.  Like  these  metals,  it  decomposes  water  at  high  tem- 
peratures. 


CHAPTER     XXXIV. 

GOLD  AND  PLATINUM. 

GOLD. 

695.  Though  generally  found  only  in  small  quantities,  gold 
is  very  widely  diffused  upon  the  surface  of  the  globe.  Traces 
of  it  may  be  found  beneath  the  sandy  beds  of  most  rivers,  and 
it  occurs  in  many  of  the  cry stalline  rocks  and  in  the  soils  result- 
ing from  their  decomposition.  Many  varieties  of  iron  pyrites 
in  particular,  contain  appreciable  quantities  of  gold,  and  silver 
is  never  found  in  nature  altogether  free  from  it.  It  occurs  in 
the  lead  and  copper  of  commerce,  as  well  as  in  the  ores  from 
which  these  metals  are  derived  and  in  many  of  the  salts  ob- 
tained from  them,  and  has  been  detected  in  various  other  met- 
als ;  it  is,  in  short,  almost  everywhere.  The  chief  source  of 
the  metal  as  an  article  of  commerce  is  native  gold  ;  this  is 
sometimes  found  in  a  condition  of  purity,  \>ut  is  usually  alloyed 
writh  more  or  less  silver.  It  is  collected,  either  directly  by  me- 
chanically washing  away  the  lighter  substances  with  which  it 
is  associated,  or,  in  the  case  of  poorer  ores,  the  gold  is  dissolved 
out  chemically  by  means  of  quicksilver,  and  is  subsequently 
recovered  from  the  amalgam  by  filtration  and  distillation. 

The  separation  of  gold  from  the  rocks  and  sands  in  which  it 


592 


PROPERTIES    OF    GOLD. 


occurs  is  a  process  attended  with  much  labor  ;  hence  gold  is 
one  of  the  costliest  of  metals.  The  price  of  an  ounce  of  gold  is 
about  sixteen  times  greater  than  that  of  an  ounce  of  silver,  and 
twice  as  great  as  that  of  an  ounce  of  platinum. 

696.  Pure  gold  is  remarkable  as  being  the  most  malleable 
of  the  metals,  and  as  being  the  only  metal  of  a  decided  yel- 
low color ;  also  for  its   softness,  which  is  nearly  as  great  as 
that  of  lead.      It  has,  however,  much  tenacity,  and  may  be 
drawn  into  extremely  fine  wire  ;  1  grm.  of  gold  can  be  made 
to  yield  as  much  as  3  kilometres  of  wire.     The  metal  can  be 
beaten  into  leaves  which  are  "not  more  than  one  ten-thousandth 
of  a  millimetre  thick.     Very  thin  leaves  of  gold  are  transpar- 
ent, transmitting  a  green,  polarized  light. 

Next  to  platinum,  gold  is  the  heaviest  of  the  ordinary  met- 
als ;  its  specific  gravity  varies  from  19.26  to  19.37,  according 
as  it  has  been  more  or  less  compressed.  Its  atomic  weight  is 
196.7.  It  melts  somewhat  less  readily  than  copper  or  silver, 
at  a  temperature  estimated  to  lie  between  1200°  and  1250°. 
Its  power  of  conducting  heat  and  electricity  is  greatly  infe- 
rior to  that  of  silver.  It  is  not  volatile  to  any  great  extent  at 
the  melting  temperature ;  but,  at  higher  temperatures,  such  as 
it  is  subjected  to  in  the  ordinary  processes  of  melting  and  re- 
fining, the  metal  wastes  considerably  ;  and  at  the  temperature 
obtained  by  the  oxy-hydrogen  blowpipe,  the  metal  goes  off  as 
a  thick  vapor. 

697.  In  the  air,  gold  undergoes  no  change  at  temperatures 
lower  than   its    melting-point ;  and  upon  this    fact,  taken  in 
connection   with  the  beautiful  color  and  lustre  of  the  metal, 
and  its  comparative  rarity,  its  principal  uses  depend. 

On  account  of  this  indestructibility,  gold  was  regarded  by 
the  earlier  chemists  as  the  king  of  metals  ;  together  with  pla- 
tinum and  silver,  it  is  still  spoken  of  as  a  noble  metal.  Few 
chemical  agents,  excepting  melted  metals,  have  any  action 
upon  gold.  None  of  the  common  acids,  when  taken  singly, 
can  dissolve  it,  though  the  metal  is  completely  soluble  in  a 
mixture  of  chlorhydric  and  nitric  acids  (§  104),  and  is  not 
completely  insoluble  in  nitric  acid  contaminated  with  nitrous 


REFINING    OF    GOLD.  593 

or  hyponitric  acid.  The  elements  chlorine  and  bromine,  how- 
ever, unite  with  it  in  the  cold,  and  when  hot,  it  is  attacked  by 
phosphorus  and  arsenic. 

As  commonly  met  with  in  coins  or  jewelry,  gold  is  far  from 
being  pure  ;  coin,  for  example,  usually  contains  at  least  10  per 
cent,  of  copper. 

In  order  to  prepare  pure  gold,  a  piece  of  coin  may  be  dissolved 
in  aquaregia  (Exp.  53),  the  solution  evaporated  to  dryness  upon  a 
water-bath,  in  order  to  expel  the  excess  of  acid,  the  residue  taken 
up  with  water  and  filtered,  to  remove  any  chloride  of  silver  which 
may  be  present,  and  the  gold  finally  precipitated  as  a  brown  pow- 
der, by  adding  to  the  solution  some  sulphate  of  protoxide  of  iron 
dissolved  in  water :  — 

2  AuCl3  +  6FeSO4  =  2 Au  +  Fe2Cl6  +  2  (Fe203 ,  3SO3) . 

The  powder  may  then  be  collected  and  dried,  and  if  desirable, 
melted  and  cast  into  solid  masses. 

Upon  the  large  scale,  fine  gold  is  obtained  from  its  alloys,  by -re- 
moving the  baser  metal,  by  means  of  either  sulphuric  or  nitric  acid. 

When  an  alloy  of  gold,  silver,  or  copper  is  boiled  with  concen- 
trated sulphuric  acid  in  iron  kettles,  the  silver  and  copper  dissolve 
with  evolution  of  sulphurous  acid,  while  the  gold  remains  undis- 
solved,  and  the  iron  vessel  is  not  acted  upon.  In  order  to  recover 
the  silver  from  the  solution  of  mixed  sulphates,  sheets  of  copper 
are  placed  in  this  solution,  and  the  silver  is  precipitated  upon  them, 
as  has  been  shown  in  Exp.  272. 

The  solution  of  sulphate  of  copper  is  then  evaporated,  and  the 
salt  obtained  in  the  crystallized  condition  fit  for  sale. 

The  treatment  of  the  alloy  of  gold  and  silver  with  nitric  acid  is 
based  upon  the  fact,  that  silver  is  soluble,  while  gold  is  insoluble,  in 
this  acid.  But  it  has  been  found  necessary,  in  order  to  obtain  a  com-  . 
plete  separation  of  the  two  metals,  that  the  proportion  of  silver  to 
that  of  the  gold  in  the  alloy,  should  be  as  much  as  2  or  3  to  1,  other- 
wise portions  of  the  silver  would,  after  a  while,  become  so  covered 
with  gold  as  to  be  protected  from  the  action  of  the  acid,  and  the 
two  metals  could  not  be  completely  separated  from  one  another. 
In  practice,  whenever  the  alloy  to  be  treated  is  found  to  contain 
more  than  a  quarter  of  its  weight  of  gold,  enough  silver  is  added 
to  reduce  it  to  this  proportion ;  hence  the  term  quartation,  by  which 
this  method  of  parting  gold  and  silver  is  commonly  known. 

Finely  divided  gold  obtained  by  precipitation,  as  above  indicated; 

45 


594  GILDING. 

is  employed  to  a  considerable  extent  for  gilding  porcelain.  The 
surface  to  be  gilt  is  first  painted  with  an  adhesive  varnish,  then 
covered  with  a  mixture  of  the  gold  powder  and  a  fusible  enamel, 
and  exposed  to  intense  heat ;  on  being  subsequently  burnished,  the 
gold  takes  a  high  polish. 

There  are  two  series  of  gold  salts,  corresponding  to  the  two 
oxides ;  a  protoxide  AuO,  and  the  teroxide  AuO3 .  These 
oxides  are  rather  acids  than  bases  ;  the  teroxide  in  particular 
unites  with  many  metallic  oxides  to  form  compounds  known 
as  aurates.  The  chlorides,  bromides,  and  iodides  of  gold  also 
readily  combine  with  other  metallic  chlorides  to  form  chlorau- 
rates,  chloraurites,  and  the  analogous  bromine  and  iodine 
compounds. 

698.  Terchloride  of  Gold  (AuCl3)  is  the  compound  of  gold 
most  commonly  emplo37ed  in  the  laboratory.     The  manner  of 
preparing  it  has  been  already  indicated  in  §  697.     It  serves  as 
a  valuable  test  for  tin. 

699.  Gilding.     There  are  several  methods  of  attaching  a 
film  of  metallic  gold  to  surfaces  of  the  baser  metals. 

In  the  old  process  of  fire-gilding,  the  object  to  be  gilt  was  first 
heated  to  redness,  then  washed  with  dilute  acid  to  cleanse  its  sur- 
face, and  with  a  solution  of  nitrate  of  mercury  in  order  to  amalga- 
mate it  slightly ;  it  was  then  rubbed  with  a  pasty  amalgam  com- 
posed of  two  parts  of  gold  and  one  part  of  mercury.  After  a 
portion  of  the  gold  amalgam  had  thus  been  attached  to  the  surface 
of  the  article  to  be  gilt,  the  latter  was  heated  to  drive  off  the  mer- 
cury, and  the  gold  left  upon  it  was  polished  with  a  burnishing  tool. 

In  the  more  modern  method  of  electro-gilding,  the  object  to  be 
gilt  is  attached  to  the  negative  pole  of  a  galvanic  battery,  a  bar  of 
gold  is  fastened  to  the  positive  pole  of  the  battery,  and  both  the 
object  to  be  gilt  and  the  bar  of  gold  are  placed  in  a  mixed  solution 
of  cyanide  of  gold  and  cyanide  of  potassium.  Under  the  action  of 
the  current,  the  solution  is  decomposed ;  gold  is  deposited  from  it 
upon  the  object  at  the  negative  pole  of  the  battery,  while  the  other 
ingredients  of  the  solution  go  to  the  positive  pole,  there  to  dissolve 
gold  from  the  bar,  and  thus  make  good  to  the  solution  the  metal  it 
has  lost.  (Compare  Exp.  358.)  Articles  of  silver,  copper,  bronze, 
brass,  or  platinum,  may  thus  be  gilt  directly;  but  with  iron,  steel, 
or  tin,  it  is  necessary  first  to  immerse  the  article  attached  to  the 


ALLOYS    OF    GOLD.  595 

battery  in  a  solution  of  cyanide  of  copper  and  of  potassium,  in  order 
to  cover  it  with  a  film  of  copper,  to  which  the  gold  may  adhere. 

Even  without  a  battery,  gold  can  be  deposited  upon  silver  or 
copper  by  placing  either  of  these  metals  in  a  hot  solution  of  the 
double  cyanide  of  gold  and  potassium.  Copper  trinkets  are  also 
sometimes  gilt  by  boiling  them  in  a  liquor  prepared  by  mixing  a 
solution  of  chloride  of  gold,  and  of  an  alkaline  carbonate. 

In  general,  the  compounds  of  gold  have  but  few  properties  which 
are  of  chemical  interest.  What  has  been  said  of  the  permanence  of 
the  metal  implies,  of  course,  that  it  is  a  weak  chemical  agent,  having 
but  little  affinity  for  other  substances. 

700.  Alloys  of  Gold.     Gold  unites  with  most  of  the  other 
metals ;  but  its  most  important  alloys  are  those  of  copper, 
silver,  and  mercury.     Pure  gold  is  so  soft  that  articles  of 
jewelry  made  of   it  would  quickly  wear  out  if  used ;    such 
articles,  as  well   as  coins  and  watches,  are  therefore  always 
made  of  gold  which  has  been  alloyed  with  copper,  in  order  to 
increase  its   hardness.     The  standard  alloy  for  coin  in  this 
country  and  in  France  is  nine  parts  by  weight  of  gold  to  one  part 
of  copper ;  in  England  it  is  eleven  parts  of  gold  to  one  of 
copper.     These  alloys,  as  well  as  the  alloys  of  silver  and  gold, 
are  more  fusible  than  pure  gold,  but  less  ductile.     Native  gold 
is  an  alloy  of  gold  and  silver,  tbe  proportion   of  the  latter 
metal  varying  from  0.2  to  62  per  cent.     Amalgams  of  gold 
play  an  important  part  in  the  metallurgy  of  gold  (§  695),  and 
in  the  process  of  fire-gilding  above  described. 

PLATINUM. 

701.  Platinum  is  a  metal  which,  like  gold,  has  little  affinity 
for  the  other  chemical  elements.     It  is  commonly  found  in  the 
native  state,  alloyed  witb  gold  and  with  other  metals.     Like 
gold,  it  is  obtained  by  washing  away  the  earth  and  sand  with 
which  it  is  found  mixed.    It  is  a  very  heavy  metal,  the  specific 
gravity  of  cast-platinum  being  21.15.     Its  atomic  weight   is 
197.4.     The   color  of  platinum   is  intermediate  between  the 
white  of  silver  and  the  gray  of  steel ;  its  lustre  is  far  less  brill- 
iant than  that  of  silver.     It  is  as  soft  as  copper,  very  mallea- 
ble and  very  tenacious  ;  it  may  be  drawn  into  wire  so  fine  that 


596  PLATINUM. 


its  diameter  is  only  y^^th  of  a  millimetre.  It  is  not  fusible 
in  ordinary  furnaces,  but  may  be  fused  in  the  blow-pipe  flame 
(Exps.  26,  207),  and  is  nowadays  melted  in  considerable 
quantities  in  lime  crucibles  by  means  of  a  blow-pipe  flame  ob- 
tained from  common  coal  gas  and  oxygen.  At  very  high  tem- 
peratures it  may  be  volatilized.  Like  wrought-iron,  platinum 
admits  of  being  forged  and  welded  at  temperatures  far  below 
its  melting-point.  When  heated,  it  expands  less  than  any 
other  metal,  and  is  hence  well  adapted  for  the  construction  of 
apparatus  in  which  metal  and  glass  must  be  fused  together. 
It  conducts  heat  and  electricity  much  less  readily  than  gold, 
silver,  or  copper,  standing  in  this  respect  not  far  from  iron. 

702.  Platinum  does  not  oxidize  in  the  air  at  any  temperature, 
nor  is  it  attacked  by  any  of  the  common  acids  taken  separately  ; 
in  aqua  regia  (§  104)  it  dissolves  slowty,  —  much  less  readily 
than  gold.  Chlorine  water  dissolves  it,  but  neither  bromine 
nor  iodine  has  any  action  upon  it.  When  heated  to  redness 
in  the  air,  in  contact  with  the  fixed  caustic  alkalies  or  alkaline 
earths,  it  is  slowly  corroded,  in  consequence  of  the  formation  of 
an  oxide  which  unites  with  the  alkali.  Phosphorus  and  arsenic 
unite  readily  with  hot  finely  divided  platinum,  forming  very 
fusible  compounds  ;  sulphur  also  combines  with  it,  though  far 
less  readily.  At  high  temperatures,  platinum  is  easily  acted 
upon  by  silicon  (Compare  §  463).  A  platinum  crucible  should 
consequently,  never  be  placed  in  direct  contact  with  a  hot  mix- 
ture of  a  carbon  compound  and  silicic  acid.  If  the  crucible  is 
to  be  heated  in  a  coal  fire,  it  should  first  be  placed  in  an  earthen 
crucible  lined  with  some  infusible  earth,  such  as  magnesia. 

With  most  of  the  other  metals  platinum  unites  readily,  form- 
ing alloys  which  in  many  instances  are  more  fusible  than  pla- 
tinum itself;  hence,  in  employing  platinum  vessels  in  chemical 
experiments,  care  must  be  taken  never  to  touch  the  platinum 
with  easily  fusible  metals,  or  to  place  in  the  vessels  any  easily 
reducible  compound  of  a  metal.  Most  of  the  alloys  of  platinum 
are  not  only  fusible,  but  they  are  also  soluble  in  acids.  Pla- 
tinum which  has  been  alloyed  with  10  or  12  times  its  weight 


CHLORIDES    OF   PLATINUM.  597 

-  of  silver,  for  example,  is  as  completely  soluble  in  nitric  acid 
as  the  silver  itself. 

From  its  comparative  inertness  as  a  chemical  agent,  taken 
in  connection  with  its  infusibility,  platinum  is  an  extremely 
useful  metal  to  the  chemist.  It  is  employed  in  the  scientific 
laboratory  for  crucibles,  evaporating  dishes,  stills,  tubes,  spat- 
ulse,  forceps,  wire,  blow-pipe  tips,  and  the  like ;  and  in  the 
manufacture  of  oil  of  vitriol,  large  platinum  stills,  together  with 
cooling  syphons  of  the  same  metal,  are  employed  in  the  process 
of  concentrating  the  acid. 

703.  A  remarkable  property  of  platinum  of  inducing  various 
gases  to  combine  chemically  one  with  the  other  has  already 
been  repeatedly  alluded  to  and  illustrated  (§§  224,  240,  387). 
This  power  of  causing  combination  is  possessed  even  by  clean 
surfaces  of  the  ordinary  solid  metal,  though  to  a  much  greater 
degree  by  spongy  platinum  (Exp.  369),  and  still  more  by  the 
very  finely  divided  powder  known  as  platinum  black. 

Platinum  forms  two  series  of  compounds,  corresponding 
respectively  to  the  protoxide  PtO  and  to  the  binoxide  PtO2 . 
Its  chlorides  are  well-defined  compounds,  but  with  the  oxygen 
acids  it  forms  comparatively  few  salts,  and  none  of  these  are 
at  present  of  much  importance. 

704.  Protochloride  of  Platinum  (PtCl2)  is  a  compound  in- 
soluble in  water,  obtained  by  carefully  heating  the  bichloride 
to  230°  upon  an  oil-bath.   It  dissolves  in  alkaline  lyes,  and  the 
solution  thus  obtained  may  be  used  for  making  platinum  black 
(§  706).     At   a  red  heat  chloride  of  platinum  is  completely 
decomposed  to  metallic  platinum  and  chlorine.  With  the  other 
metallic  chlorides,  protochloride  of  platinum  unites  to  form 
compounds  known  as  chloroplatinites  ;  the  general  formula  of 
these  compounds  is  2MCl,PtCl2. 

705.  Bichloride  of  Platinum  (PtCl4)  is  the  platinum  com- 
pound most  commonly  employed  in  the  laboratory.    It  is  a  deli- 
quescent  substance,   readily  soluble   in    water,   alcohol,   and 
ether  ;  the  aqueous  solution  is  of  a  reddish-brown  color.    When 
heated  to  230°,  or  thereabouts,  the  salt  loses  half  its  chlorine, 
as  has  been  already  stated.     The  aqueous  solution   of  biclilo- 


598  PLATINUM    SPONGE. 

ride  of  platinum  is  much  used  as  a  test  for  potassium  and 
ammonium,  and  for  preparing  certain  organic  compounds  suit- 
able for  analysis. 

Exp.  366.  — Cut  half  a  gramme,  or  more,  of  worn-out  platinum 
foil  or  wire  into  small  fragments,  and  boil  them  with  a  teaspoonful 
of  aqua  regia  so  long  as  the  metal  appears  to  be  acted  upon,  then 
decant  the  liquid  into  a  porcelain  dish,  add  to  the  fragments  of 
platinum  another  teaspoonful  of  aqua  regia,  and  proceed  as  before, 
repeating  the  treatments  until  all  the  metal  has  dissolved.  By  the 
repeated  action  of  successive  small  portions  of  the  solvent,  plati- 
num, and  other,  comparatively  speaking,  insoluble  substances,  can 
be  dissolved  much  more  readily  than  if  all  the  liquid  necessary  for 
its  solution  were  added  at  once.  Evaporate  the  solution  to  dryness 
upon  a  water-bath,  take  up  the  residue  with  water,  and  preserve 
the  solution  in  a  bottle  provided  with  a  glass  stopper. 

Exp.  367.  —  Pour  a  teaspoonful  of  a  solution  of  chloride  of  potas- 
sium, or  of  almost  any  other  salt  of  potassium,  into  a  test-tube, 
acidulate  the  liquid  with  chlorhydric  acid,  and  add  to  it  a  drop  of 
the  solution  of  bichloride  of  platinum  obtained  in  the  preceding  ex- 
periment. A  yellow,  insoluble  powder  will  soon  be  precipitated. 
It  is  a  double  chloride  of  potassium  and  platinum,  and  its  formula 
may  be  written  2KC1,  PtCl4.  This  test  serves  to  distinguish  potas- 
sium from  sodium,  and,  if  need  be,  to  separate  potassium  from  solu- 
tions in  which  it  is  mixed  with  sodium ;  for  the  double  chloride 
produced  with  chloride  of  sodium  and  bichloride  of  platinum  is 
easily  soluble  in  water. 

Exp.  368.  —  Repeat  Exp.  367,  but  substitute  chloride  of  ammo- 
nium for  the  chloride  of  potassium.  A  yellow  precipitate,  similar  to 
that  obtained  in  Exp.  367,  will  separate  immediately,  or  if  the  so- 
lutions employed  are  dilute,  after  a  short  time.  The  composition 
of  this  precipitate  may  be  represented  by  the  formula  2NH4C1,  PtCl4. 
Again  repeat  the  experiment,  and  this  time  take  enough  of  the 
platinum  solution  and  of  the  chloride  of  ammonium  to  make  half 
a  teaspoonful  of  the  yellow  precipitate,  taking  care  that .  at  last 
there  shall  be  a  slight  excess  of  free  chloride  of  ammonium  rather 
than  of  chloride  of  platinum  in  the  supernatant  liquid.  Allow  the 
precipitate  to  settle,  separate  it  from  the  clear  liquor  by  decanta- 
tion,  and  dry  it  partially  at  a  gentle  heat.  When  the  precipitate 
has  acquired  the  consistence  of  slightly  moistened  earth,  transfer  it 
to  a  cup-shaped  piece  of  platinum  foil,  and  heat  it  to  redness  in  the 
gas  flame,  as  long  as  fumes  of  chloride  of  ammonium  continue  to 


PLATINUM    BLACK.  599 

• 

escape.  All  the  chlorine,  hydrogen,  and  nitrogen  will  be  driven 
off,  and  there  will  remain  upon  the  foil  a  gray,  loosely  coherent, 
sponge-like  mass  of  metallic  platinum  ;  it  is  called  platinum  sponge. 

Exp.  369.  —  Hold  the  dry  platinum  sponge  of  Exp.  368  in  a 
stream  of  hydrogen  or  of  common  illuminating  gas  issuing  from  a 
fine  jet.  The  metal  will  soon  begin  to  glow,  and  in  a  moment  will 
become  hot  enough  to  inflame  the  mixture  of  air  and  gas  in  con- 
tact with  it.  Before  friction  matches  were  employed,  this  property 
of  spongy  platinum,  of  inflaming  hydrogen,  was  sometimes  made 
use  of  for  striking  a  light.  The  mode  of  action  of  the  platinum  in 
this  experiment  is  obscure ;  it  has  already  been  alluded  to  in  §  387. 

From  platinum  sponge,  solid  articles  of  platinum  may  be  manufac- 
tured by  compression.  If  the  spongy  platinum  be  first  rubbed  to  pow- 
der under  water,  the  particles  of  metal  of  which  it  is  composed  can 
be  readily  compacted  into  solid  bars  by  subjecting  the  powder  to 
powerful  pressure  in  appropriate  moulds.  The  pressed  bar  is  then 
heated  intensely  in  a  coke-fire  with  strong  draught,  and  forged  by 
striking  it  with  the  hammer  upon  its  ends ;  the  process  of  heating 
and  forging  being  several  times  repeated,  until  the  bar  has  become 
sufficiently  condensed.  The  metal  may  then  be  wrought  into  any 
desired  shape  by  heating  and  hammering,  in  the  same  way  as  any 
other  malleable  metal.  This  process  of  working  platinum  was  for  a 
long  time  the  common  method,  and  is  still  employed  to  a  certain 
extent. 

706.  Platinum  Black  is  a  term  applied  to  metallic  platinum, 
even  more  finely  divided  than  the  sponge  above  described. 

By  dissolving  protochloride  of  platinum  in  hot,  concentrated  pot- 
ash-lye, and  pouring  into  the  hot  liquor  alcohol,  by  small  successive 
portions,  platinum  will  be  thrown  down  as  a  black  powder  looking 
like  soot.  The  powder  should  be  freed  from  the  supernatant  liquor 
by  decantation,  and  then  boiled  successively  with  alcohol,  chlorhy- 
dric  acid,  potash-lye,  and  water,  in  order  to  free  it  from  all  impu- 
rities. 

A  capacious  vessel  must  be  chosen  for  the  reaction  of  the  alcohol 
upon  the  alkaline  solution  of  chloride  of  platinum,  for  much  car- 
bonic acid  is  generated  while  the  components  of  the  alcohol  are  re- 
ducing the  solution  of  platinum,  so  that  lively  effervescence  occurs. 
Platinum  black  is  capable  not  only  of  absorbing  and  storing  up 
many  times  its  own  bulk  of  oxygen  gas ;  it  is  also  capable  of  giv- 
ing away  this  oxygen  to  many  other  substances.  If  easily  oxidiz- 
able  liquids,  such  as  alcohol  or  ether,  are  dropped  upon  platinum 


600  THE    PLATINUM    METALS. 

• 

black  which  has  previously  been  exposed  to  the  air,  the  liquids  will 
be  oxidized  and  converted  into  new  substances,  while  the  powder 
becomes  red-hot  from  the  heat  evolved  during  the  act  of  oxida- 
tion. 

707.  Besides  forming  with  the  chlorides  of  potassium  and 
ammonium,  the  insoluble  compounds,  above  described,  bichlo- 
ride of  platinum  unites  with  many  other  chlorides,  both  of  metals 
and  of  organic  radicals,  to  form  analogous  salts  of  the  general 
formula  2MCl,PtCl4,  or  MCl2,PtCl4.     These   compounds   are 
commonly  called  chloroplatinates  ;  by  means  of  them,  the  com- 
position and  combining  weights  of  many  organic  compounds 
have  been  determined.  It  is  only  necessary  to  ignite  a  weighed 
portion  of  the  chloroplatinate,  and  to  weigh  the  residue  of 
pure  platinum  which  is  left  after  the  organic  matter  has  all 
been  driven  off,  in  order  to  ascertain  how  much  platinum  is 
contained  in  the   compound.     This  fact  having  been  deter- 
mined, the  quantity  of  the  organic  radical,  or  rather  of  the 
chloride  of  the  radical,  which  was  combined  with  the  chloride 
of  platinum  in  the  chloroplatinate,  may  be  readily  calculated. 

708.  With  gold  and  platinum  are  classed  several  rare  metals, 
which  are  never  found  except  in  association  with  platinum, 
and  which  closely  resemble  that  metal.     They  are  commonly 
called   platinum    metals,    and   the   group  may   be   appropri- 
ately termed  the   platinum  group.      The   whole   group   con- 
sists of  Rhodium  (atomic  weight  =  104),  Ruthenium  (104), 
Palladium  (106.5),  Gold  (196.7),  Platinum  (197.4),  Indium 
(198),  and  Osmium    (199).     Palladium  is  used  to  impart  to 
brass  gas-fixtures  a  peculiar  reddish  tint,  sometimes  called 
salmon-bronze.     Iridium  is  used  for  the  very  hard  tips  of  gold 
pens.     Osmium  forms,  among  other  oxides,  a  volatile  com- 
pound  OsO4,  whose  vapors   are   intensely  poisonous.     The 
metals   of  this   group  are  noble  metals  ;  the}r  withstand  the 
action  of  the  atmosphere ;  none  of  them  are  acted  upon  by 
nitric  acid,  though  they  dissolve  in  chlorine  and  in  aqua  regia. 
Their  oxides  part  with  all  their  oxygen  when  simply  heated, 
leaving  the  metal  behind. 


ATOMIC    WEIGHTS    OF    THE    ELEMENTS. 


601. 


An  alphabetical  list  of  the  sixty-five  recognized  elements,  with  their  symbols 
and  atomic  weights,  is  here  given  for  convenience  of  reference.  The  names  of 
those  elements  which  are  so  rare  as  to  be  at  present  of  little  importance  are 
printed  in  italics :  — 


Aluminum, 

Al 

-       27.4 

Molybdenum,  - 

Mo 

-       96 

Antimony, 

Sb 

-     122 

Nickel,        - 

-     Ni 

-       58.8 

Arsenic,      - 

As 

-      75 

.JiTitrogen, 

N 

-       14 

Barium, 

Ba 

-     137 

Norium, 

-    No 

? 

v  Bismuth,    - 

Bi 

-     210 

Osmium, 

Os 

-     199 

^Boron,    - 

Bo 

-       11 

_£xygen,      - 

-    0 

-       16 

"Bromine,    - 

Br 

-       80 

Palladium, 

Pd 

-     106.5 

Cadmium, 

Cd 

-     112 

—Phosphorus, 

-    P 

-       31 

CfBsium,      - 

Cs 

-     133 

Platinum, 

Pt 

-     197.4 

Calcium, 

Ca 

-       40 

,  Potassium, 

-        -    K 

-       39.1 

Carbon,       - 

C 

-       12 

Rhodium, 

-.         Rh 

-     104 

Cerium,  -         -         - 

Ce 

-       92 

Rubidium,  - 

-    Rb 

-       85.7 

-Chlorine,    - 

Cl 

-       35.5 

Ruthenium, 

Ru 

-     104 

r*v» 

Cr 

52.5 

0     ? 

-     Se 

Cobalt,        -         -  '      - 

Co 

-       58.8 

Silicon,  - 

Si 

-       28 

Columbium  (Niobium), 

Ni 

94 

Silver, 

-        -  .Ag 

-     108 

Copper,       ... 

Cu 

-       63.4 

Sodium, 

Na 

-       23 

Didymium, 

D 

-       95 

Strontium,  - 

-     Sr 

-       87.5 

Erbium,       - 

E 

? 

Sulphur, 

S 

-       32 

--Fluorine, 

Fl 

-       19 

Tantalum, 

-    Ta 

-     137.6 

Glucinum,  -         -         - 

Gl 

-       14 

—^Pellurium, 

Te 

-     128 

Gold,      - 

Au 

-     196.7 

Terbium,      - 

-    Tb 

? 

Hydrogen, 

H 

1 

Thallium, 

Tl 

-     204 

Indium, 

In  - 

35.9  (?) 

Thorium,     - 

-  Th  - 

231.5  (?) 

Iodine,        ... 

I 

-     127 

Tin, 

Sn 

-     118 

Iridium, 

Ir 

-     198 

Titanium,  - 

-    Ti 

-       50 

Iron, 

Fe 

-       56 

Tungsten, 

W 

-     184 

Lanthanum,    - 

La 

-       92.8 

Uranium,    - 

-    Ur 

-     120 

Lead, 

Pb 

-     207 

Vanadium, 

V 

-     137 

Lithium, 

Li 

7 

Yttrium,     - 

-     Yt 

-       68 

Magnesium, 

Mg 

-       24 

Zinc, 

Zn 

-       65 

Manganese,     - 

Mn 

-       55 

Zircomium, 

-     Zr 

-    90(?) 

Mercury,    -         -         - 

Hg 

-     200 

46 

602 


NATURAL    GROUPS. 


In  the  following  table  the  elements  are  arranged  in  what  are  believed  to  be 
natural  groups.  Without  accepting  any  one  infallible  criterion  of  classification, 
or  insisting  upon  any  systematic  arrangement  of  the  elements  in  groups  with  that 
strenuousness  which  is  apt  to  make  classification  rather  a  hindrance  than  a  help, 
the  student  may  provisionally  use  this  subdivision  of  the  elements  into  groups,  as 
a  help  in  remembering  facts,  as  a  guide  to  the  prompt  recognition  of  general  prop- 
erties and  general  laws,  and  as  a  suggestive  compend  of  his  whole  chemical 
knowledge :  — 


Fluorine,     - 

19 

Glucinum, 

-      14 

Chlorine, 

35  5 

Aluminum 

-                             27.5 

Bromine, 

80 

52.2 

Iodine,    - 

-        -        -        -    127 

Manganese, 

55 

/ 

Iron, 

-      56 

Oxygen, 

16 

Sulphur, 

.-  •  '  -"    '.  j  '••'   -      32 

Nickel     - 

Selenium,    - 

-        -  '    -          79.5 

Yttrium, 

68 

Tellurium, 

,.  '      -  "      -  "      -128 

Erbium, 

? 

Terbium 

? 

Nitrogen,    - 
Phosphorus, 

-       >-   >-    ?       14 
-      31 

Zirconium, 
Norium, 

-         -         -  90  (?) 
? 

Arsenic, 

75 

Cerium,  - 

92 

Antimony, 
Bismuth, 

-    122 
210 

Lanthanum, 
Didymium, 

92.8 
-      95 

Carbon,   - 

-      12 

Uranium,     - 

120 

Boron, 

11 

Thorium,      __    - 

-       231.5  (?) 

Silicon,    - 

-      28 

Copper, 

63.4 

Hydrogen,  - 

.  -         -         -             1 

Mercury, 

-    200 

Lithium, 
Sodium, 
Potassium, 

7 
23 
-      39.1 

Titanium,    - 
Columbium,     - 
Tin,     - 

-       --           50 
-        -         -      94 
-.  .'     «,         .         ;Q§ 

Rubidium,  - 
Silver,     - 

85.7 
-    108 

Tantalum, 

•         -         -    137.6 

133 

Molybdenum 

Qfi 

Thallium, 

-    204 

~                  «/O 

...    137 

Tungsten,    - 

.-  .      -         -         184 

Calcium,  -    - 

40 

c 

07  c 

otrontium, 
Barium, 

-             -             -             -         O  i  .0 

137 

Ruthenium, 

-•        104 

Lead, 

-  207 

Palladium, 

-    106.5 

Gold    - 

Magnesium, 

24 

Platinum, 

-    197.4 

Zinc, 

-      65 

Iridium, 

198 

Cadmium,    - 

-        -         112 

Osmium, 

-         -    199 

APPENDIX. 


CHEMICAL    MANIPULATION. 

1.  Glass-tubing.  Two  qualities  of  glass-tubing  are  used  in  chemical 
experiments,  that  which  softens  readily  in  the  flame  of  a  gas  or  spirit- 
lamp,  and  that  which  fuses  with  extreme  difficulty  in  the  flame  of  the 
blast-lamp.  These  two  qualities  are  distinguished  by  the  terms  soft 
and  hard  glass.  Soft  glass  is  to  be  preferred  for  all  uses  except  the 
intense  heating,  or  ignition,  of  dry  substances.  Fig.  I.  represents  the 
most  convenient  sizes  of  glass-tubing,  both  hard  and  soft,  and  shows  also 
the  proper  thickness  of  the  glass  walls  for  each  size. 

FIG.  I. 


2.  Cutting  and  cracking  glass.  Glass-tubing  and  glass  rod  must 
generally  be  cut  to  the  length  required  for  any  particular  apparatus. 
A  sharp  triangular  file  is  used  for  this  purpose.  The  stick  of  tubing, 
or  rod,  to  be  cut  is  laid  upon  a  table,  and  a  deep  scratch  is  made  with 
the  file  at  the  place  where  the  fracture  is  to  be  made.  The  stick  is 
then  grasped  with  the  two  hands,  one  on  each  side  of  the  mark, 
while  the  thumbs  are  brought  together  just  at  the  scratch.  By 
pushing  with  the  thumbs  and  pulling  in  the  opposite  direction  with  the 
fingers,  the  stick  is  broken  squarely  at  the  scratch,  just  as  a  stick  of 
candy  or  dry  twig  may  be  broken.  The  sharp  edges  of  the  fracture 
should  invariably  be  made  smooth,  either  with  a  wet  file,  or  by  soften- 
ing the  end  of  the  tube  or  rod  in  the  lamp.  (See  Appendix,  §  3.) 
Tubes  or  rods  of  sizes  four  to  eight  inclusive  may  readily  be  cut  in 
this  manner ;  the  larger  sizes  are  divided  with  more  difficulty,  and  it  is 
often  necessary  to  make  the  file-mark  both  long  and  deep.  An  even 


11  CUTTING  AND  CRACKING  GLASS. 

fracture  is  not  always  to  be  obtained  with  large  tubes.     The  lower  ends 
of  glass  funnels,  and    those  ends  of  gas  delivery-tubes  which  enter 
the  bottle  or  flask  in  which  the  gas  is  generated,  should  be  filed  off, 
FlQ  jj  or  ground  off  on  a  grindstone,  obliquely  (Fig.  II.),  to 

facilitate  the  dropping  of  liquids  from  such  extremi- 
ties. 

In  order  to  cut  glass  plates  the  glazier's  diamond 
must  be  resorted  to.  For  the  cutting  of  exceedingly 
thin  glass  tubes  and  of  other  glass  ware,  like  flasks, 
retorts,  and  bottles,  still  other  means  are  resorted  to,  based  upon  the 
sudden  and  unequal  application  of  heat.  The  process  divides  itself 
into  two  parts,  the  producing  of  a  crack  in  the  required  place,  and  the 
subsequent  guiding  of  this  crack  in  the  desired  direction.  To  produce 
a  crack,  a  scratch  must  be  made  with  the  file,  and  to  this  scratch  a 
pointed  bit  of  red-hot  charcoal,  or  the  jet  of  flame  produced  by  the 
mouth  blowpipe,  or  a  very  fine  gas-flame,  or  a  red-hot  glass-rod  may  be 
applied.  If  the  heat  does  not  produce  a  crack,  a  wet  stick  or  file  may 
be  touched  upon  the  hot  spot.  Upon  any  part  of  a  glass  surface  ex- 
cept the  edge,  it  is  not  possible  to  control  perfectly  the  direction  and 
extent  of  this  first  crack ;  at  an  edge  a  small  crack  may  be  Started  with 
tolerable  certainty  by  carrying  the  file-mark  entirely  over  the  edge. 
To  guide  the  crack  thus  started,  a  pointed  bit  of  charcoal  or  slow- 
match  may  be  used.  The  hot  point  must  be  kept  on  the  glass  from 
1  c.  m.  to  0.5  c.  m.  in  advance  of  the  point  of  the  crack.  The  crack  will 
follow  the  hot  point,  and  may  therefore  be  carried  in  any  desired 
direction.  By  turning  and  blowing  upon  the  coal  or  slow-match  the 
point  may  be  kept  sufficiently  hot.  Whenever  the  place  of  experiment 
is  supplied  with  common  illuminating  gas,  a  very  small  jet  of  burning 
gas  may  be  advantageously  substituted  for  the  hot  coal  or  slow  match. 
To  obtain  such  a  sharp  jet,  a  piece  of  hard  glass  tube,  No.  5,  10  c.  m. 
long,  and.  drawn  to  a  very  fine  point  (see  Appendix,  §  3),  should  be 
placed  in  the  caoutchouc  tube  which  ordinarily  delivers  the  gas  to  the 
gas-lamp,  and  the  gas  should  be  lighted  at  the  fine  extremity.  The 
burning  jet  should  have  a  fine  point,  and  should  not  exceed  1.5  c.  m. 
in  length.  By  a  judicious  use  of  these  simple  tools,  broken  tubes, 
beakers,  flasks,  retorts,  and  bottles  may  often  be  made  to  yield  very 
useful  articles  of  apparatus.  No  sharp  edges  should  be  allowed  to 
remain  upon  glass  apparatus.  The  durability  of  the  apparatus  itself, 
and  of  the  corks  and  caoutchouc  stoppers  and  tubing  used  with  it,  will 
be  much  greater,  if  all  sharp  edges  are  removed  with  the  file,  or,  still 
better,  rounded  in  the  lamp. 

3.  Bending  and  closing  glass  tubes.     Tubing  of  sizes  five  to  eight  in. 


BENDING  AND  CLOSING  GLASS  TUBES.  iii 

elusive  can  generally  be  worked  in  the  common  gas  or  spirit-lamp ;  for 
larger  tubes  the  blast-lamp  is  necessary.  (See  Appendix,  §  6.)  Glass 
tubing  must  not  be  introduced  suddenly  into  the  hottest  part  of  the 
flame,  lest  it  crack.  Neither  should  a  hot  tube  be  taken  from  the 
flame  and  laid  at  once^upon  a  cold  surface.  Gradual  heating  and  grad- 
ual cooling  are  alike  necessary,  and  are  the  more  essential  the  thicker 
the  glass ;  very  thin  glass  will  sometimes  bear  the  most  sudden  changes 
of  temperature,  but  thick  glass  and  glass  of  uneven  thickness  absolutely 
require  slow  heating  and  annealing.  When  the  end  of  a  tube  is  to  be 
heated,  as  in  rounding  sharp  edges,  more  care  is  required  in  consequence 
of  the  great  facility  with  which  cracks  start  at  an  edge.  A  tube 
should,  therefore,  always  be  brought  first  into  the  current  of  hot 
air  beyond  the  actual  flame  of  the  gas  or  spirit-lamp,  and  there 
thoroughly  warmed,  before  it  is  introduced  into  the  flame  itself.  If 
a  blast-lamp  is  employed,  the  tube  may  be  warmed  in  the  smoky 
flame,  before  the  blast  is  turned  on,  and  may  subsequently  be 
annealed  in  the  same  manner ;  the  deposited  soot  will  be  burnt  off 
in  the  first  instance,  and  in  the  last,  may  be  wiped  off  when  the  tube 
is  cold.  In  heating  a  tube,  whether  for  bending,  drawing,  or  closing, 
the  tube  must  be  constantly  turned  between  the  fingers,  and  also  moved 
a  little  to  the  right  and  left,  in  order  that  it  may  be  uniformly  heated 
all  round,  and  that  the  temperature  of  the  neighboring  parts  may  be 
duly  raised.  If  a  tube,  or  rod,  is  to  be  heated  at  any  part  but  an  end, 
it  should  be  held  between  the  thumb  and  first  two  fingers  of  each  hand 
in  such  a  manner,  that  the  hands  shall  be  below  the  tube  or  rod,  with 
the  palms  upward,  while  the  lamp-flame  is  between  the  hands.  When 
the  end  of  a  tube  or  rod  is  to  be  heated,  it  is  best  to  begin  by  warming 
the  tube  or  rod  about  2  c.  m.  from  the  end,  and  from  thence  to  proceed 
slowly  to  the  end. 

The  best  glass  will  not  be  blackened  or  discolored  during  heating. 
The  blackening  which  often  occurs  is  generally  caused  by  the  reduc- 
tion to  the  metallic  state  of  the  oxide  of  lead  contained  in  common 
flint  glass.  Glass  containing  much  oxide  of  lead  is  not  well  adapted 
for  chemical  uses.  The  blackening  may  sometimes  be  removed  by 
putting  the  glass  in  the  upper  or  outer  part  of  the  flame,  where  the 
reducing  gases  are  consumed,  and  the  air  has  the  best  access  to  the 
glass.  The  blackening  may  be  altogether  avoided  by  always  keeping 
the  glass  in  the  oxidizing  part  of  the  flame. 

Glass  begins  to  soften  and  bend  below  a  visible  red  heat.  The  con- 
dition of  the  glass  is  judged  of  as  much  by  the  fingers  as  the  eye  ;  the 
hands  feel  the  yielding  of  the  glass,  either  to  bending,  pushing,  or 
pulling,  better  than  the  eye  can  see  the  change  of  color  or  form.  It 


IV  BENDING  AND  CLOSING  GLASS  TUBES. 

may  be  bent  as  soon  as  it  yields  in  the  hands,  but  can  be  drawn  out 
only  when  much  hotter  than  this.  Glass-tubing,  however,  should  not 
be  bent  at  too  low  a  temperature;  the  curves  made  at  too  low  a  heat 
are  apt  to  be  flattened,  of  unequal  thickness  on  the  convex  and  con- 
cave sides,  and  brittle. 

In  bending  tubing  to  make  gas  delivery-tubes  and  the  like,  attention 
should  be  paid  to  the  following  points:  1st,  the  glass  should  be  equally 
hot  on  all  sides ;  2d,  it  should  not  be  twisted,  pulled  out,  or  pushed 
together  during  the  heating ;  3d,  the  bore  of  the  tube  at  the  bend 
should  be  kept  round,  and  not  altered  in  size  ;  4th,  if  two  or  more 
bends  be  made  in  the  same  piece  of  tubing  (Fig.  III.  a),  they  should  all 
be  in  the  same  plane,  so  that  the  finished  tube  will  lie  flat  upon  the 
level  table. 

When  a  tube  or  rod  is  to  be  bent  or  drawn  close  to  its  extremity,  a 
temporary  handle  may  be  attached  to  it  by  softening  the  end  of  the 
tube  or  rod,  and  pressing  against  the  soft  glass  a  fragment  of  glass  tube, 
which  will  adhere  strongly  to  the  softened  end.  The  handle  may  sub- 
sequently be  removed  by  a  slight  blow,  or  by  the  aid  of  a  file.  If  a 
considerable  bend  is  to  be  made,  so  that  the  angle  between  the  arms 
will  be  very  small  or  nothing,  as  in  a  syphon,  for  example,  the  curva- 
ture cannot  be  well  produced  at  one  place  in  the  tube,  but  should  be 
made  by  heating,  progressively,  several  centimetres  of  the  tube,  and 
bending  continuously  from  one  end  of  the  heated  portion  to  the  other 
FlG  m  (Fig.  III.  &).  Small  and  thick  tube,  may  be 

bent  more  sharply  than  large  or  thin  tube. 

In  order  to  draw  a  glass  tube  down  to  a 
finer  bore,  it  is  simply  necessary  to  thor- 
oughly soften  on  all  sides  one  or  two  centime 
ters'  length  of  the  tube,  and  then  taking  the 
glass  from  the  flame,  pull  the  parts  asunder  by  a  cautious  move- 
ment of  the  hands.  The  larger  the  heated  portion  of  glass,  the  longer 
will  be  the  tube  thus  formed.  Its  length  and  fineness  also  increase 
with  the  rapidity  of  motion  of  the  hands.  If  it  is  desirable  that  the 
finer  tube  should  have  thicker  walls  in  proportion  to  its  bore  than  the 
original  tube,  it  is  only  necessary  to  keep  the  heated  portion  soft  for 
two  or  three  minutes^  before  drawing  out  the  tube,  pressing  the  parts 
slightly  together  the  while.  By  this  process  the  glass  will  be  thickened 
at  the  hot  ring. 

To  obtain  a  tube  closed  at  one  end,  it  is  best  to  take  a  piece  of 
tubing,  open  at  both  ends,  and  long  enough  to  make  two  closed  tubes. 
In  the  middle  of  the  tube  a  ring  of  glass,  narrow  as  possible,  must  be 
made  thoroughly  soft.  The  hands  are  then  separated  a  little,  to  cause 


BLOWING  BULBS.  V 

a  contraction  in  diameter  at  the  hot  and  soft  part.  The  point  of  the 
flame  must  now  be  directed,  not  upon  the  narrowest  part  of  the  tube, 
but  upon  what  is  to  be  the  bottom  of  the  closed  tube.  This  point  is 
indicated  by  the  line  a  in  Fig.  IV.  By  with- 
drawing the  right  hand,  the  narrow  part  of 
the  tube  is  attenuated,  and  finally  melted  off, 
leaving  both  halves  of  the  original  tube  closed 
at  one  end,  but  not  of  the  same  form;  the  right- 
hand  half  is  drawn  out  into  a  long  point,  the 

other  is  more  roundly  closed.  It  is  not  possible  to  close  handsomely 
the  two  pieces  at  once.  The  tube  is  seldom  perfectly  finished  by  the 
operation  ;  a  superfluous  knob  of  glass  generally  remains  upon  the  end. 
If  small,  it  may  be  got  rid  of  by  heating  the  whole  end  of  the  tube,  and 
blowing  moderately  with  the  mouth  into  the  open  end.  The  knob, 
being  hotter,  and  therefore  softer  than  any  other  part,  yields  to  the 
pressure  from  within,  spreads  out  and  disappears.  If  the  knob  is  large, 
it  may  be  cut  off  with  scissors  while  red-hot,  or  drawn  off  by  sticking 
to  it  a  fragment  of  tube,  and  then  softening  the  glass  above  the 
junction.  The  same  process  may  be  applied  to  the  too  pointed  end  of 
the  right-hand  half  of  the  original  tube,  or  to  any  misshapen  result  of 
an  unsuccessful  attempt  to  close  a  tube,  or  to  any  bit  of  tube  which  is 
too  short  to  make  two  closed  tubes.  When  the  closed  end  of  a  tube  is 
too  thin,  it  may  be  strengthened  by  keeping  the  whole  end  at  a  led 
heat  for  two  or  three  minutes,  turning  the  tube  constantly  between  tha 
fingers.  It  may  be  said  in  general  of  all  the  preceding  operations 
before  the  lamp,  that  success  depends  on  keeping  the  tube  to  be  heated 
in  constant  rotation,  in  order  to  secure  a  uniform  temperature  on  all 
sides  of  the  tube. 

4.  Blowing  Bulbs  and  Piercing  Holes  in  Tubing.  If  the  bulb  desired 
is  large  in  proportion  to  the  size  of  the  tube  on  which  it  is  to  be  made, 
the  walls  of  the  tube  must  be  thickened  by  rotation  in  the  flame 
before  the  bulb  can  be  blown.  If  the  bulb  is  to  be  blown  in 
the  middle  of  a  piece  of  tubing,  this  thickening  is  effected  by  gently- 
pressing  the  ends  of  the  tube  together  while  the  glass  is  red-hot 
in  the  place  where  the  bulb  is  to  be ;  if  the  bulb  is  to  be  placed 
at  the  end  of  a  tube,  this  end  is  first  closed,  and  then  suitably 
thickened  by  pressing  the  hot  glass  'up  with  a  piece  of  metal 
until  enough  has  been  accumulated  at  the  end.  The  thickened  portion 
of  glass  is  then  to  be  heated  to  a  cherry-red,  suddenly  withdrawn  from 
the  flame,  and  expanded  while  hot  by  steadily  blowing,  or 
rather  pressing  air,  into  the  tube  with  the  mouth ;  the  tube  must  be 
constantly  turned  on  its  axis,  not  only  while  in  the  flame,  but  also 


VI 


LAMPS. 


FIG.  V. 


while  the  bulb  is  being  blown.  If  too  strong  or  too  sudden  a  pressure 
be  exerted  with  the  mouth,  the  'bulb  will  be  extremely  thin  and 
quite  useless.  By  watching  the  expanding  glass,  the  proper  mo- 
ment for  arresting  the  pressure  may  usually  be  determined.  If  the  bulb 
obtained  be  not  large  enough,  it  may  be  reheated  and  enlarged  by  blow- 
ing into  it  again,  provided  that  a  sufficient  thickness  of  glass  remain. 

It  is  sometimes  necessary  to  make  a  hole  in  the  side  of  a  tube  or 
other  thin  glass  apparatus.  This  may  be  done  by  directing  a  pointed 
flame  from  the  blast-lamp  upon  the  place  where  the  hole  is  to  be, 
until  a  small  spot  is  red-hot,  and  then  blowing  forcibly  into  one  end  of 
the  tube  while  the  other  end  is  closed  by  the  finger ;  at  the  hot  spot 
the  glass  is  blown  out  into  a  thin  bubble,  which  bursts  or  may  be  easily 
broken  oflf,  leaving  an  aperture  in  the  side  of  the  tube. 

It  is  hoped  that  these  few  directions  will  enable  the  attentive  student 
to  perform,  sufficiently  well,  all  the  manipulations  with  glass  tubes 
which  the  experiments  described  in  the  manual  require.  Much  prac- 
tice will  alone  give  a  perfect  mastery  of  the  details  of  glass-blowing. 

5.  Lamps.     The   common   glass   spirit-lamp  will   be 
understood  without  description  from  the  figure  (Fig.  V.). 
This  lamp  does  not  give  heat  enough  for  most  ignitions  ; 
for  such  purposes  a  lamp  with  circular  wick,  of  some 
one  of  the  numerous  forms  sold  under  the  name  of 
Berzelius's  Argand  Spirit  Lamp  (Fig.  VI.),  is  necessary. 
These  argand  lamps  are  usually  mounted  on  a  lamp- 
stand  provided  with  three  brass  rings,  but  the  fittings  of 
these  lamps  are  all  made  slender,  in  order  not  to  carry 
off    too    much    heat.      When 
it  is  necessary  to  heat  heavy 
vessels,  other  supports  must  be 
used. 

Whenever  gas  can  be  ob- 
tained, gas-lamps  are  greatly  to 
be  preferred  to  the  best  spirit- 
lamps.  For  all  the  experiments 
of  this  book,  except  a  few  for 
which  ignition-tubes  must  be 
prepared  or  in  which  consider- 
able lengths  of  tubing  must  be 
heated,  the  gas-lamp  known  as 
Bunsen's  burner  will  be  suffi- 
cient. The  cheapest  and  best 
construction  of  the  lamp  may 


FIG.  VI. 


LAMPS. 


Vll 


FIG.  VII. 


be  learned  from  the  following  description  with  the  accompanying  figures. 
(Fig  VII.)  The  single  casting  of  brass  ab  comprises  the  tube  Ahrou^h 
which  the  gas  enters,  and  the  block  a 
from  which  the  gas  escapes  by  two  or 
three  fine  vertical  holes  passing  through 
the  screw  d,  and  issuing  from  the  upper 
face  of  d,  as  shown  at  e.  The  length  of 
the  tube  b  is  4.5  c.  m.  and  its  outside 
diameter  varies  from  0.5  c.  m.  at  the 
outer  end  to  1  c.  m.  at  the  junction 
with  the  block  a.  The  outside  di- 
ameter of  the  block  a  is  1 .6  c.  m.,  and 
its  outside  height  without  the  screws 
is  1.8  c.  m.  By  the  screw  c,  the 
piece  ab  is  attached  to  the  iron  foot  g,  which  may  be  6  c.  m.  in 
diameter.  By  the  screw  d,  the  brass  tube /is  attached  to  the  casting  ab. 
The  diameter  of  the  face  e,  and  therefore  the  internal  diameter  of  the 
tube  /"should  be  8  m.  m.  The  length. of  the  tube /is  9  c.  m.  Through 
the  wall  of  this  tube,  four  holes  5  m.  m.  in  diameter,  are  to  be  cut  at 
such  a  height  that  the  bottom  of  each  hole  will  come  1  m.  m.  above  the 
face  e  when  the  tube  is  screwed  upon  ab.  These  holes  are  of  course 
opposite  each  other  in  pairs.  The  finished  lamp  is  also  shown  in 
the  figure  8.  To  the  tube  b  a  caoutchouc  tube  of  5  to  7  m.  m.  internal 
diameter  is  attached  ;  this  flexible  tube  should  be  about  1  m.  long,  and 
its  other  extremity  should  be  connected  with  the  gas-cock  through  the 
intervention  of  a  short  piece  of  brass  gas-pipe  screwed  into  the  cock. 

A  lamp  to  give  a  powerful  flame  8  or  10  c.  m.  long,  suitable  for  heat- 
ing tubes,  may  be  very  simply  constructed  by  boring  two  holes,  enter- 
ing the  side  and  issuing  at  the  upper  face,  through  a  block  of  compact 
hard  wood,  10  c.  m.  by  6.5  c.  m.  by  3.5  c.  m.,  and  fitting  short  pieces 
of  brass  tubing  into  the  holes  so  formed.  To  the  tubes  at  the  side  are 
attached  the  caoutchouc  tubes  which  deliver  the  gas,  and  from  the 
tubes  at  the  top  the  gas  issues  under  a  sheet-iron 
funnel  closed  at  the  top  with  wire-gauze.  Above 
this  gauze,  the  mixture  of  gas  and  air  is  to  be  lighted. 
The  iron  funnel  will  be  readily  understood  from 
the  accompanying  figure,  and  the  following  dimen- 
sions; length  of  the  wire-gauze  10  c.  m. ;  width  of 
the  gauze  5  c.  m. ;  width  at  a  &,  9  c.  m. ;  height  of 
the  line  a  b  from  the  table,  8.5  c.  m. ;  whole  height 
of  the  funnel,  21  c.  m.  A  partition  parallel  to  a  6 
divides  the  funnel  into  two  equal  parts  from  the  gauze  to  the  level  of  a  b. 


FIG.  VHI. 


Vlll 


BLAST-LAMPS    AND    BLOWERS. 


FIG.  IX. 


6.  Bfast-lamps  and  Blowers.    For  drawing,  bending,  and  closing  large 
glass  tubes,  a  blast-lamp  is  necessary.     The  best  form  is  that  sold  under 

the  name  of  Bunsen's  Gas  Blow-pipe. 
Its  construction  and  the  method  ot 
using  it  may  be  learned  from  the  ac- 
companying figure;  a  &  is  the  pipe 
through  which  the  gas  enters,  c  is  the 
tube  for  the  blast  of  air ;  the  relation 
of  the  air-tube  to  the  external  gas  tube 
is  shown  at  d;  there  is  an  outer 
sliding  tube  by  which  the  form  and 
volume  of  the  flame  can  be  regulated. 
If  gas  is  not  to  be  had,  a  lamp  burn- 
ing oil  or  naphtha  must  be  employed. 
Figure  X.  represents  a  common  tin, 
glass-blower's  lamp,  suitable  for  burn- 
ing oil.  A  large  wick  is  essential, 
whether  oil  or  naphtha  be  the  com- 
bustible. 

For  every  blast-lamp  a  blowing-machine  of  some  sort  is  necessary. 
To  supply  a  constant  blast  it  is  essential  that  the  bellows  be  of  that 
FIG.  X.  construction    called    double.      Figures 

XL  and   XII.  represent  two  forms  ot 
blowpipe-table;  their  height  is  that  ot 
an  ordinary  table,  from  which  dimen- 
sion the  other  proportions  may  be  in- 
ferred.    A  small  double-acting  bellows 
is  now  made  for  the  use  of  dentists,  which  is  available  at  any  table  by 
FIG.  XL  *he  help  °f  a  caoutchouc  tube  to  conduct 

the  blast  to  the  jet.  These  bellows  are  too 
small  to  give  a  steady  flame  of  large  size, 
but  will,  nevertheless,  answer  for  most  01 
the  glass-blowing  necessary  in  the  execution 
of  the  experiments  described  in  this  manual. 
Where  an  abundant  supply  of  water  is 
at  command,  the  following  blowing  appa- 
ratus is  very  convenient.  A  tin  pipe  a  b 
(Fig.  XIII.)  about  one  metre  long  and 
about  13  m.  m.  in  diameter,  has  two  smaller 
pipes,  12  to  16  c.  m.  long,  soldered  into 
it;  these  small  pipes  are  8  m.  m.  in  di- 
ameter; one,  c  d.is  inserted  at  right  angles  12  c.  m.  from  the  end, 


BLOWERS. 


IX 


the  other,  ef,  2.5  c.  m.  lower,  at  an  angle  of  45°.    The  lower  end  of  the 
tube  passes  through  the  cork  of  a  wide-mouthed  five-litre  bottle,  extend- 

FIG.  XIII. 
FIG.  XII.  ~  i  « 

d\ 


ing  rather  more  than  half  way  down.  Two  glass  tubes  also  pass 
through  the  cork  of  the  bottle,  —  a  short  small  tube  #,  No.  4,  which 
should  reach  some  16  c.  m.  above  the  cork,  but  should  not  project  into 
the  bottle,  and  a  larger  tube  h,  No.  2,  extending  to  the  bottom  of  the 
bottle.  The  outer  end  of  the  tube  h  bends  over  and  is  connected  by 
caoutchouc  tubing  with  a  straight  tube  of  equal  diameter.  This  last 
arrangement  forms  the  siphon.  To  the  tube  g  a  caoutchouc  tube,  g  i, 
is  attached  to  convey  the  blast  to  any  desired  point.  To  produce  a- 
blast,  the  water-cock  is  connected  with  the  tube  c  d  by  means  of  a 
caoutchouc  tube.  When  the  water  is  turned  on,  the  caoutchouc  tube 
g  i  is  closed  for  a  moment  with  the  thumb  and  finger.  This  starts  the 
water  through  the  siphon,  and  imme'diately  a  continuous  and  powerful 
blast  of  air  rushes  out  through  the  tube  g  i,  and  may  be  carried  directly 
to  the  blowpipe.  The  siphon  must  be  capable  of  carrying  off  a  larger 
stream  of  water  than  that  which  is  allowed  to  enter,  so  that  there  shall 
never  be  more  than  3  or  4  c.  m.  of  water  in  the  bottle.  By  regulating 
the  water-cock,  the  proper  supply  of  water  may  be  determined. 


X  CAOUTCHOUC    TUBING. 

The  same  apparatus  may  be  used  as  an  aspirator.  When  the  instru- 
ment is  to  be  used  to  draw  air  through  any  apparatus,  the  tube  g  i  is 
closed  by  inserting  a  glass  rod  ;  the  upper  end  of  the  tube  a  &  is  closed 
with  a  cork,  and  the  tube  ef  is  connected  with  the  apparatus  through 
which  the  current  of  air  is  to  be  drawn.  The  force  of  the  current 
of  air  is  to  a  certain  degree  affected  by  the  size  of  the  tube  a  b ;  to 
diminish  the  effective  calibre  of  this  tube,  in  case  a  gentle  current  of 
air  is  required,  a  glass  rod  as  long  as  the  tube  may  be  passed  down 
the  tube  through  a  cork  inserted  at  a.  The  same  apparatus  may  thus 
be  made  to  produce  a  gentle  or  a  powerful  current  of  air. 

7.   Caoutchouc.     Vulcanized  caoutchouc  is  a  most  useful  substance 
in  the  laboratory,  on  account  of  its  elasticity  and  because  it  resists  so 
well  most  of  the  corrosive  substances  with  which  the  chemist  deals.     It 
is  used  in  three  forms:  (1),  in  tubing  of  various  diameters  comparable 
with  the  sizes  of  glass  tubing;  (2),  in  stoppers  of  various  sizes  to  re- 
place  corks;    (3),  in   sheets.      Caoutchouc    tubing  may  be    used   to 
conduct  all  gases  and  liquids  which  do  not  corrode  its  substance,  pro- 
vided that   the  pressure  under  which  the  gas  or  liquid  flows  be  not 
greater  or  their  temperature  higher,  than  the  texture  of  the  tubing  can 
endure.     The  flexibility  of  the  tubing  is  a  very  obvious  advantage  in  a 
great  variety  of  cases.     Short  pieces  of  such  tubing,  a  few  centimetres 
in  length,  are  much  used,  under  the  name  of  connectors,  to  make  flex- 
ible joints  in  apparatus,  of  which  glass  tubing  forms  part ;  flexible  joints 
add  greatly  to  the  durability  of  such  apparatus,  because  long  glass 
tubes  bent  at  several  angles  and  connected  with  heavy  objects,  like 
globes,  bottles  or  flasks  full  of  liquid,  are  almost  certain  to  break  even 
with  the  most  careful  usage;  gas  delivery-tubes,  and  all  considerable 
lengths  of  glass  tubing  should  invariably  be  divided  at  one  or  more 
places,  and  the  pieces  joined  again  with  caoutchouc  connectors.     The 
ends  of  glass  tubing  to  be  thus  connected  should  be  squarely  cut,  and 
then  rounded  in  the  lamp,  in  order  that  no  sharp  edges  may  cut  the 
caoutchouc  ;  the  internal  diameter  of  the  caoutchouc  tube  must  be  a  little 
smaller  than  the  external  diameter  of  the  glass  tubes ;  the  slipping  on 
of  the  connector  is  facilitated  by  wetting  the  glass.     In  some  cases  of 
delicate  quantitative  manipulations,  in  which  the  tightest  possible  joints 
must  be  secured,  the  caoutchouc  connector  is  bound  on  to  the  glass 
tube  with  a  silk  or  smooth  linen  string  ;  the  string  is  passed  as  tightly 
as  possible  twice  round  the  connector  and  tied  with  a  square  knot ; 
the  string  should  be  moistened  in  order  to  prevent  it  from  slipping  while 
the  knot  is  tying. 

Caoutchouc  stoppers  of  good  quality  are  much  more  durable  than 
corks,  and  are  in  every  respect  to  be  preferred.     The  German  stoppers 


CORKS.  xi 

are  of  excellent  shape  and  quality ;  the  American,  being  chiefly  in- 
tended for  wine  bottles,  are  apt  to  be  too  conical.  Caoutchouc  stoppers 
can  be  bored,  like  corks  (see  the  next  section),  by  means  of  suitable 
cutters,  and  glass  tubes  can  be  fitted  into  the  holes  thus  made  with  a 
tightness  unattainable  with  corks.  German  stoppers  may  be  bought 
already  provided  with  one,  two,  and  three  holes.  It  is  not  well  to  lay 
in  a  large  stock  of  caoutchouc  stoppers,  for  though  they  last  a  long 
time  when  in  constant  use,  they  not  infrequently  deteriorate  when 
kept  in  store,  becoming  hard  and  somewhat  brittle  with  age.  These 
stoppers  must  not  be  confounded  with  the  very  inferior  caps  which 
were  in  use  a  few  years  ago. 

Pieces  of  thin  sheet  caoutchouc  are  very  conveniently  used  for 
making  tight  joints  between  large  tubes  of  two  different  sizes,  or 
between  the  neck  of  a  flask,  or  bottle,  and  a  large  tube  which  enters  it, 
or  between  the  neck  of  a  retort  and  the  receiver  into  which  it  enters. 
A  sufficiently  broad  and  long  piece  of  sheet  caoutchouc  is  considerably 
stretched,  wrapped  tightly  over  the  glass  parts  adjoining  the  aperture 
to  be  closed,  and  secured  in  place  by  a  string  wound  closely  about  it 
and  tied  with  a  square  knot. 

8.  Corks.  It  is  often  very  difficult  to  obtain  sound,  elastic  corks  of 
fine  grain  and  of  size  suitable  for  large  flasks  and  wide-mouthed  bottles. 
On  this  account,  bottles  with  mouths  not  too  large  to  be  closed  with  a 
cork  cut  across  the  grain  should  be  chosen  for  chemical  uses,  in  prefer- 
ence to  bottles  which  require  large  corks  or  bungs  cut  with  the  grain, 
and  therefore  offering  continuous  channels  for  the  passage  of  gases,  or 
even  liquids.  The  kinds  sold  as  champagne  corks  and  as  satin  corks 
for  phials  are  suitable  for  chemical  use.  The  best  corks  generally  need 
to  be  softened  before  using ;  this  softening  may  be  effected  by  rolling  the 
cork  under  a  board  upon  the  table,  or  under  the  foot  upon  the  clean 
floor,  or  by  gently  squeezing  it  on  all  sides  with  the  well-known  tool 
expressly  adapted  for  this  purpose,  and  thence  called  a  cork-squeezer. 
Steaming  also  softens  the  hardest  corks. 

Corks  must  often  be  cut  with  cleanness  and  precision  ;  a  sharp,  thin . 
knife,  such  as  shoemakers  use,  is  desirable  for  this  purpose.  When  a 
cork  has  been  pared  down  to  reduce  its  diameter,  a  flat  file  may  be 
employed  in  finishing ;  the  file  must  be  fine  enough  to  leave  a  smooth 
surface  upon  the  cork  ;  in  filing  a  cork,  a  cylindrical,  not  a  conical 
form  should  be  aimed  at. 

In  boring  holes  through  corks  to  receive  glass  tubes,  a  hollow  cylinder 

of  sheet  brass  sharpened  at  one  end  is  a  very  convenient  tool.     Figure 

XIV.  represents  a    set   of   such   little  cylinders  of  graduated    sizes, 

'  slipping  one  within  the  other  into  a  very  compact-form ;  a  stout  wire,  of 


Xll  PUTTING    TUBES    THROUGH    CORKS. 

the  same  length  as  the  cylinders,  accompanies  the  set,  and  serves  a  double 
purpose,  —  passed  transversely  through  two  holes  in  the  cap  which  ter- 
Fia.  xiv.  minates  each  cylinder,  it  gives  the  hand  a  better 

grasp  of  the  tool  while  penetrating  the  cork  ; 
and  when  the  hole  is  made,  the  wire  thrust 
through  an  opening  in  the  top  of  the  cap  expels 
the  little  cylinder  of  cork  which  else  would  re- 
main in  the  cutting  cylinder  of  brass.  That  cujtter, 
whose  diameter  is  next  below  that  of  the  glass 
tube  to  be  inserted  in  the  cork,  is  always  to  be 
selected,  and  if  the  hole  it  makes  is  too  small, 
a  round  file  must  be  used  to  enlarge  the  aper- 
ture ;  the  round  file,  also,  often  comes  in  play  to 
smooth  the  rough  sides  of  a  hole  made  by  a  dull 
cork-borer.  A  pair  of  small  calipers  is  a  very 
convenient,  though  by  no  means  essential  tool 
in  determining  which  size  of  cutter  to  employ. 
A  flask  which  presents  sharp  or  rough  edges  at  the  mouth  can 
seldom  be  tightly  corked,  for  the  cork  cannot  be  introduced  into  the 
neck  without  being  cut  or  roughened;  such  sharp  edges  must  be 
rounded  in  the  lamp.  In  thrusting  glass  tubes  through  bored  corks,  the 
following  directions  are  to  be  observed  :  (1.)  The  end  of  the  tube  must 
not  present  a  sharp  edge  capable  of  cutting  the  cork.  (2.)  The  tube 
should  be  grasped  very  close  to  the  cork,  in  order  to  escape  cutting  the 
hand  which  holds  the  cork,  should  the  tube  break ;  by  observing  this 
precaution  the  chief  cause  of  breakage,  viz.,  irregular  lateral  pressure, 
will  be  at  the  same  time  avoided.  (3.)  A  funnel-tube  must  never  be  held 
by  the  funnel  in  driving  it  through  a  cork,  nor  a  bent  tube  grasped  at  the 
bend,  unless  the  bend  comes  immediately  above  the  cork.  (4.)  If  the 
tube  goes  very  hard  through  the  cork,  the  application  of  a  little  soap 
and  water  will  facilitate  its  passage,  but  if  soap  is  used  the  tube  can 
seldom  be  withdrawn  from  the  cork  after  the  latter  has  become  dry. 
(5.)  The  tube  must  not  be  pushed  straight  into  the  cork,  but  screwed 
in,  as  it  were,  with  a  slow  rotary  as  well  as  onward  motion.  Joints 
made  with  corks  should  always  be  tested  before  the  apparatus  is  used 
by  blowing  into  the  apparatus  and  at  the  same  time  stopping  up  all 
legitimate  outlets.  Any  leakage  is  revealed  by  the  disappearance  of  the 
pressure  created.  To  the  same  end,  air  may  be  sucked  out  of  an  appa- 
ratus and  its  tightness  proved  by  the  permanence  of  the  partial  vacuum. 
To  attempt  to  use  a  leaky  cork  is  generally  to  waste  time  and  labor  and 
to  insure  the  failure  of  the  experiment.  When,  however,  a  leak  is  only 
discovered  during  the  actual  progress  of  the  experiment,  it  is  sometimes 


SUPPORTS    FOR    VESSELS.  xiii 

possible  to  save  the  experiment  by  using  a  lute ;  for  this  purpose  wax 
applied  with  a  warm  knife,  or  a  paste  made  of  rye-meal  and  water  may 
be  used ;  common  sealing-wax  also  is  sometimes  a  useful  make-shift. 

9.  Iron-stand,  Sand-bath,  and  Wire-gauze.  To  support  vessels  over 
the  gas-lamp,  an  iron  stand  is  used  consisting  of  a  stout  vertical  rod 
fastened  into  a  heavy,  cast-iron  foot,  and  three  iron  rings  of  graduated 
sizes  secured  to  the  vertical  rod  with  binding  screws ;  all  the  rings  may 
be  slipped  off  the  rod,  or  any  ring  may  be  adjusted  at  any  convenient 
elevation.  The  general  arrangement  is  not  unlike  that  of  the  stand 
which  makes  part  of  the  Berzelius  lamp  (Fig.  VI.),  although  simpler 
and  cheaper.  As  a  general  rule,  it  is  not  best  to  apply  the  direct 
flame  of  the  lamp  to  glass  and  porcelain  vessels;  hence  a  piece  of 
wire-gauze  is  stretched  loosely  over  the  largest  ring,  and  bent  down- 
wards a  little  for  the  reception  of  round-bottomed  vessels ;  on  this 
gauze,  flasks,  retorts,  and  porcelain  dishes  are  usually  supported.  In 
a  few  cases,  in  which  a  very  gradual  and  equable  heat  is  required,  the 
wire-guage  is  replaced  by  a  small,  shallow  pan,  beaten  out  of  sheet- 
iron,  and  filled  with  dry  sand.  This  arrangement  is  called  a  sand-bath. 
With  the  aid  of  annealed  iron  wire,  the  iron-stand  may  be  made  avail- 
able for  supporting  tubes  over  the  lamp.  Crucibles,  or  dishes,  too  small 
for  the  smallest  ring  belonging  to  the  stand,  are  conveniently  supported 
on  an  equilateral  triangle  made  of  three  pieces  of  soft  iron  wire  twisted 
together  at  the  apices ;  this  triangle  is  laid  on  one  of  the  rings  of  the 
stand.  An  iron  tripod,  —  that  is,  a  stout  ring  supported  on  three  legs.  — 
may  often  be  used  instead  of  the  stand  above  described,  but  it  is  not.so 
generally  useful  because  of  the  difficulty  of  adjusting  it  at  various 
heights ;  with  a  sufficiency  of  wooden  blocks  wherewith  to  raise  the 
lamp  or  the  tripod  as  occasion  may  require,  it  may  be  made  available. 

10.  Pneumatic  Trough.  The  pneumatic  trough  is  a  contrivance 
which  enables  us  to  collect  and  confine  gases  in  suitable  vessels,  and 
to  decant  them  from  one  vessel  to  another.  Its  efficiency  depends  on 
the  pressure  of  the  atmosphere,  which  as  we  know  is  capable  of  sup- 
porting a  column  of  water  10.33  metres  long  or  a  column  of  mercury 
76  c.  m.  long  (see  §  7),  provided  that  the  liquid  column  be  so  ar- 
ranged that  the  atmospheric  pressure  shall  be  fully  felt  upon  the 
foot  of  the  column,  but  not  at  all  upon  its  head.  If  a  tube,  closed 
at  one  end  and  open  at  the  other,  and  of  any  length  less  than 
10.33  m.,  be  completely  filled  with  water,  and  then  inverted  so 
that  its  open  end  shall  dip  beneath  some  water  held  in  a  basin  or 
saucer,  the  tube  will  remain  full  of  water  when  the  thumb  or  cork, 
which  closed  the  open  end  while  the  inversion  was  effected,  is  with- 
drawn. What  is  true  of  a  tube  is  equally  true  of  a  bell,  or  other 


XIV  PNEUMATIC    TROUGH. 

vessel  closed  at  one  end,  of  any  diameter  or  shape,  provided  its  height  be 
not  greater  than  10.33  m. ;  and  the  principle  which  applies  to  water  is 
equally  applicable  to  mercury,  except  that  the  height  of  the  mercury 
column,  which  the  average  atmospheric  pressure  can  hold  up,  is  only 
76  c.  m.,  because  mercury  is  13.596  times  heavier  than  water.  If  a  few 
bubbles  of  any  gas  insoluble  in  water  should  be  delivered  beneath  the 
open  end  of  a  tube,  or  bell,  thus  standing  full  of  water  in  apparent  defi- 
ance of  gravitation,  the  gas  would  rise  to  the  top  of  the  tube,  by  virtue  of 
being  lighter  than  the  water,  and  the  exact  volume  of  water  displaced 
by  the  gas,  small  or  large,  would  drop  into  the  basin  or  saucer  beneath. 
If  the  gas  were  thus  delivered  continuously  beneath  the  tube  or  bell, 
we  should  finally  get  the  tube  or  bell  full  of  gas,  without  admixture  of 
air,  and  sealed  at  the  bottom  by  the  water  in  the  basin  or  saucer.  If 
mercury  were  the  liquid,  the  operation  would  be  precisely  the  same, 
except  as  regards  the  height  of  xthe  tube  or  bell.  Even  this  difference 
of  possible  height  is  not  noticeable  in  practice,  because  bells  and  bottles 
more  than  50  c.  m.  high  are  very  seldom  used  with  either  liquid.  On 
account  of  its  costliness,  mercury  is  rarely  used,  unless  the  gas  to  be 
collected,  or  experimented  upon,  be  soluble  in  water.  A  trough  for 
mercury  is  made  as  small  as  possible  for  the  same  reason.  It  is  obvious 
that  the  object  of  a  pneumatic  trough  may  be  accomplished  under  a 
great  variety  of  forms.  Any  bucket  or  tub  with  a  hanging  shelf  in  it 
may  be  made  to  serve.  It  will  be  sufficient  to  describe  two  convenient 
forms  of  the  apparatus. 

The  apparatus  used  throughout  this  manual  as  a  pneumatic  trough, 
FIG.  xv.  under  the  name  of   the  water-pan    (Fig. 

XV.),  consists  of  two  pieces,  1st,' a  stone- 
ware pan,  about  30  c.  m.  in  diameter  on 
the  bottom,  with  sides  sloping  slightly  out- 
wards and  rising  to  the  height  of  about  10 
c.  m. ;  2d,  a  deep  flower-pot  saucer  about 
15  c.  m.  in  diameter  with  one  hole  bored 
through  the  middle  of  the  bottom,  and  a 
second  arched  hole  nipped  out  of  its  rim ; 
this  saucer  is  inverted  in  the  pan.  If  this 
second  piece  be  made  expressly  for  this  purpose,  it  should  be  made 
about  5  c.  m.  high,  and  its  interior  should  be  rounded  to  the  hole  in  the 
centre,  while  the  outside  is  left  flat  like  the  flower-pot  saucer.  To 
use  this  apparatus,  the  pan  is  filled  with  water  to  a  level  about  2  c.  m 
above  the  top  of  the  inverted  saucer;  the  bottle,  cylinder,  or  bell  which 
is  to  receive  the  gas  is  completely  filled  with  water  from  a  pitcher  or 
water-cock,  then  closed  with  the  hand  of  the  operator  or  with  a  flat  piece 


PNEUMATIC    TROUGH. 


XV 


of  glass  or  wood,  inverted  into  the  pan,  and  placed  on  the  saucer  over  the 
hole  in  its  centre ;  the  end  of  the  gas  delivery-tube  is  thrust  through  the 
side  hole  in  the  saucer,  and  the  gas  rising  through  the  centre  hole 
bubbles  up  into  the  bottle  or  cylinder  placed  to  receive  it.     While  one 
bottle  is  filling  with  gas,  another  is  made  ready  to  replace  it,  and  when 
the  first  is  full,  it  is  pushed  off  the  centre  hole  of  the  saucer,  and  the 
second  bottle  is  brought  over  the  hole.     A  bottle  full  of  gas  may  be 
removed  from  the  trough  by  slipping  beneath  the  mouth  of  the  bot- 
tle  a   shallow   plate,  or  dish,  and  then  lifting  plate   and   bottle  out 
of  the  pan   together   in   such   a   manner  that  water  enough  to  seal 
the  mouth  of  the  bottle  shall  remain  in  the  plate.     The  gas  in  one 
bottle  may  be  decanted  upwards  into  another,  by  filling  the  second 
bottle  with  water,  and  then  carefully  inclining  the  bottle  containing  the 
gas  so  as  to  bring  its  mouth  under  the  mouth  of  the  bottle  which  is  full 
of  water,  keeping  the  mouths  of  both  bottles  all  the  time  beneath  the 
surface  of  the  water  in  the  pan.     If  the  gas  which  has  been  collected  is 
heavier  than  air,  a  bottle  of  it  may  be  withdrawn  from  the  water-pan 
and  got  at  for  use,  by  simply  slipping  a  flat  piece  of  glass  or  wood 
beneath  its  mouth  so  as  to  close  it  rather  tightly,  and  then  standing  the 
bottle,  mouth  upward,  upon  the  table.     If  the  cover  be'  then  removed 
from  the  bottle,  the  gas  will  not  flow  out,  though  it  will  slowly  diffuse 
into  the  air.     As  the  water  with  which  the  bottles  or  cylinders  are 
filled  falls  into  the  pan  when  displaced  by  gas,  it  is  possible  that  the 
pan  may  become  inconveniently  full  if  many  large  bottles  are  used ; 
this  difficulty  must  be  remedied  by  dipping  water  out  of  the  pan,  and 
so  restoring  the  true  level. 

Where  considerable  quantities  FlG  Xvi. 

of  gas  are  frequently  to  be  han- 
dled, and  large  vessels  are  there- 
fore necessary,  a  large  apparatus, 
shown  in  figure  XVI.,  is  much 
more  convenient  than  the  small 
pan,  which  suffices  for  all  common 
experiments.  The  form  of  this 
larger  pneumatic  trough  and  the 
mode  of  using  it  will  readily  be 
understood  from  the  figure ;  the  depth  and  width  of  the  tank  or  well 
must  be  determined  by  the  size  of  the  bells  and  cylinders  which  are  to 
be  sunk  in  it,  and  the  length  and  breadth  of  the  shallow  part  or  shelf 
by  the  number  of  bells  or  jars  of  gas  which  are  likely  to  be  in  use  at 
any  one  time.  The  deep  groove  in  the  shelf  permits  a  glass  or 
caoutchouc  tube  to  pass  without  compression  under  a  bell  whose  rim 


XVI  COLLECTING    GASES. 

projects  over  the  groove.  Such  a  trough  is  best  made  of  zinc  or  lead. 
It  is  very  convenient  to  have  it  sunk  in  a  table,  and  permanently 
provided  with  a  water-cock  and  drain-pipe.  A  chief  merit  of  this  instru- 
ment is  that  the  glass  vessels  used  can  be  filled  with  water  by  sinking  them 
in  the  well  much  more  conveniently  than  from  a  pitcher  or  water-cock. 

A  pneumatic  trough  for  mercury  may  be  made  either  of  wood,  iron, 
or  stone.  For  all  common  uses,  it  is  very  well  cut  out  of  a  solid  block  of 
compact,  hard  wood,  which  will  not  split.  Small  cylinders  or  bells  only 
can  be  used,  and  the  well  of  the  trough  should  be  scooped  out  but  a 
little  larger  than  the  bell  or  cylinder  selected;  with  its  principal  dimen- 
sion horizontal,  and  its  bottom  curved  to  fit  the  cylindrical  bell  which 
is  to  be  laid  in  it ;  the  shelf,  too,  should  have  but  a  small  area,  sufficient 
only  for  four  or  five  bells  of  3  or  4  c.  m.  diameter. 

In  using  a  pneumatic  trough,  of  any  construction  or  dimensions,  the 
student  should  be  on  his  guard  against  two  difficulties  of  possible  occur- 
rence, —  against  the  sucking  back  of  the  liquid  in  the  trough  into  the  gas- 
generating  apparatus,  and  against  the  leakage  sometimes  induced  by 
the  pressure  created  by  thrusting  the  gas  delivery-tube  deep  under 
water  or  mercury.  The  first  of  these  difficulties  is  the  most  serious. 
When  the  flow  of  gas  from  a  heated  flask  or  tube  is  suddenly  arrested, 
in  consequence  of  some  reduction  of  temperature,  or  from  any  other 
cause,  it  often  happens  that  the  volume  of  gas  in  the  generating  appa- 
ratus contracts,  and  the  cold  water  or  mercury  from  the  trough  rises  in 
the  delivery-tube  to  fill  the  void ;  if  the  contraction  is  so  considerable 
as  to  suffer  the  cold  liquid  to  penetrate  into  the  hot  flask  or  tube,  an 
explosion  almost  inevitably  ensues  which  fractures  the  apparatus,  if  it 
does  no  worse  damage.  In  collecting  over  water  a  gas  somewhat  solu- 
ble in  that  liquid,  this  danger  is  especially  imminent.  The  occurrence 
of  such  accidents  may  be  effectually  guarded  against  by  paying  atten- 
tion to  the  following  directions:  (1.)  Whenever  it  is  proposed  to  stop 
an  evolution  of  gas  which  has  been  going  on  from  a  hot  flask  or  tube, 
withdraw  the  delivery-tube  from  the  water  before  extinguishing  the 
lamp,  and  shake  off*  from  the  bent  end  of  the  tube  the  drops  of  water 
which  are  apt  to  adhere  to  it ;  the  lamp  may  then  be  safely  put  out,  for 
air  can  enter  the  apparatus  through  the  open  tube.  (2.)  When  the  flow 
of  gas  from  a  hot  apparatus  is  observed  to  slacken,  watch  closely  the 
escape  of  the  gas  from  the  delivery-tube,  and  as  soon  as  any  tendency 
to  reflux  of  water  is  detected,  lift  the  delivery-tube  quickly  out  of  the 
water,  or,  better,  slip  off  the  caoutchouc  connector,  which  should  always 
be  found  between  the  flask  and  the  water-pan  on  every  such  piece  of 
apparatus ;  if  there  be  no  connector,  the  cork  must  be  loosened  in  the 
neck  of  the  flask.  Air  will  thus  be  admitted  to  the  hot  flask  or  tube. 


SAFETY-TDBES. 


XV11 


These  precautions  apply  more  particularly  to  the  cases  where  gas  is 
evolved  from  dry  materials,  as  in  making  oxygen  or  nitrous  oxide  ; 
when  a  liquid  is  contained  in  the  generating  flask,  a  safety-tube  is  a 
sure  protection  against  the  danger  of  sucking  back.  The  atmospheric 
pressure  can  force  air  into  a  flask,  in  which  a  partial  vacuum  has  been 
created,  through  the  safety-tube,  by  lifting  and  displacing  a  column  of 
the  liquid  whose  height  is  the  length  of  that  portion  of  the  safety-tube 
which  dips  beneath  the  liquid.  Unless  the  liquid  in  the  flask  be  extraor- 
dinarily dense,  the  force  required  to  do  this  will  be  very  much  less  than 
that  required  to  lift  a  column  of  water  whose  height  is  determined  by 
the  elevation  of  the  highest  point  of  the  delivery-tube  above  the  level 
of  the  water  in  the  pan. 

When  the  gas  coming  from  the  generating  flask  has  to  force  out  and 
keep  out  of  the  delivery-tube  a  column  of  water  measured  from  the 
lowest  point  of  the  tube  to  the  surface  of  the  water  in  the  pan,  a 
pressure  determined  by  the  height  of  this  column  is  established  upon 
the  interior  of  the  flask  and  upon  every  joint  of  the  apparatus.  Hence  an 
apparatus  will  sometimes  leak,  and  FIG.  xvn. 

refuse  to  deliver  gas  at  the  desired 
point,  when  its  delivery-tube  is  deep- 
ly immersed,  while  it  does  not  leak 
if  the  tube  merely  dip  beneath  the 
surface  of  the  water.  With  mer- 
cury the  pressure  of  a  few  centime- 
tres is  very  considerable  on  account 
of  the  high  specific  gravity  of  the 
fluid,  so  that  this  difficulty  is  more 
likely  to  occur  with  this  metal  than 
with  water.  Tight  joints  prevent 
the  occurrence  of  this  difficulty.  A 
partial  remedy  is  to  dip  the  delivery- 
tube  as  little  as  possible  below  the 
surface  of  the  fluid  in  the  trough. 

11.  Gas-holders.  A  small  gas- 
holder, very  convenient  for  many 
uses,  is  made  from  a  common  glass 
bottle  in  the  following  manner:  A 
(Fig.  XVII.)  is  a  bottle  of  4-6  litres 
capacity ;  through  the  cork  in  its 
neck  pass  two  glass  tubes  (No.  6),  of 
which  one  reaches  the  bottom  of  the  bottle,  while  the  other  merely  pene- 
trates the  cork ;  with  the  outer  end  of  the  first  tube  a  caoutchouc  tube  c 


XV'lll 


GAS-HOLDERS. 


is  connected,  with  the  outer  end  of  the  second  a  common  gas-cock  a. 
The  bottle  being  first  completely  filled  with  water,  the  apparatus  which 
generates,  or  contains,  the  gas  to  be  introduced  into  the  holder  is  con- 
nected with  the  tube  carrying  the  cock  a ;  this  cock  is  open.  As  the 
gas  presses  in,  the  water  mounts  in  the  long  tube,  and  flows  out  by  the 
syphon  c.  In  order  to  relieve  the  gas  from  this  pressure  at  the  begin- 
ning, it  is  only  necessary  to  suck  a  little  at  c.  The  tube  c  should  of 
course  be  thrust  into  a  sink  or  drain-pipe. 

To  get  gas  out  of  the  bottle,  thus  charged,  the  cock  a  is  closed,  and 
the  flexible  tube  c  is  lifted  up  and  connected,  as  shown  in  the  figure, 
with  a  bottle  of  water  B  placed  on  a  shelf,  or  stand,  somewhat  above 
the  bottle  A.  When  the  cock  b  is  opened,  the  gas  in  A  is  pressed  upon 
by  the  weight  of  the  superincumbent  column  of  water,  and  may  there- 
fore be  made  to  issue  at  will  from  the  cock  a.  The  higher  B  is  placed 
above  ^4,  the  greater  will  be  the  force  with  which  the  gas  will  issue. 
If  a  moderate,  or  easily  regulated  water-pressure  is  at  hand,  supplied 
by  city  water-works  or  a  reservoir  in  the  upper  part  of  the  building, 
the  bottle  B  is  unnecessary,  and  the  flexible  tube  c  may  be  connected 
with  such  a  water-supply,  whenever  gas  is  to  be  pressed  out  of  the 
gas-holder,  A. 

When  larger  quantities  of  gas  are  to  be  stored  for  use,  a  metallic 
gas-holder,  whose  construction  and  propor- 
tions are  shown  in  Fig.  XVIIL,  is  advan- 
tageously employed.  The  open  cistern  B 
is  supported  over  the  vessel  A  on  two  col- 
umns c  c,  and  two  tubes  a  and  b ;  of  these 
tubes  the  first,  a,  reaches  from  the  bottom 
of  B  nearly  to  the  bottom  of  A,  while  the 
second,  &,  starts  from  the  bottom  of  B  and 
just  enters  the  arched  top  of  A  without 
projecting  into  it ;  c?  is  a  short,  large  tube, 
sloping  upwards  and  outwards,  and  capable 
of  being  tightly  closed  with  a  cork  or  caout- 
chouc stopper  ;  g  is  a  glass  gauge  to  show 
the  height  of  the  water  in  the  vessel  A  ;  e 
is  the  discharge-pipe.  To  fill  the  gas-holder 
with  water,  close  d,  open  the  stop-cocks  a, 
b,  and  e,  and  pour  water  into  the  cistern  B : 
the  water  entering  A  will  expel  the  air 
through  b  and  e;  when  the  water  begins 
to  flow  through  e,  close  that  stop-cock  and 
expel  the  rest  of  the  air  through  b.  The  gas-holder  may  now  be  filled 


FIG.  XVIII. 


GAS-HOLDERS.  xix 

with  gas  by  displacing  the  water  in  the  following  manner :  —  Close  all 
the  stop-cocks,  withdraw  the  cork  or  stopper  from  d,  and  introduce  the 
tube  which  delivers  the  gas  through  that  opening;  a  short  piece  of 
caoutchouc  tubing  makes  the  best  end  for  the  gas  delivery-tube,  but 
glass  tubing  will  answer  the  purpose  if  the  end  be  slightly  bent  upward  ; 
the  water  flows  out  at  d  as  fast  as  the  gas  enters,  and  the  gas-holder 
should  therefore  stand  in  a  shallow  metal  tray  provided  with  a  drain- 
pipe. When  the  desired  quantity  of  gas  has  been  introduced,  close  d. 
To  draw  gas  out  of  a  gas-holder  of  this  construction,  the  cistern  B  is 
filled  with  water  and  the  cork  a  is  opened ;  under  the  pressure  thus 
established  the  gas  may  be  drawn  off  through  e,  or  allowed  to  rise 
through  b  into  bottles  or  bells  filled  with  water  and  held  over  the 
mouth  of  the  tube  b  in  the  cistern  B ;  in  this  last  case  B  answers  the 
purpose  of  a  pneumatic  trough. 

This  gas-holder  may  be  cheaply  made  of  zinc ;  any  gas-fitter  can 
supply  the  necessary  stop-cocks ;  care  must  be  taken  that  the  glass 
tube  which  constitutes  the  gauge  is  fitted  air-tight  to  the  gas-holder. 
The  stop-cock  e  need  not  end  in  a  screw ;  tubes  may  be  as  well  con- 
nected with  it  by  caoutchouc.  The  available  pressure,  under  which 
the  gas  in  the  holder  streams  out  at  e,  is  of  course  limited  by  the  ele- 
vation of  B  above  A,  which  must  always  be  moderate.  When  a 
stronger  pressure  is  desirable,  as  in  getting  the  oxy-hydrogen  blowpipe 
flame,  for  example,  a  heavier  water-column  may  be  obtained  by  screw- 
ing a  tall  tube  with  a  capacious  funnel  on  top  of  it  into  the  tube  a, 
where  it  opens  into  the  bottom  of  the,  cistern  B.  A  piece  of  common 
iron  or  copper  gas-pipe  about  a  metre  long,  answers  this  purpose  very 
well ;  the  funnel  at  the  top  should  hold  two  or  three  litres,  and  must  be 
kept  full  of  water  from  a  cask  or  tub  provided  with  a  cork  and  placed 
just  above  the  funnel.  Where  a  water-supply,  with  moderate  pressure, 
is  obtainable,  it  may  be  used  to  keep  the  funnel  full,  or  to  replace 
the  funnel  altogether,  if  directly  connected  with  the  tube  a.  A  gas- 
holder, measuring  not  more  than  50  c.  m.  in  total  height,  is  not  too  heavy 
to  be  portable,  and  during  the  process  of  filling  may  be  placed  over  a 
tub  ;  but  a  gas-holder  of  much  larger  proportions  is  better  made  a  fix- 
ture, and  provided  in  a  permanent  manner  with  drain-pipe  and  water- 
supply.  The  gas-holder  thus  described  is  that  which  is  the  most  gener- 
ally useful  ;  it  may  be  charged  from  any  glass  flask,  retort,  or  bottle, 
without  any  pressure  being  exerted  upon  the  glass  vessel ;  and  un- 
used gas  contained  in  any  sort  of  bell,  bottle,  or  flask,  can  be  very 
readily  transferred  to  such  a  gas-holder  without  waste  and  with  very 
little  trouble. 

A  cheaper  gas-holder  may  be  made  on  the  plan  of  the  large  gas- 


XX 


DEFLAGRATING    SPOON. 


holders,  improperly  called  gasometers,  used  in  gae-works.     Fig.  XIX. 
FIG.  XIX.  represents  a  gas-holder  of  this  sort.     Over  a 

tank  of  water,  which  may  be  a  cylinder  of 
zinc  as  shown  in  the  figure,  or  a  headless  pork- 
or  oil-barrel,  or  any  other  water-tight  tub,  is 
balanced  by  pulleys  and  weights  a  tight  bell 
of  zinc,  not  too  large  for  complete  immersion 
in  the  tank.  The  U-tube,  shown  in  the  figure, 
which  may  be  either  of  lead  or  brass,  serves 
both  to  introduce  and  deliver  the  gas.  To  fill 
such  a  gasometer,  open  the  cock,  lift  the  coun- 
terbalancing weight,  and  let  the  bell  sink  into 
the  water ;  then  connect  the  vessel  from  which 
the  gas  is  delivered  with  the  tube  of  the 
holder,  counterpoise  the  bell,  and  the  gas 
coming  from  the  generator  will  gradually  lift 
the  bell  from  out  the  water.  To  force  the  gas 
out  of  the  holder  it  is  only  necessary  to  remove 
the  counterbalancing  weight ;  the  weight  of 
the  bell  forces  out  the  gas,  and  if  this  pressure 
be  not  sufficient,  additional  weights  may  be  placed  on  the  top  of  the 
bell.  Gas-holders  of  this  construction,  unless  very  small,  are  too  heavy, 
when  filled  with  water,  to  be  carried  about ;  but  this  difficulty  may  be 
obviated,  when  economy  is  not  specially  to  be  regarded,  by  placing 
within  the  lower  cylinder,  or  tank,  a  second  air-tight  cylinder  as  a  core, 
so  as  to  leave  only  a  narrow  space  between  the  inner  and  outer  cylin- 
ders for  the  water  into  which  the  upper  bell  dips.  Elegant,  but  not 
cheap,  gas-holders  are  thus  made,  which  are  convenient  for  some  uses, 
but  are  not  so  generally  to  be  recommended  as  those  of  the  construc- 
tion first  described.  The  vessel  from  which  a  gas-holder  with  counter- 
poised bell  is  charged,  is  always  subjected  to  some  pressure,  slight  if 
the  pulleys,  cords,  and  weights  are  in  perfect  order,  but  more  fre- 
quently considerable  on  account  of  the  difficulty  of  maintaining  such 
an  apparatus  in  perfect  condition. 

12.  Deflagrating  Spoon.  The  little  cup  which  holds  combustible 
material,  to  be  burnt  in  a  bottle  or  jar  of  gas,  is  called  a  deflagrating 
spoon  ;  it  may  be  cheaply  made  by  hollowing  a  hemispherical  cup  out 
of  a  cube  of  chalk  about  2  c.  m.  on  a  side,  and  attaching  a  stout  iron 
or  brass  wire  to  the  chalk,  in  such  a  manner  that  the  cup  will  be  right 
side  up  when  hung  by  the  wire  in  a  jar  of  gas ;  the  upper  end  of  this 
wire  should  be  straight,  that  it  may  be  thrust  through  the  cork  or  piece 
of  wood  which  covers  the  mouth  of  the  bottle  or  jar.  A  small  cupel  is 


P  .ATINUM. FILTERING.  XXI 

a  convenient  ready-made   substitute  for  the  chalk  cup.     Brass  defla- 
grating spoons  are  also  to  be  had  of  philosophical  instrument-makers. 

13.  Platinum  Foil  and  Wire.     A  piece  of  platinum  foil  about  3  c.  m. 
square  will  suffice  for  all  the  experiments  in  this  manual  in  which  such 
foil  is  used,  and  for  all  the  applications  of  platinum-foil  in  qualitative 
analysis  as  well.     The  foil  should  be  at  least  so  thick  that  it  does  not 
crinkle  when  wiped;  and  it  is' more  economical  to  get  foil  which  is  too 
thick   than   too   thin,   for   it    requires   frequent    cleaning.     A   bit   of 
platinum-wire,  not  thicker  than  a  No.  10  needle,  and  20  c.  m.  long,  will 
last  a  long  time  with  careful  usage.    No  other  metal,  and  no  mixture  of 
substances  from  which  a  metal  can  be  reduced,  must  ever  be  heated  on 
platinum-foil  or  wire,  for  platinum  forms  alloys  with  other  metals  which 
are  much  more  fusible  than  itself.     If  once  alloyed  with" a  baser  metal, 
the  platinum  ceases  to  be  applicable  to  its  peculiar  uses.     Platinum 
may  be  cleaned  by  boiling  it  in  either  nitric  or  chlorhydric  acid,  by 
fusing  the  acid  sulphate  of  sodium  or  potassium  upon  it,  or  by  scouring 
it  with  fine  sand.      Aqua-regia  (§  104)   and  chlorine- water    dissolve 
platinum  ;  the  sulphides,  cyanides,  and  oxides  of  sodium  and  potassium, 
when  fused  in  platinum  vessels,  slowly  attack  the  metal. 

14.  Filtering.     Filtration  is  resorted  to  in  order  to  separate  a  finely 
divided  solid  from  a  liquid.     The  filter  may  be  made  of  paper,  cloth, 
tow,  cotton,  asbestos,  and  other  substances.     Paper  is  the  substance 
oftenest  used.     A  good  filtering-paper  must  be  porous  enough  to  filter 
rapidly,  and    yet    sufficiently   close    in    texture   to   retain    the   finest 
powders ;  and  it  must  also  be  strong  enough  to  bear,  when  wet,  the 
pressure  of  the  liquid  which  must  be  poured  upon  it.     For  delicate 
experiments  it  is  also  necessary  that  filtering-paper  should  contain  no 
soluble  salts,  and  but  a  very  small  proportion  of  incombustible  mate- 
rial, which  would  remain  as  ash  were  the  filter  to  be  burned.     Filter- 
ing-paper, which  is  generally  sold  in  sheets,  is  first  cut  into  circles  of 
various   diameters,    adapted    to   the   various   scales 

of  operation  and  quantities  of  liquids  to  be  filtered. 
To  prepare  a  filter  for  use,  one  of  these  circles  is 
folded  over  on  its  own  diameter,  and  the  semi- 
circle thus  obtained  is  folded  once  upon  itself 
into  the  form  of  a  '  quadrant ;  the  paper  thus 
folded  is  opened  so  that  three  thicknesses  shall 
come  upon  one  side,  and  one  thickness  upon  the 
other,  as  shown  in  Fig.  XX. ;  the  filter  is  then 
placed  in  a  glass  funnel,  the  angle  of  which  should 
be  precisely  that  of  the  opened  paper,  viz ,  60°  ; 
after  being  wetted,  it  is  ready  to  receive  the  liquid  to  be  filtered. 


XXII  DRYING    GASKS. 

The  paper  may  be  so  folded  as  to  fit  a  funnel  whose  angle  is  more  or 
less  than  60°,  but  this  is  the  most  advantageous  angle,  and  glass 
funnels  should  be  selected  with  reference  to  their  correctness  in  this 
respect. 

Coarse  and  rapid  filtering  can  be  effected  with  cloth  bags ;  also  by 
plugging  the  neck  of  a  funnel  loosely  with  tow  or  cotton.  If  a  very 
acid  or  very  caustic  liquid,  which  would  Destroy  paper,  cotton,  tow,  or 
wool,  is  to  be  filtered,  the  best  substances  wherewith  to  plug  the  neck 
of  the  funnel  are  asbestos  and  gun-cotton,  neither  of  which  is  attacked 
by  such  corrosive  liquids. 

The  glass  funnel  which  holds  the  filter  generally  requires  an  inde- 
pendent support,  for  it  is  seldom  judicious,  or  possible,  to  support  the 
funnel  directly  upon  the  vessel  which  receives  the  filtrate,  as  the  clear 
FIG.  xxi.       liquid  which  runs  through  the  filter  is  called.     The  iron- 
stand  may  be  used  for  this  purpose  ;  and  wooden  stands, 
of  a  similar  construction,  adapted  expressly  for  holding 
funnels,  are    very  convenient  and  not  expensive.     In 
general,  care  should  be  taken  that  the  lower  end  of  the 
funnel  touch  the  side  or  edge  of  the  vessel  into  which 
*the  filtrate  descends,  in  order  that  the  liquid  may  not 
fall  in  drops,  but  run  quietly  down  without  splashing. 
Sometimes  there  is  no  objection  to  thrusting  a  funnel 
directly  into  the  neck  of  a  bottle  or  flask,  but  in  this 
case  an  ample  exit  for  the  air  in  the  bottle  must  be 
provided  (Fig.  XXL). 

15.  Drying  gases.  It  is  often  desirable  to  remove  the  aqueous  vapor 
which  is  mixed  with  gases  collected  over  water,  or  prepared  from 
materials  containing  water.  It  very  seldom  happens  that  a  gas  can  be 
prepared  at  one  operation  in  so  dry  a  state  as  to  contain  no  vapor  of 
water  ;  this  vapor  must  ordinarily  be  removed  by  a  -subsequent  or 
additional  process.  Experience  has  shown  that  some  gases  are  more 
easily  dried  than  others ;  thus  air,  hydrogen,  and  common  oxygen  are 
thoroughly  dried  with  great  ease,  but  gases  which  contain  antozone 
only  with  great  difficulty  (compare  §  179);  chlorine  is  three  times  as 
hard  to  dry  as  carbonic  acid.  These  and  similar  facts  must  be  borne 
in  mind  in  constructing  drying  apparatus.  The  common  drying  process 
depends  upon  bringing  the  moist  gas  into  contact  with  some  liquid  or 
solid  which  greedily  and  rapidly  absorbs  aqueous  vapor.  The  three 
substances  most  used  for  this  purpose  are  concentrated  sulphuric  acid, 
chloride  of  calcium,  and  dry  quick-lime.  Sulphuric  acid  may  be  used 
in  two  ways :  the  gas  may  be  made  to  bubble  through  a  few  centi- 
metres' depth  of  the  liquid  acid,  or  it  may  be  forced  to  pass  through 


DRYING-TUBES. 


XXU1 


the  interstices  of  a  column  of  broken  pumice-stone  which  has  been 
previously  soaked  in  the  acid.  The  latter  method  is  the  most  effectual, 
because  it  secures  a  more  thorough  contact  of  the  gas  with  the  hygro- 
scopic acid  than  is  possible  during  the  rapid  bubbling  of  the  light  °gas 
through  a  shallow  layer  of  the  dense 
liquid.  The  column  of  fragments 
of  pumice-stone  may  be  held  in  a 
U-tube,  arranged  like  that  shown 
in  Fig.  XXII.;  but  the  vertical 
cylinder  shown  in  the  same  figure 
is  better  adapted  for  this  use,  be- 
cause the  acid,  as  it  become  dilute 
from  absorption  of  moisture,  grad- 
ually trickles  from  the  pumice-stone, 
and  is  apt  to  collect  in  such  quan- 
tity at  the  bottom  of  the  U-tube  as 
to  completely  close  the  tube.  In 
preparing  the  upright  cylinder  for  use,  the  portion  below  the  contrac- 
tion is  not  filled  with  pumice-stone  ;  it  receives  the  drippings  from  the 
pumice-stone  column.  The  gas  to  be  dried  enters  by  the  lower  lateral 
opening,  ancj,  goes  out  at  the  top  of  the  cylinder.  Though  especially 
adapted  to  the  column  of  acid-soaked  pumice-stone,  this  cylinder  may 
very  well  be  used  with  either  of  the  other  drying  agents,  chloride  of 
calcium  or  quick-lime.  Either  of  the  forms  of  drying-tube  represented 
in  Fig.  XXII.  may  be  employed  with  these  latter  substances  ;  in  charg- 
ing the  horizontal  tubes,  bits  of  loose  cotton-wool  should  first  be  placed 
against  the  exit-tube  to  prevent  any  particles  of  the  chloride  of  calcium, 
or  quick-lime,  from  entering  that  tube ;  pieces  of  the  perfectly  dry 
solid  are  then  introduced  in  such  a  way  that  the  tube  may  be  com- 
pactly filled  with  fragments  which  leave  room  for  the  gas  to  pass  very 
deviously  between  them,  but  offer  no  direct  channels  through  which 
the  gas  could  find  straight  and  quick  passage.  Quick-lime  must  be 
charged  much  more  loosely  than  chloride  of  calcium,  because  of  its 
great  expansion  when  moistened.  Fused  chloride  of  calcium  is  not  so 
well  adapted  for  drying  gases  as  the  unfused  substance.  It  is  not  at  all 
necessary  that  the  fragments  of  chloride  of  calcium,  or  quick-lime, 
should  be  of  uniform  size.  When  the  tube  is  nearly  full,  a  plug  of 
loose  cotton  should  be  inserted  before  putting  in  the  cork.  A  chloride 
of  calcium  tube,  once  filled,  will  often  serve  for  many  experiments ; 
whenever  out  of  use,  its  outlets  should  be  covered  with  paper  caps ;  or, 
better,  caoutchouc  connectors  may  be  slipped  upon  the  exit-tubes,  and 
bits  of  glass  rod  thrust  into  these  connectors.  The  moisture  of  the  air 


XXIV 


CHLORIDE    OF    CALCIUM    U-TUBE. 


is  thus  kept  from  the  chloride  of  calcium.  The  dimensions  of  drying- 
tubes  are  of  course  very  various ;  the  bulb-tube  shown  in  Fig.  XXII. 
is  seldom  used  with  a  greater  length  than  25  c.  m. ;  when  this  form 
of  tube  is  employed  the  gas  should  invariably  enter  by  the  end  without 
a  cork,  where  the  small  size  of  the  tube  permits  direct  connection 
with  a  common  gas  delivery-tube  by  means  of  a  caoutchouc  connector ; 
the  other  horizontal  tube,  shown  in  the  figure,  may  be  of  any  length, 
FIG.  xxin.  kut  whenever  a  great  extent  of  drying  surface  is  neces- 
sary, U-tubes  have  the  advantage  of  compactness,  for 
many  can  be  hung  upon  one  short  frame  ;  the  upright 
cylinder  may  be  from  25  c.  m.  to  40  c.  m.  in  height. 
A  good  U-tube,  with  an  addition  which  has  the  merit 
of  economizing  chloride  of  calcium,  is  shown  in  Fig. 
XXIII. ;  the  addition  consists  of  a  short  test-tube,  into 
which  the  tube  by  which  the  gas  comes  in  barely 
enters ;  a  quantity  of  water  is  often  caught  in  this 
test-tube  which  otherwise  would  wet  and  spoil  a  con- 
siderable amount  of  chloride  of  calcium ;  the  little  test- 
tube  may,  of  course,  be  taken  out  and  emptied  at  will. 

The  choice  between  one  or  other  of  the  three  drying  substances  is 
determined  in  each  special  case  by  the  chemical  relations  pf  the  gas  to 
be  dried ;  thus  ammonia-gas,  which  is  absorbed  by  sulphuric  acid  and 
by  chloride  of  calcium,  must  be  dried  by  passing  it  over  quick-lime, 
while  sulphurous  acid  gas,  which  would  combine  with  quick-lime,  must 
be  dried  by  contact  with  sulphuric  acid. 


FIG.  xxv. 


FIG.  XXIV. 


16.  Spring-clip  and  Screw-compressor.  These  are  very  convenient 
substitutes  for  the  ordinary  stop-cock,  and  as  such  are  in  constant  use 
in  the  laboratory.  Their  form,  and  the  manner  of  their  use,  will  be 
readily  understood  from  the  figures.  As  glass  stop-cocks  are  expensive 
and  fragile,  and  metal  stop-cocks  are  usually  out  of  the  question,  be- 
cause so  many  gases  and  liquids  attack  the  common  metals,  these  excel- 
lent substitutes  are  used  whenever  a  caoutchouc  tube  is  not  inadmis- 
sible ;  they  cannot  be  used  unless  a  bit  of  elastic  tubing  can  be  inserted 
into  the  apparatus  which  requires  a  cock. 


WATER-BATH. 


XXV 


Another  effective  mode  of  temporarily  stopping,  or  partially  closing, 
a  caoutchouc  tube,  is  to  slip  over  the  tube  a  common  brass  ring  of  about 
the  same  diameter  as  that  of  the  tube,  and  then  to  thrust  a  slightly  conical 
plug  of  hard  wood  or  ivory  between  the  ring  and  the  flexible  tube. 

17.  Water-bath.     It  is  often  necessary  to  evaporate  solutions  at  a 
moderate  temperature  which  can  permanently  be  kept  below  a  certain 
known  limit ;  thus,  when  an  aqueous  solution  is  to  be  quietly  evapor- 
ated without  spirting  or  jumping,  the  temperature  of  the  solution  must 
never  be  suffered  to  rise  above  the  boiling-point  of 

water,  nor  even  quite  to  reach  this  point.  This 
quiet  evaporation  is  best  effected  by  the  use  of  a 
water-bath,  —  a  copper  cup  whose  top  is  made  of 
concentric  rings  of  different  diameters  to  adapt  it 
to  dishes  of  various  size  (Fig.  XXVL).  This  cup, 
two-thirds  full  of  water,  is  supported  on  the  iron- 
stand  over  the  lamp,  and  the  dish  containing  the 
solution  to  be  evaporated  is  placed  on  that  one  of 
the  several  rings  which  will  permit  the  greater  part  of  the  dish  to  sink 
into  the  copper  cup.  The  steam  rising  from  the  water  impinges  upon 
the  bottom  of  the  dish,  and  brings  the  liquid  within  it  to  a  temperature 
which  insures  the  evaporation  of  the  water,  but  will  not  cause  any 
actual  ebullition.  The  water  in  the  copper  cup  must  never  be  allowed 
to  boil  away.  Wherever  a  constant  supply  of  steam  is  at  hand,  as  in 
buildings  warmed  by  steam,  the  copper  cup  above  described  may  be 
converted  into  a  steam-bath  by  attaching  it  to  a  steam-pipe  by  means 
of  a  small  tube  provided  with  a  stop-cock. 

The  same  copper  vessel  may  be  conveniently  employed,  when  it  is 
required  to  expose  substances  to  a  constant  temperature  higher  than 
100°.  For  this  purpose  the  cup  is  filled  with  oil,  wax,  paraffine,  or  a 
solution  of  chloride  of  zinc  or  chloride 
of  calcium ;  the  flask  or  dish  contain- 
ing the  substance  to  be  heated  should, 
in  this  case,  be  immersed  in  the  fluid  to 
about  two-thirds  of  its  depth ;  a  ther- 
mometer must  be  used  to  indicate  the 
temperature  of  such  a  bath.  When 
oil,  wax,  or  paraffine  is  used,  the  tem- 
perature must  not  be  carried  so  high 
as  to  burn  or  decompose  these  organic 
matters,  else  a  very  disgusting  vapor  will  be  produced. 

18.  Iron  Retort.     A  retort,  made  of  iron,  of  the  form  showed  in 
Fig.  XXVII.,  is  a  very  convenient  tool  in  making  large  quantities  of 


FIG.  XXVII. 


XXVI 


SELF-REGULATING    GAS-GENERATOR. 


FIG.  XXVIII. 


oxygen,  and  in  preparing  illuminating  gas  or  marsh  gas.  The  iron 
top  is  fitted  to  the  retort  with  a  ground  joint,  fastened  by  a  screw- 
clamp.  When  the  top  is  removed,  the  whole  inner  surface  of  the  retort 
is  exposed,  —  a  decided  advantage  wherever  the  residue  left  in  the 
retort  after  use  is  solid.  A  retort  of  about  300  c.  c.  capacity  is  amply 
large  for  all  the  applications  of  such  a  retort  suggested  in  this  manual. 
19.  Self-regulating  Gas-generator.  An  apparatus  which  is  always 
ready  to  deliver  a  constant  stream  of  hydrogen,  and  yet  does  not 
generate  the  gas  except  when  it  is  immediately  wanted  for  use,  is  a 
great  convenience  in  an  active  laboratory  or  on  a  lecture-table.  The 
same  remark  applies  to  the  two  gases,  sulphydric  acid  and  carbonic 
acid,  which  are  likewise  used  in  considerable  quantities,  and  which  can 
be  conveniently  generated  in  precisely  the 
same  form  of  apparatus  which  is  advanta- 
geous for  hydrogen.  Such  a  generator 
may  be  made  of  divers  dimensions.  The 
following  directions,  with  the  accompany- 
ing figure  (Fig.  XXVIII.),  will  enable  the 
student  to  construct  an  apparatus  of  con- 
venient size  Procure  a  glass  cylinder  20 
or  25  c.  m.  in  diameter  and  30  or  35  c.  m. 
high ;  ribbed  candy  jars  are  sometimes  to 
be  had  of  about  this  size  ;  procure  also  a 
stout  tubulated  bell-glass  10  or  12  c.  m. 
wide  and  5  or  7  c.  m.  shorter  than  the  cyl- 
inder. Get  a  basket  of  sheet-lead  7.5  c.  m. 
deep  and  2.5  c.  nu  narrower  than  the  bell- 
glass,  and  bore  a  number  of  small  holes  in 
the  sides  and  bottom  of  this  basket.  Cast 

a  circular  plate  of  lead  7  m.  m.  thick  and  of  a  diameter  4  c.  m.  larger 
than  that  of  the  glass  cylinder ;  on  what  is  intended  for  its  under  side 
solder  three  equidistant  leaden  strips,  or  a  continuous  ring  of  lead,  to 
keep  the  plate  in  proper  position  as  a  cover  for  the  cylinder.  Fit 
tightly  to  each  end  of  a  good  brass  gas-cock  a  piece  of  brass  tube  8 
c.  m.  long,  1.5  to  2  c.  m.  wide,  and  stout  in  metal.  Perforate  the  centre 
of  the  leaden  plate,  so  that  one  of  these  tubes  will  snugly  pass  through 
the  orifice,  and  secure  it  by  solder,  leaving  5  c.  m.  of  the  tube  project- 
ing below  the  plate.  Attach  to  the  lower  end  of  this  tube  a  stout  hook 
on  which  to  hang  the  leaden  basket.  By  means  of  a  sound  cork  and 
common  sealing-wax,  or  a  cement  made  of  oil  mixed  with  red  and 
white  lead,  fasten  this  tube  into  the  tubulature  of  the  bell-glass  air- 
tight, and  so  firmly  that  the  joint  will  bear  a  weight  of  several  pounds. 


GLASS-WABE. 

Hang  the  basket  by  means  of  copper  wire  within  the  bell  5  c.  m. 
above  the  bottom  of  the  latter.  To  the  tube  which  extends  above  the 
stop-cock  attach  by  a  good  cork  the  neck  of  a  tubulated  receiver  of 
100  or  150  c.  c.  capacity,  the  interior  of  which  has  been  loosely  stuffed 
with  cotton.  Into  the  second  tubulature  of  the  receiver  fit  tightly  the 
delivery-tube  carrying  a  caoutchouc  connector  ;  into  this  connector  can 
be  fitted  a  tube  adapted  to  convey  the  gas  in  any  desired  direction. 
This  apparatus  is  charged  by  placing  the  zinc,  sulphide  of  iron,  or 
marble,  as  the  case  may  be,  in  the  basket,  hanging  the  basket  in  the, 
bell,  and  then  putting  the  bell-glass  full  of  air  into  its  place  and  closing 
the  stop-cock ;  the  cylinder  is  then  filled  with  dilute  acid  to  within  4 
c.  m.  of  the  top.  On  opening  the  cock,  the  weight  of  the  acid  expels 
the  air  from  the  bell,  the  acid  comes  in  contact  with  the  solid  in  the 
basket,  and  a  steady  supply  of  gas  is  generated  until  either  the  acid  is 
saturated  or  the  solid  dissolved :  if  the  cock  be  closed,  the  gas  accu- 
mulates in  the  bell,  and  pushes  the  acid  below  the  basket  so  that  all 
action  ceases.  In  cold  weather  the  apparatus  must  be  kept  in  a  warm 
place.  For  generating  hydrogen,  sulphuric  acid  diluted  with  four  or 
five  parts  of  water  is  used ;  for  sulphydric  acid,  sulphuric  acid  is  diluted 
with  fourteen  parts  of  water;  for  carbonic  acid,  chlorhydric  acid 
diluted  with  two  or  three  parts  of  water  is  to  be  preferred. 

20.  Glass  Retorts,  Flasks,  Beakers,  Test-tubes  and  Test-glasses.  All 
glass  vessels  which  are  meant  for  use  in  heating  liquids  must  have  uni- 
formly thin  bottoms.  Tubulated  retorts  are  much  more  generally  use- 
ful than  those  without  a  tubulature  ;  as  retorts  are  expensive  in  com- 
parison with  flasks,  they  are  less  used  than  formerly. 

The  neck  of  a  flask  should  have  such  a  form  that  it  can  be  tightly 
closed  by  a  cork,  and  the  lip  must  be  strengthened  to  resist  the  force 
used  in  pressing  in  the  cork,  either  by  a  rim  of  glass  added  on  the 
outside,  or  better  by  causing  the  rim  itself  to  flare  outward.  The 
actual  edge  of  the  rim  must  never  be  sharp  or  rough,  but  always  smooth 
and  rounded  by  partial  fusion.  The  thin-bottomed  flasks  in  which 
olive-oil  is  sometimes  imported  from  Italy  are  excellent  for  chemical- 
uses ;  their  edges  always  require,  however,  to  be  rounded  in  the  lamp. 
These  Florence  flasks,  which  are  very  much  to  be  recommended  both 
on  the  ground  of  cheapness  and  of  durability,  may  be  cleaned  by 
soaking  them  24  hours  in  a  weak  caustic  lye,  and  then  washing  them 
with  boiling  water. 

Beakers  are  thin  flat-bottomed  tumblers  with  a  slightly  flaring  rim. 
They  are  to  be  bought  in  sets  or  nests  which  sometimes  include  a  large 
range  of  sizes.  The  small  sizes  are  very  useful  vessels ;  the  large  are 
so  fragile  as  to  be  almost  worthless.  Up  to  the  capacity  of  about  a 


XXV111  TEST-TUBES    AND    THEIR 

litre,  beakers  are  to  be  recommended  for  heating  liquids  whenever 
it  is  an  object  to  have  the  whole  interior  of  the  vessel  readily  acces- 
sible. 

Test-tubes  are  little  cylinders  of  thin  glass,  with  round,  thin  bottoms, 
and  lips  slightly  flared.  Their  length  may  be  from  12  c.  m.  to  18  c.  in., 
and  their  diameter  1  c.  m.  to  2  c.  m. ;  they  should  never  have  a 
diameter  so  large  that  the  open  end  cannot  be  closed  by  the  ball  of  the 
thumb.  To  hold  the  tubes  upright  a  wooden  rack  is  necessary ;  be- 
sides the  row  of  holes  to  receive  a  dozen  test-tubes  bottom  down,  the 
rack  should  have  a  row  of  pegs  on  which  the  test-tubes  may  be  inverted 
when  not  in  use  ;  in  this  position  the  water  in  which  they  are  rinsed 
drains  off,  and  dust  cannot  be  deposited  within  the  tubes.  Test-tubes 
are  much  used  for  heating  small  quantities  of  liquid  over  the  gas  or 
spirit-lamp ;  they  may  generally  be  held  by  the  upper  end  in  the 
fingers  without  inconvenience,  but  if  a  liquid  is  to  be  boiled  long  in  a 
test-tube,  the  tube  must  be  held  in  wooden  nippers  (see  Fig.  1),  or  in  a 
strip  of  thick  folded  paper,  nipped  round  the  tube  and  grasped  between 
the  thumb  and  forefinger  just  outside  the  tube.  The  woodeo  nippers, 
above  mentioned,  are  made  of  two  bits  of  wood  about  a  foot  long 
hinged  together  at  the  back,  and  at  once  connected  and  kept  apart  by 
a  sliding  steel  or  brass  spring,  somewhat  like  those  used  on  certain 
pruning-shears  and  some  kinds  of  steel  nippers.  When  a  liquid  is 
boiling  actively  in  a  test-tube,  it  sometimes  happens  that  portions  of 
the  hot  liquid  are  projected  out  of  the  tube  with  some  force ;  the 
operator  should  always  be  careful  not  to  direct  a  tube,  which  he  is  thus 
using,  either  towards  himself  or  towards  any  other  person  in  his 
neighborhood.  Test-tubes  are  cleaned  by  the  aid  of  cylindrical 
brushes,  made  of  bristles  caught  between  twisted  wires,  like  those 
used  for  cleaning  lamp-chimneys :  they  should  have  a  round  end  of 
bristles. 

Two  precautions  are  invariably  to  be  observed  in  heating  glass  and 
porcelain  vessels  of  whatever  form  :  first,  the  outside  of  the  vessel  to  be 
heated  must  be  made  perfectly  dry ;  secondly,  the  temperature  must 
not  be  raised  too  rapidly.  When  a  large  flask  or  beaker  containing  a 
cold  liquid  is  first  warmed  over  a  lamp,  moisture  almost  invariably  con- 
denses upon  the  bottom  of  the  vessel :  this  moisture  should  be  wiped 
off  with  a  cloth. 

Glasses  such  as  are  represented  in  Figs.  9  and  1 7  are  convenient  for 
many  uses  not  involving  the  application  of  heat.  They  are  called  test- 
glasses,  and  may  be  had  of  various  shapes  and  sizes.  It  is  obvious  that 
cheap  wine  or  beer-glasses  and  common  jelly-tumblers  would  answer 
the  purposes  which  these  test-glasses  serve. 


MEASURING-GLASSES    AND    BURETTES. 


XXIX 


FIG.  XXIX. 


21.  Measuring-glasses  and  Burettes.  Measuring  glasses,  divided 
into  cubic  centimetres,  are  made  in  the  cylindrical  form,  and  also  in 
the  flaring  shape  common  in  druggists'  measuring- 
glasses  ;  the  cylindrical  form  is  to  be  preferred.  Such 
a  glass  of  250  c.  c.,  or  better  of  500  c.  c.  capacity  is 
a  very  useful  implement ;  a  flask  holding  just  one 
litre  when  filled  to  a  mark  upon  its  neck  is  also  con- 
venient. Smaller  quantities  of  liquid  are  measured 
with  burettes.  Mohr's  burette  is  the  most  generally 
useful  of  all  forms  of  this  instrument  (see  fig.  XXIX., 
the  right-hand  instrument)  ;  it  is  a  graduated  tube 
drawn  to  a  small  bore  at  the  bottom ;  a  caoutchouc 
connector  is  slipped  upon  the  bottom  of  the  tube,  a 
short  bit  of  tube  drawn  to  a  fine  point  is  thrust  into 
the  lower  end  of  the  connector,  and  a  spring-clip  nips 
the  connector  between  the  two  glass  tubes.  The 
spring-clip  closes  the  bottom  of  the  burette,  but  it 
can  of  course  be  opened  at  will  to  permit  the  liquid 
in  the  burette  to  flow  or  drop  out.  The  caoutchouc  affects  injuri- 
ously some  of  the  liquids  which  are  used  in  burettes,  so  that  this 
common  form  of  Mohr's  burette  is  not  always  applicable.  To  avoid 
this  difficulty  the  instrument  may  be  made  with  a  glass  cock,  but  is 
then  rather  costly.  Gay-Lussac's  burette  is  available  whenever  the 
caoutchouc  in  Mohr's  burette  is  objectionable.  The  construction  of 
Gay-Lussac's  burette  is  plainly  to  be  seen  in  the  figure  (see  Fig.  XXIX., 
the  left-hand  instrument)  ;  a  narrow  tube  runs  up  beside  the  large  grad- 
uated tube  to  the  top  of  the  latter,  and  the  liquid  can  be  poured  out  in 
drops  by  gently  inclining  the  instrument.  Its  fragility  is  a  serious  objec- 
tion to  this  form  of  burette  ;  the  danger  of  breaking  off  the  small  tube  is 
lessened,  if  a  small  piece  of  cork  be  inserted  between  the  two  tubes  at 
the  top,  and  a  string  tied  round  them  both.  A  wooden  foot  in  which 
it  may  stand  upright  upon  a  table  is  a  convenient  addition  to  Gay- 
Lussac's  burette.  Mohr's  burette  must  be  held  upright  in  a  .suitable 
screw-clamp,  or  fastened  against  a  wooden  frame  in  such  a  manner  that 
the  tube  shall  be  vertical,  firm,  and  at  the  same  time  easily  detached. 
The  fineness  of  the  graduation  upon  a  burette  may  be  increased  with- 
out loss  of  legibility  by  diminishing  the  bore  of  the  tube.  For  delicate 
work,  burettes  divided  into  tenths  of  a  cubic  centimetre  are  employed. 
The  way  in  which  the  reading-off  is  effected  is  a  matter  of  importance 
in  using  a  burette;  it  is  essential,  1st,  that  the  burette  should  be 
vertical"  2d,  that  the  eye  should  be  brought  to  a  level  with  the  surface 
of  the  fluid ;  3d,  that  a  fixed  standard  should  be  adopted  of  what  is  to 


XXX  READING    BURETTES. 

be  considered  the  surface.  If  a  burette,  partly  filled  with  a  liquid,  be 
held  between  the  eye  and  a  white  wall,  the  surface  of  the  liquid  pre- 
sents a  light  line  which  is  nearly  level,  and  just  below  this  line  a  second 
line,  which  is  dark  and  curved  with  the  convexity  downward.  If  a 
sheet  of  white  paper  be  held  immediately  behind  the  tube,  these 
two  lines,  though  somewhat  altered  in  appearance,  are  still  distinctly 
visible.  They  may  be  made  still  mor«  distinct  by  using  instead  of  white 
paper  a  card  half  white,  half  black,  with  a  straight  dividing  line  between 
the  two  colors.  On  holding  this  card  with  the  white  half  uppermost, 
and  the  border  line  between  white  and  black  from  2  to  3  m.  ni.  below 
the  lowest  visible  dark  line,  two  zones  are  brought  out,  a  light  zone  and 
a  dark  zone,  and  the  lower  limit  of  the  dark  zone  is  made  very  distinct. 
Care  must  be  taken  to  hold  the  card  invariably  in  the  same  relative 
position,  since,  if  it  be  held  lower  down,  the  "lower  border  of  the  dark 
zone  will  move  higher  up.  In  practice  the  lower  border  of  the  dark 
zone  is  read  off  as  the  surface  of  the  liquid,  this  being  the  most  distinctly 
marked  line.  There  is  one  exception  to  this  rule ;  when  an  opaque 
solution  of  permanganate  of  potassium  is  to  be  measured  in  Gay- 
Lussac's  burette,  the  upper  border  of  the  dark  zone  must  be  held  to  be 
the  surface  of  the  liquid:  in  this  case  it  is  best  to  place  the  burette 
against  a  white  background. 

The  zero  of  the  graduated  scale  on  a  burette  is  always  near  the  top 
of  the  tube.  In  order  to  fill  Mohr's  burette,  the  point  of  the  instrument 
is  dipped  into  the  liquid,  the  spring-clip  opened,  and  a  little  liquid,  suffi- 
cient at  least  to  reach  into  the  burette  tube,  is  sucked  up  by  applying 
the  mouth  to  the  upper  end  ;  the  spring-clip  is  then  closed,  and  the 
liquid  poured  into  the  burette  through  the  upper  end  until  it  has  risen 
a  little  above  the  zero-line.  By  opening  the  spring-clip,  the  liquid  is 
then  allowed  to  drop  out  until  the  exact  level  of  the  zero-line  is  reached. 
The  instrument  is  then  ready  for  use.  When  a  quantity  of  liquid  has 
been  allowed  to  flow  out  of  a  burette  thus  filled,  and  the  operator 
desires  to  read  off  the  amount  used,  he  must  wait  a  few  moments  to 
give  the  particles  of  fluid  adhering  to  the  sides  of  the  emptied  portion 
of  the  tube  time  to  run  down.  This  remark  applies  to  all  forms  of 
burette.  Erdmann's  swimmer  is  an  excellent  addition  to  Mohr's  burette. 
It  is  a  cylindrical  glass  float  of  such  a  width  as  nearly  to  fill  the  burette, 
but  yet  so  loosely  as  to  float  freely  up  and  down  with  the  liquor.  To 
set  the  instrument  at  zero  a  ring  cut  round  the  swimmer  is  brought  to 
coincide  with  the  line  0°  engraved  on  the  burette.  The  absolute 
height  of  the  liquor  in  the  burette  is  to  be  disregarded.  In  order  to 
read  the  height  of  the  liquor  in  the  burette  at  any  time,  it  is  only 
necessary  to  note  that  degree  on  the  scale  with  which  the  line  cut 
round  the  float  coincides. 


#> 


PORCELAIN    WARE.  XXXI 

22.  Pipettes.     Pipettes  are  tubes  drawn  to  a  point,  and  sometimes 
furnished  with  a  bulb  or  a  cylindrical  enlargement.     They  are  chiefly 
used  to  suck  small  quantities  of  fluid  out  of  a  vessel  without  disturbing 
the  bulk  of  the  liquid.     Figure  XXX.  represents  three      FlQ  xxx 
tbrms  of  pipette ;  the  form  with  the  lower  end  bent  up- 
wards is  used  to  introduce  liquids  into  a  bell  or  bottle  of. 

gas  standing  over  mercury.  Pipettes  graduated  into  cubic 
centimetres,  or  holding  a  certain  number  of  cubic  centime- 
tres when  filled  to  a  mark  on  the  stem,  are  often  convenient. 

23.  Wash-bottle.     A  wash-bottle  is  a  flask  with  a  uni- 
formly thin  bottom,  closed  with  a  sound  cork  or  caoutchouc 
stopper,  through  which  pass  two  glass-  tubes  as  shown  in 
Fig.  XXXI.     The  outer  end  of  the  longer  tube  is  drawn 

to  a  moderately  fine  point.     By  blowing  into  the  short  tube,  a  stream 
of  water  will  be  driven  out  of  the  long  tube  with  considerable  force. 
This  force  with  which  the  stream  is  projected  adapts  the  apparatus  to  re- 
moving precipitates  from  the  sides  of  vessels  as  well  as  to       FlG  Xxxi 
washing  them  on  filters.     As  the  wash-bottle  is  often  filled 
with  hot  or  even  boiling  waterr  it  may  be  improved  by 
binding  about  its  neck  a  ring  of  cork,  or  winding  the 
neck  closely  with  smooth  cord.    It  may  then  be  handled 
without  inconvenience,  when  hot. 

24.  Porcelain  Dishes   and   Crucibles.      Open   dishes, 
which  will  bear  heat  without  cracking,  are    necessary 
implements  in  the  laboratory  for  conducting  the  evapo- 
ration of  liquids.     The  best  evaporating  dishes  are  those 
made  of  Berlin  porcelain,  glazed  both  inside  and  out, 
and  provided  with  a  little  lip  projecting  beyond   the 
rim.      The  dishes  made  of  Meissen  porcelain  are  not 

glazed  on  the  outside,  and  are  not  so  durable  as  those  of  Berlin 
manufacture  ;  but  they  are  much  cheaper,  and  with  proper  care  last  a 
long  time.  The  small  Berlin  dishes  will  generally  bear  an  evaporation 
to  dryness  on  the  wire-gauze  over  the  open  flame  of  the  gas-lamp ;  the 
Meissen  dishes  do  not  so  well  endure  this  severe  treatment.  Evaporat- 
ing-dishes  are  made  of  all  diameters  from  3  c.  m.  to  45  c.  m. ;  they 
should  be  ordered  by  specifying  the  diameter  desired.  The  large  sizes 
are  expensive,  and  not  very  durable ;  they  should  never  be  used 
except  on  a  sand-bath.  Dishes  of  German  earthenware  are  as  good  as 
porcelain  for  many  uses,  and  are  much  to  be  recommended  in  place  of 
the  large  sizes  of  porcelain  dishes. 

Very  thin,  highly  glazed  porcelain  crucibles  with  glazed  covers  are 
made  both  at  Berlin  and  at  Meissen  near  Dresden;  they  are  indispen- 


XXX11  CRUCIBLES. 

sable  implements  to  the  chemist.  In  general,  the  Meissen  crucibles  are 
thinner  than  the  Berlin,  but  the  Berlin  crucibles  are  somewhat  less 
liable  to  crack ;  both  kinds  are  glazed  inside  and  out,  except  on  the 
outside  of  the  bottom.  Crucibles  should  be  ordered  by  specifying  the 
diameters  of  the  sizes  desired ;  they  are  to  be  had  of  nearly  a  dozen 
different  sizes,  with  diameters  varying  from  2  c.  m.  to  9  c.  m.  The 
smallest  and  largest  sizes  are  little  used ;  for  most  purposes  the  best 
sizes  are  those  between  3  c.  m.  and  5  c.  m.  in  diameter.  As  the  covers 
are  much  less  liable  to  be  broken  than  the  crucibles,  it  is  advantageous 
to  buy  more  crucibles  than  covers,  whenever  it  is  possible  so  to  do. 
Porcelain  crucibles  are  supported  over  the  lamp  on  an  iron-wire  triangle ; 
they  must  always  be  gradually  heated,  and  never  brought  suddenly  in 
contact  with  any  cold  substance  while  they  are  hot. 

25.  Rings  to  support  round-bottomed  vessels.     It  is  often  necessary  to 
support  globes,  round-bottomed  flasks,  evaporating  dishes,  and  round 
receivers  in  a  stable  manner  upon  the  table  or  other  flat  surface.     For 
this  purpose  rings  are  used,  made  of  braided  straw,  or  of  straw  wound 
about  a  core  of  straw,  or  of  tin  wound  with  listing  or  coarse  woollen 
cloth.     The  material  of  which  these  rings  are  made,  or  with  which  they 
are  covered,  ought  to  be  a  substance  which  does  not  conduct  heat  well, 
because  one  of  the  chief  uses  of  these  rings  is  to  receive  hot  vessels  just 
removed  from   the   lamp   or  sand-bath.     A  hot  flask  or  dish  would 
almost  certainly  be  broken,  if  it  were  placed  upon  the  cold  surface  of  a 
good  conductor  of  heat.     The  student  must  never  touch  a  hot  vessel 
with   cold  water,  or  bring  it  into  sudden  contact  with  a  surface  of 
marble,  iron,  copper,  or  other  conductor  of  heat. 

26.  Crucibles.     For  use  in  a  coal  fire  there  are  three  good  kinds  of 
crucible,  each  of  which  has  its  own  merits  which  recommend  it  for 
certain  purposes.   The  Hessian  crucibles  are  sold  in  nests  containing  from 
3  to  10  crucibles ;  there  are  10  sizes,  which  vary  from  3  to  25  c.  m.  in 
height.    They  have  a  triangular  form,  and  will  withstand  a  very  high  tem- 
perature, if  they  are  warmed  before  being  put  in  the  fire.     They  have  no 
covers,  but  a  triangular  piece  of  soapstone  may  be  very  conveniently  used 
as  a  cover  for  these  crucibles.     Hessian  crucibles  are  cheaper  than  any 
other  kind,  and  are  therefore  the  most  used.     The  French  crucibles, 
called  Beaufay  crucibles,  are  admirable,  but  too  dear  for  common  use. 
They  have  a  tall,  narrow  form,  a  smooth  surface,  and  a  small  lip ;  covers 
for  the  crucibles  are  sold  separately.     The  crucibles  are  sold,  not  in 
nests,  but  singly  ;  there  are  22  sizes,  which  vary  from  4  to  40  c.  m.  in 
height.     They  are  highly  refractory.     Plumbago  crucibles  are  used  for 
the  fusion  of  the  most  refractory  metals,  gold,  silver,   copper,  brass, 
steel,  iron,  and  so  forth  ;  they  resist  better  than  any  other  crucibles  the 


MORTARS.  xxxiii 

combined  action  of  a  very  high  temperature  and  a  strong  flux,  and  as 
they  are  not  liable  to  crack,  they  may  often  be  used  several  times 
without  risk.  Their  first  cost  is  higher  than  that  of  any  other  crucibles. 
Crucibles  are  mainly  used  for  the  fusion  and  reduction  of  metals,  but 
there  are  also  many  chemical  compounds  which  can  only  be  prepared 
at  the  very  high  temperatures  which  by  the  use  of  crucibles  we  are 
able  to  com nand.  Although  crucibles  often  withstand  the  most  sudden 
changes  of  temperature,  it  is,  nevertheless,  expedient  as  a  general  rule 
to  heat  up  a  crucible  gradually,  and  to  previously  warm  a  charge 
which  is  to  be  placed  in  a  crucible  already  hot.  If  a  cold  crucible  is  to 
be  introduced  into  a  fire,  it  should  first  be  placed  in  the  coldest  part  of 
the  fire  and  gradually  brought  into  the  hottest  part. 

27.  Tongs  and  Pincers.     Hot  crucibles  are  handled  by  means  of 
tongs  of  various  shapes  and  sizes,  according  to  the  weight  and  nature 
of  the  vessels  to  be  lifted.     Fig.  XXXII.  represents  two  good  forms  of 
stout  iron  tongs  for  lifting  large  FlG  XXXII 

crucibles  out  of  a  coal  fire.  The 
manner  of  using  them  is  readily 
understood  from  the  figure. 

Small  porcelain  crucibles  are 
handled,  when  hot,  by  means  of 
small  steel  or  iron  tongs,  such  as 
are  represented  in  Fig.  XXXIII. 

Small  steel  pincers  (jewellers'  tweezers)  are  applied  in  the  labora- 
tory to  a  great  variety  of  uses. 

28.  Mortars.    Iron,   porcelain,  and  agate  EIG.  xxxm. 
mortars  are  used  by  chemists  to  reduce  solids 

to  powder.  An  iron  mortar  is  useful  for 
coarse  work,  and  for  effecting  the  first  rough  breaking  up  of 
substances  which  are  subsequently  powdered  in  the  porcelain  or 
agate  mortar.  If  there  be  any  risk  of  fragments  being  thrown  out  of 
the  mortar,  it  should  be  covered  with  a  cloth  or  piece  of  stiff*  paper, 
having  a  hole  in  the  middle  through  which  the  pestle  may  be  passed. 
Pieces  of  stone,  minerals,  and  lumps  of  brittle  metals  may  be  safely 
broken  into  fragments  suitable  for  the  mortar  by  wrapping  them  in 
strong  paper,  laying  them  so  enclosed  upon  an  anvil  and  striking  them 
with  a  heavy  hammer.  The  paper  envelope  retains  the  broken  parti- 
cles which  might  otherwise  fly  about  in  a  dangerous  manner,  and  be 
lost. 

The  best  porcelain  mortars  are  those  known  by  the  name  of  Wedge- 
wood-ware,  but  there  are  many  cheaper  substitutes.  Porcelain  mortars 
will  not  bear  sharp  and  heavy  blows ;  they  are  intended  rather  for 


XXXIV  PULVERIZING. 

grinding  and  trituration  than  for  hammering ;  the  pestle  may  either  be 
formed  of  one  piece  of  porcelain,  or  a  piece  of  porcelain  cemented  to  a 
wooden  handle ;  the  latter  is  the  less  desirable  form  of  pestle.  Unglazed 
porcelain  mortars  are  to  be  preferred.  In  selecting  mortars,  the  following 
points  should  be  attended  to,  —  1st,  the  mortar  should  not  be  porous  ;  it 
ought  not  to  absorb  strong  acids  or  any  colored  fluid,  even  if  such  liquids 
be  allowed  to  stand  for  hours  in  the  mortar ;  2d,  it  should  be  very  hard, 
and  its  pestle  should  be  of  the  same  hardness  ;  3d,  it  should  be  sound  ;  4th, 
it  should  have  a  lip  for  the  convenience  of  pouring  out  liquids  and  fine 
powders.  As  a  rule,  porcelain  mortars  will  not  endure  sudden  changes 
of  temperature.  They  may  be  cleaned  by  rubbing  in  them  a  little  sand 
soaked  in  nitric  or  sulphuric  acid,  or  if  acids  are  not  appropriate,  in 
caustic  soda. 

Agate  mortars  are  only  intended  for  trituration ;  a  blow  would  break 
them.  They  are  exceedingly  hard,  and  impermeable.  The  material 
is  so  precious  and  so  hard  to  work,  that  agate  mortars  are  always  small. 
The  pestles  are  generally  inconveniently  short,  —  a  difficulty  which 
may  be  remedied  by  fitting  the  agate  pestle  into  a  wooden  handle. 

In  all  grinding  operations  in  mortars,  whether  of  porcelain  or  agate, 
it  is  expedient  to  put  only  a  small  quantity  of  the  substance  to  be 
powdered  into  the  mortar  at  once.  The  operation  of  powdering  will 
be  facilitated  by  sifting  the  matter  as  fast  as  it  is  powdered,  returning 
to  the  mortar  the  particles  which  are  too  large  to  pass  through  the 
sieve. 

29.  Spatula.     For  transferring  substances  in  powder,  or  in  small 
grains  or  crystals,  from  one  vessel  to  another,  spatulae  and  scoops  made 
of  horn  or  bone  are  convenient  tools.     A  coarse  bone  paper-knife 
makes  a  good  spatula  for  laboratory  use.     Cards,  free  from  glaze  and 
enamel,  are  excellent  substitutes  for  spatulae. 

30.  Thermometers.     Thermometers  intended  for  chemical  use  must 
have  no  metal,  and  no  wood  or  other  organic  material  upon  their  outer 
surfaces  ;  their  external  surfaces  must  be  wholly  of  glass.    The  best  ther- 
mometers are  straight  glass  tubes,  of  uniform  diameter,  with  cylindrical 
instead  of  spherical  bulbs,  and  having  the  scale  engraved  upon  the 
glass  ;  such  instruments  can  be  passed  tightly  through  a  cork,  and  are 
free  from  many  liabilities  to  error  to  which  thermometers  with  paper  or 
metal  scales  are  always  exposed.     A  cheaper  kind  of  thermometer, 
having  a  paper  scale  enclosed  in  a  glass  envelope,  will  answer  for  most 
of  the  experiments  of  this  manual. 

31.  Furnaces.    For  all  common  fusions,  an  anthracite  or  coke  fire  in 
an  ordinary  cylinder  stove  will  suffice.     The  chafing-dish,  or  open 
portable  stove,  such  as  is  used  by  plumbers  for  example,  is  very  con 


THE    METRICAL    SYSTEM.  XXXV 

venient  for  operations  which  require  less  heat.  The  clay  buckets  used 
as  open  furnaces  are  better  than  the  iron  ones,  because  they  hold  the 
heat  better. 

Charcoal  is  the  fuel  used  in  these  open  fires.  A  very  useful  accom- 
paniment to  these  portable  furnaces  is  a  piece  of  straight  stove-pipe, 
about  60  c.  m.  long  and  10  c.  m.  wide,  and  flaring  out  below  like  a 
funnel  until  it  is  wide  enough  to  cover  the  top  of  the  furnace.  This 
contrivance  powerfully  increases  the  draught,  and  is  used  to  urge  the 
fire  during  kindling,  or  to  intensify  it  while  a  fusion  is  in  progress. 
With  a  furnace  of  this  description  there  is  no  difficulty  in  keeping  a 
small  crucible  white-hot  for  a  short  time. 


THE  METRICAL  SYSTEM  OF  WEIGHTS  AND  MEASURES. 

The  metrical  system,  employed  in  the  affairs  of  e very-day  life  by 
most  of  the  nations  of  continental  Europe  and  by  scientific  writers 
throughout  the  world,  is  based  upon  a  fundamental  unit,  or  measure,  of 
length,  called  a  metre.  This  metre  is  defined  as  the  40-millionth  part 
of  the  circumference  of  the  earth,  or,  in  other  words,  of  a  "  great 
circle "  or  meridian ;  its  length  was  originally  determined  by  actual 
measurement  of  a  considerable  arc  of  a  meridian,  but  the  various 
measurements  heretofore  made  of  the  length  of  the  earth's  meridian 
differ  slightly  from  each  other,  and  it  is  to  be  expected,  and  indeed 
hoped,  that  the  steady  improvements  of  methods  and  instruments  will 
make  each  successive  determination  of  the  length  of  the  meridian  better 
than,  and  therefore  different  from,  the  preceding.  It  is,  therefore, 
necessary  to  define  the  standard  of  length,  by  legislation,  to  be  a  certain 
rod  of  metal,  deposited  in  a  certain  place  under  specified  guaranties, 
and  to  secure  the  uniformity  and  permanence  of  the  standard  by  the 
multiplication  of  exact  copies  in  safe  places  of  deposit. 

From  this  single  quantity,  the  metre,  all  other  measures  are  deci- 
mally derived.  Multiplied  or  divided  by  10,  100,  1000,  and  so  forth, 
the  metre  supplies  all  needed  linear  measures,  and  the  square  metre 
and  cubic  metre,  with  their  decimal  multiples,  supply  all  needed 
measures  of  area  or  surface,  011  the  one  hand,  and  of  solidity  or  capacity 
on  the  other. 

From  the  unit  of  measure  to  the  unit  of  weight  the  transition  is 
admirably  simple  and  convenient.  The  cube  of  the  1-hundredth  of 
the  linear  metre  is,  of  course,  the  millionth  of  the  cubic  metre  ;  its 
bulk  is  about  that  of  a  large  die  of  the  common  back-gammon  board. 
This  little  cube  of  pure  water  is  the  universal  unit  of  weight,  a  gramme, 


XXXVI  THE    METRICAL    SYSTEM. 

which,  decimally  multiplied  and  divided,  is  made  to  express  all  weights. 
The  numbers  expressing  all  weights,  from  the  least  to  the  greatest,  find 
direct  expression  in  the  decimal  notation ;  the  weights  used  in  different 
trades  only  differ  from  each  other  in  being  different  decimal  multiples 
of  the  same  fundamental  unit ;  and  in  comparing  together  weights  and 
volumes,  none  but  easy  decimal  computations  are  ever  necessary. 

The  nomenclature  of  the  metrical  system  is  extremely  simple ;  one 
general  principle  applies  to  each  of  the  following  tables.  The  Greek 
prefixes  for  10,  100,  and  1000,  viz.,  deca,  hecto,  and  kilo,  are  used  to 
signify  multiplication,  while  the  Latin  prefixes  for  10,  100,  and  1000, 
viz.,  deci,  centi,  and  milli,  are  employed  to  express  subdivision.  Of  the 
names  thus  systematically  derived  from  that  of  the  unit  in  each  table, 
many  are  not  often  used  ;  the  names  in  common  use  are  those  printed 
in  small  capitals.  Thus  in  the  table  for  linear  measure,  only  the  metre, 
kilometre,  centimetre,  and  millimetre  are  in  common  use,  —  the  first 
for  such  purposes  as  the  English  yard  subserves,  the  second  instead  of 
the  English  mile,  the  third  and  fourth  in  lieu  of  the  fractions  of  the 
English  foot  and  inch. 

LINEAR    MEASURE. 

Metre. 

(  MILLIMETRE,  0.001  or  1-1,000  of  a  metre. 

Divisions . .  <  CENTIMETRE,  ==  0.01   or  1-100  u 

(Decimetre,  =  0.1      or  1-10 

Unit METRE,  1. 

( Decametre,  =  10. 

Multiples  . .  <  Hectometre,  100. 

(  KILOMETRE,  1,000. 

SURFACE   MEASURE. 

(  Millimetre  square,  0.000,001  of  a  metre  square. 

Divisions . .  <  Centimetre  square,         ==  0.000,1        " 

(  Decimetre  square,  0.01 

Unit METRE  SQUARE,  1. 

CUBIC  MEASURE. 

Cubic  Metre. 

(  Cubic  Millimetre  =  0.000,000,001 

Divisions  . .  /  Cubic  Centimetre  =  0.000,001 

(  Cubic  Decimetre  0.001 

Unit CUBIC  METRE  =  1. 

!  Cubic  Decametre  =  1,000. 

Cubic  Hectometre  =  1,000,000. 

Cubic  Kilometre  1,000,000,000. 

The  table  for  land  measure  we  omit,  as  having  no  connection  with 
our  subject.  For  the  measurement  of  wine,  beer,  oil,  grain,  and  simi- 
lar wet  and  dry  substances,  a  smaller  unit  than  the  cubic  metre  is 
desirable.  The  cubic  decimetre  has  been  selected  as  a  special  standard 
of  capacity  for  the  measurement  of  substances  such  as  are  bought  and 
sold  by  the  English  wet  and  dry  measures.  The  cubic  decimetre  thus 
used  is  called  a  litre. 


THE    METRICAL    SYSTEM.  XXXV ii 

CAPACITY   MEASURE. 

Litres.  Cubic  Metres. 

I  Millilitre        =            0.001  =      0.000,001  =1  cubic  centimetre. 

Divisions . .  <!  Centilitre       =            0.01  =      0.000,01 

(  Decilitre        ==            0.1  =      0.000,1 

Unit LITRE            ==           1.  =      0.001        =  1  cubic  decimetre. 

{Decalitre        ==          10.  =      0.01. 

HECTOLITRE  =        100.  =  0.1 

Kilolitre         =     1,000.  =      1.              =  1  cubic  metre. 

The  table  of  weights  bears  an  intimate  relation  to  this  table  of 
capacity.  As  already  mentioned,  the  weight  of  that  die-sized  cube,  a 
cubic  centimetre  or  millilitre  of  distilled  water  (taken  at  4°,  its  point  of 
greatest  density)  constitutes  the  metrical  unit  of  weight.  This  weight 
is  called  a  gramme.  From  the  very  definition  of  the  gramme,  and 
from  the  table  of  capacity-measure,  it  is  clear  that  a  litre  of  distilled 
water  at  4°  will  weigh  1000  grammes. 

WEIGHTS. 
Grammes. 

(  MILLIGRAMME     =         0.001 
Divisions  . .  <  CENTIGRAMME     =          0.01 
[  DECIGRAMME       =          0.1 

Unit GRAMME  '  '  *•          1.     =  1  cubic  centimetre  of  water  at  4°. 

(  Decagramme        =        10. 
Multiples  . .  <  Hectogramme       =      100. 

(Kilogramme         =    1,000.      =  1  cubic  decimetre  of  water  at  4°. 

The  simplicity  and  directness  of  the  relations  between  weights  and 
volumes  in  the  metrical  system,  can  now  be  more  fully  explained. 
The  chemist  ordinarily  uses  the  gramme  as  his  unit-weight,  and  for  his 
unit  of  volume  a  cubic  centimetre,  which  is  the  bulk  of  a  gramme  of 
water.  For  coarser  work,  the  kilogramme  becomes  the  unit  of  weight, 
and  the  corresponding  unit  of  measure  is  the  litre,  which  is  the  bulk  of 
a  kilogramme  of  water.  In  commercial  dealings,  in  manufacturing 
processes,  and  above  all  in  scientific  investigations,  these  simple  rela- 
tions between  weights  and  measures  have  been  found  to  be  an  inesti- 
mable advantage.  The  numerical  expressions  for  metrical  weights  and 
measures  may  always  be  read  as  decimals.  Thus  5.126  metres  will  be 
read  five  metres  and  one  hundred  and  twenty-six  thousandths,  and  not 
five  metres,  one  decimetre,  two  centimetres,  and  six  millimetres.  The 
expression  10.5  grammes  is  read  ten  and  five-tenths  grammes;  just  as 
.we  say  one  hundred  and  five  dollars,  not  ten  eagles  and  five  dollars  ;  or 
sixty-five  cents,  not  six  dimes  and  five  cents.  All  computations  under 
the  metrical  system  are  made  with  decimals  alone. 

The  abbreviations  commonly  met  with  in  chemical  literature  are  :  — 

m.  m.  for  millimetre  :  c.  m.  for  centimetre  ; 

m.  for  metre  ;  o.  c.  for  cubic  centimetre  ; 

grm.  for  gramme  ;  kilo,  for  kilogramme. 


xxxvni 


THERMOMETERS    COMPARED. 


TABLE  —  For  the  Conversion  of  Degrees  on  the  Centigrade  Thermometer  into 
Degrees  of  Fahrenheit's  Scale. 


Cent.     Fahr. 

Cent.     Fahr. 

Cent.     Fahr. 

Cent.     Fahr. 

0          0 

_50  —58.0 

o        o 
—  3     26  6 

0             0 

44      111.2 

0              0 

91      195.8 

—  49  —  56.2 

—  2     28.4 

45      113.0 

92     197.6 

—  48   —  54.4 

—  1     30.2 

46      114.8 

93     199.4 

—  47  —52.6 

0     32.0 

47      116.6 

94     201.2 

—  46  —50.8 

-j-  1     33.8 

48      118.4 

95     203.0 

—  45   -49.0 

2     35.6 

49      120.2 

96     204.8 

—  44   -47.2 

3     37.4 

50      122.0 

97     206.6 

—  43   —45.4 

4     39.2 

51      123.8 

98     208.4 

—  42   —43.6 

5     41.0 

52      125.6 

99     210.2 

—  41   —41.8 

6     42.8 

53      127.4 

100     212.0 

—  40   —40.0 

7     44.6 

54      129.2 

101     213.8 

—  39  —38.2 

8     46.4 

55      131.0 

102     215.6 

—  38  —36.4 

9     48.2 

56      132.8 

103     217.4 

—  37   —34.6 

10     50.0 

57      134.6 

104     219.2 

—  36  —  32.8 

11     51.8 

58      136.4 

105     221.0 

—  35  —31.0 

12     53.6 

59      138.2 

106     222.8 

—  34   -  29.2 

13     55.4 

60      140.0 

107     224.6 

—  33   -27.4 

14     57.2 

61      141.8 

108     226.4 

—  32  —25.6 

15     59.0 

62      143.6 

109     228.2 

—  31  —23.8 

16     60.8 

63      145.4 

110     230.0 

—  30  —  22.0 

17  '   62.6 

64      147.2 

111     231.8 

—  29  —  20.2 

18     64.4 

65      149.0 

112     233.6 

—  28  —18.4 

19     66.2 

66      150.8 

113     235.4 

—  27   -16.6 

20     68.0 

67      152.6 

114     237.2 

—  26   -14.8 

21     69.8 

68      154.4 

115     239.0 

—  25   —  13.0 

22     71.6 

69      156.2 

116     240.8 

—  24  —11.2 

23     73.4 

70      158.0 

117     242.6 

—  23   —  9.4 

24     75.2 

71      159.8 

118     244.4 

—  22  —  7.6 

25     77.0 

72      161.6 

119     246.2 

—  21   —  5.8 

26     78.8 

73      163.4 

120     248.0 

—  20    -  4.0 

27     80.6 

74      165.2 

121     249.8 

—  19  —  2.2 

28     82.4 

75      167.0 

122     251.6 

—  18  —  0.4 

29     84.2 

76      168.8 

123     253.4 

—  17  +  1.4 

30     86.0 

77      170.6 

124     255.2 

—  16      3.2 

31     87.8 

78      172.4 

125     257.0 

—  15      5.0 

32     89.6 

79      174.2 

126     258.8 

—  14      6.8 

33     91.4 

80      176.0 

127     260.6 

—  13      8.6 

34     93.2 

81      177.8 

128     262.4 

—  12     10.4 

35     95.0 

82      179.6 

129     264.2 

—  11     12.2 

36     96.8 

83      181.4 

130     266.0 

—  10     14.0 

37     98.6 

84      183.2 

131     267.8 

—  9     15.8 

38    100.4 

85      185.0 

132     269.6 

—  8     17.6 

39    102.2 

86      186.8 

133     271.4 

—  7     19.4 

40    104.0 

87      188.& 

134     273.2 

—  6     21.2 

41    105.8 

88      190.4 

135     275.0 

—  5     23.0 

42    107.6 

89      192.2 

136     276.8 

—  4     24.8 

43    109.4 

90      194.0 

137     278.6 

THERMOMETERS    COMPARED. 


XXXIX 


TABLE  —  Continued. 


Cent.     Fahr. 

Cent.     Fahr. 

Cent.     Fahr. 

Cent.     Fahr. 

o         o 

O            0 

o         o 

o  i       o 

138     280.4 

185     365.0 

232     449.6 

279     534.2 

139     282.2 

186     366.8 

233     451.4 

280     536.0 

140     284.0 

187     368.6 

234     453.2 

281     537.8 

141     285.8 

188     370.4 

235     455.0 

282     539.6 

142     287.6 

189     372.2 

236     456.8  . 

283     541.4 

143     289.4 

190     374.0 

237     458.6 

284     543.2 

144     291.2 

191     375.8 

238     460.4 

285     545.0 

145     293.0 

192     377.6 

239     462.2 

286     546.8 

146     294.8 

193     379.4 

240     464.0 

287     .048.6 

147     296.6 

194     381.2 

241     465.8 

288     550.4 

148     298.4 

195     383.0 

242     467.6 

289     552.2 

149     300.2 

196     384.8 

243     469.4 

290     554.0 

150     302.0 

197     386.6 

244     471.2 

291     555.8 

151     303.8 

198     388.4 

245     473.0 

292     557.6 

152     305.6 

199     390.2 

246     474.8 

293     559.4 

153     307.4 

200     392.0 

247     476.6 

294     561.2 

154     309.2 

201     393.8 

248     478.4 

295     563.0 

155     311.0 

202     395.6 

249     480.2 

296     564.8 

156     312.8 

203     397.4 

250     482.0 

297     566.6 

157     314.6 

204     399.2 

251     483.8 

298     568.4 

158     316.4 

205     401.0 

252     485.6 

299     570.2 

159     318.2 

206     402.8 

253     487.4 

300     572.0 

160     320.0 

207     404.6 

254     489.2 

301     573.8 

161     321.8 

208     406.4 

255     491.0 

302  '   575.6 

162     323.6 

209     408.2 

256     492.8 

303     577.4 

163     325.4 

210     410.0 

257     494.6 

304     579.2 

164     327.2 

211     411.8 

258     496.4 

305     581.0 

165     329.0 

212     413.6 

259     498.2 

306     582.8 

166     330.8 

213     415.4 

260     500.0 

307     584.6 

167     332.6 

214     417.2 

261     501.8 

308     586.4 

168     334.4 

215     419.0 

262     503.6 

309     588.2 

169     336.2 

216     420.8 

263     505.4 

310     590.0 

170     338.0 

217     422.6 

264     507.2 

311     591.8 

171     339.8 

218     424.4 

265     509.0 

312     593.6 

172     341.6 

219     426.2 

266     510.8 

313     595.4 

173     343.4 

220     428.0 

267     512.6 

314     597.2 

174     345.2 

221     429.8 

268     514.4 

315     599.0 

175     347.0 

222     431.6 

269     516.2 

316     600.8 

176     348.8 

223     433.4 

270     518.0 

317     602.6 

177     350.6 

224     435.2 

271     519.8 

318     604.4 

178     352.4 

225     437.0 

272     521.6 

319     606.2 

179     354.2 

226     438.8 

273     523.4 

320     608.0 

180     356.0 

227     440.6 

274     525.2 

181     357.8 

228     442.4 

275     527.0 

182     359.6 

229     444.2 

276     528.8 

-183     361.4 

230     446.0 

277     530.6 

184     363.2 

231     447.8 

278     532.4 

xl 


THE    METRICAL    SYSTEM. 


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0 

UNIVERSITY  OF  CALIFORNIA   LIBRARY 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


JAN 


NOV    3  1915 


DEC  20 
flpR  2   J918 

$W  T^  rS\o   *"" 
AUG  16  1920 

SEP    v 
MAR  31 1928 

DEO    3  1 


FEB    27  1932 


MAY  28  1940 


30m-6,'14 


>>pcs»>>i> 


YB   16815 


I 


